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Blog Quizzes Top SQL MCQs With Answers

Top SQL MCQs With Answers

By: Jaya

Last Updated: October 5th, 2026

Read Time: 30:00 Minutes

1. Top SQL MCQs for Interview Preparation

2. Your Quiz Result

Preparing for your next SQL interview? This interactive SQL MCQ list is your ultimate resource. It includes the most frequently asked SQL multiple-choice questions, covering key topics like queries, joins, indexing, and more. Test your skills and identify areas for improvement!

Note: "Score 60% or more and unlock an exclusive chance to grab up to 50% off on all self-paced SQL courses."

Let's Start!

Top SQL MCQs for Interview Preparation

1. What does SQL stand for?

A) Standard Query Language
B) Structured Query Language
C) Sequential Query Language
D) System Query Language

2. Which SQL statement is used to retrieve data from a database?

A) GET
B) FETCH
C) SELECT
D) READ

3. Which SQL clause is used to filter rows before grouping?

A) HAVING
B) WHERE
C) GROUP BY
D) ORDER BY

4. Which clause is used to group rows having the same values?

A) ORDER BY
B) WHERE
C) DISTINCT
D) GROUP BY

5. Which SQL clause filters grouped results after aggregation?

A) HAVING
B) WHERE
C) FILTER
D) GROUP

6. Which keyword removes duplicate rows from a SELECT result?

A) UNIQUE
B) REMOVE
C) DISTINCT
D) DEDUP

7. Which clause is commonly used to sort query results?

A) SORT BY
B) ORDER BY
C) GROUP BY
D) ARRANGE BY

8. What is the default sort direction of ORDER BY in standard SQL?

A) RANDOM
B) DESC
C) NONE
D) ASC

9. Which operator is used to search for a pattern in SQL?

A) LIKE
B) MATCHES
C) SEARCH
D) FIND

10. Which wildcard represents any sequence of zero or more characters with LIKE?

A) _
B) ?
C) %
D) *

11. Which LIKE wildcard represents exactly one character?

A) %
B) _
C) *
D) #

12. Which operator checks whether a value is within a specified range?

A) WITHIN
B) RANGE
C) BETWEENED
D) BETWEEN

13. Which operator is used to compare a value against a list of values?

A) IN
B) LIST
C) ANYLIST
D) MATCH

14. Which condition correctly checks whether a column contains NULL?

A) column = NULL
B) column == NULL
C) column IS NULL
D) column NULL

15. Which SQL expression is used to replace NULL with another value?

A) REPLACE_NULL
B) COALESCE
C) NULLREPLACE
D) DEFAULTNULL

16. Which function returns the number of rows in a result set?

A) TOTAL()
B) ROWS()
C) NUMBER()
D) COUNT()

17. Which aggregate function returns the average of numeric values?

A) AVG()
B) MEAN()
C) AVERAGE()
D) MID()

18. Which aggregate function returns the largest value?

A) HIGH()
B) TOP()
C) MAX()
D) LARGEST()

19. Which aggregate function returns the smallest value?

A) LOW()
B) BOTTOM()
C) SMALLEST()
D) MIN()

20. Which aggregate function calculates the total of numeric values?

A) TOTAL()
B) SUM()
C) ADD()
D) PLUS()

21. Which keyword is used to assign an alias to a column or table?

A) RENAME
B) LABEL
C) AS
D) ALIAS

22. Which SQL statement is used to add new rows to a table?

A) INSERT
B) ADD
C) APPEND
D) CREATE ROW

23. Which SQL statement modifies existing rows?

A) MODIFY
B) CHANGE
C) ALTER ROW
D) UPDATE

24. Which SQL statement removes selected rows from a table?

A) REMOVE
B) DELETE
C) DROP ROW
D) ERASE

25. Which statement removes an entire table definition and its data?

A) DELETE TABLE
B) REMOVE TABLE
C) DROP TABLE
D) CLEAR TABLE

26. Which statement removes all rows while retaining the table structure in common SQL implementations?

A) TRUNCATE TABLE
B) DROP TABLE
C) DELETE DATABASE
D) CLEAR DATABASE

27. Which SQL command is used to create a new table?

A) NEW TABLE
B) MAKE TABLE
C) BUILD TABLE
D) CREATE TABLE

28. Which command modifies the structure of an existing table?

A) CHANGE TABLE
B) ALTER TABLE
C) UPDATE TABLE
D) MODIFY TABLE

29. Which constraint uniquely identifies each row in a table?

A) UNIQUE
B) CHECK
C) PRIMARY KEY
D) DEFAULT

30. What is a foreign key primarily used for?

A) Establishing a relationship between tables
B) Encrypting a table
C) Sorting rows
D) Creating indexes automatically

31. Which constraint prevents NULL values in a column?

A) UNIQUE
B) CHECK
C) DEFAULT
D) NOT NULL

32. Which constraint ensures that values in a column are unique?

A) DISTINCT
B) UNIQUE
C) DIFFERENT
D) SINGLE

33. Which constraint is used to enforce a condition on column values?

A) RULE
B) VALIDATE
C) CHECK
D) ASSERT

34. Which constraint supplies a value when no value is explicitly provided?

A) DEFAULT
B) AUTO
C) FALLBACK
D) VALUE

35. Which type of join returns matching rows from both tables?

A) LEFT JOIN
B) CROSS JOIN
C) FULL JOIN
D) INNER JOIN

36. Which join returns all rows from the left table and matching rows from the right table?

A) RIGHT JOIN
B) LEFT JOIN
C) INNER JOIN
D) CROSS JOIN

37. Which join returns all rows from the right table and matching rows from the left table?

A) LEFT JOIN
B) INNER JOIN
C) RIGHT JOIN
D) CROSS JOIN

38. What does a CROSS JOIN produce?

A) Cartesian product
B) Only matching rows
C) Only unmatched rows
D) Duplicate-free rows

39. Which join can return matched and unmatched rows from both tables?

A) INNER JOIN
B) LEFT JOIN
C) CROSS JOIN
D) FULL OUTER JOIN

40. Which SQL join is useful when a table is joined to itself?

A) CROSS JOIN
B) SELF JOIN
C) NATURAL JOIN
D) FULL JOIN

41. Which set operator combines results and removes duplicate rows?

A) JOIN
B) MERGE
C) UNION
D) COMBINE

42. Which set operator combines results while retaining duplicates?

A) UNION ALL
B) UNION DISTINCT
C) MERGE ALL
D) JOIN ALL

43. Which set operator returns rows present in both query results?

A) UNION
B) DIFFERENCE
C) MERGE
D) INTERSECT

44. Which SQL concept is used to query the result of another SELECT statement?

A) Trigger
B) Subquery
C) Index
D) Constraint

45. What is a correlated subquery?

A) A subquery that has no SELECT statement
B) A subquery that always returns one row
C) A subquery that references columns from the outer query
D) A subquery used only with DELETE

46. Which keyword is commonly used to test whether a subquery returns at least one row?

A) EXISTS
B) FOUND
C) PRESENT
D) AVAILABLE

47. Which SQL object stores a reusable query definition?

A) Trigger
B) Index
C) Cursor
D) View

48. Which command is generally used to create a view?

A) MAKE VIEW
B) CREATE VIEW
C) BUILD VIEW
D) NEW VIEW

49. What is a common reason for using a database view?

A) To permanently duplicate every table
B) To replace database backups
C) To simplify complex queries and restrict exposed data
D) To automatically increase storage

50. What is an index primarily designed to improve?

A) Query data retrieval performance
B) Table storage size
C) SQL syntax validation
D) User authentication

51. What is a common trade-off of adding indexes?

A) They always increase duplicate rows
B) They disable SELECT queries
C) They prevent constraints
D) They consume storage and can slow writes

52. Which index can contain multiple columns?

A) Scalar index
B) Composite index
C) Atomic index
D) Single index

53. In a composite index on (A, B), which query often benefits most directly from the index prefix?

A) A query filtering only on B
B) A query filtering only on an unrelated column
C) A query filtering on A
D) A query containing no predicates

54. What does a query execution plan show?

A) How the database optimizer plans to execute a query
B) Only table definitions
C) User passwords
D) Database backup contents

55. Which practice is generally useful when diagnosing a slow SQL query?

A) Remove every index
B) Add random indexes
C) Disable constraints permanently
D) Inspect the execution plan and query statistics

56. Which SQL feature assigns a sequential value or ranking within a result set?

A) GROUP BY
B) Window function
C) Foreign key
D) Trigger

57. Which window function assigns a unique sequential number to rows?

A) RANK()
B) DENSE_RANK()
C) ROW_NUMBER()
D) NUMBER_RANK()

58. What happens to ranking values after a tie when using RANK()?

A) Rank numbers can contain gaps
B) Every row receives a different rank
C) All ranks become zero
D) Ranking stops

59. How does DENSE_RANK() differ from RANK()?

A) It cannot handle ties
B) It only works with strings
C) It always produces random order
D) It does not leave gaps after ties

60. Which clause defines the rows over which a window function operates?

A) WINDOW BY
B) OVER
C) RANGE BY
D) PARTITION

61. Which keyword divides rows into groups for a window function?

A) GROUP WINDOW
B) GROUP BY
C) PARTITION BY
D) SPLIT BY

62. Which window function is commonly used to calculate a running total?

A) SUM() OVER()
B) TOTAL OVER()
C) RUNNING_SUM()
D) SUM_RUNNING()

63. Which function returns a value from a following row in a window?

A) PREVIOUS()
B) NEXTROW()
C) FOLLOWING()
D) LEAD()

64. Which function returns a value from a preceding row in a window?

A) PREV()
B) LAG()
C) BEFORE()
D) PRIORROW()

65. Which SQL expression is commonly used for conditional logic?

A) IF ELSE
B) WHEN IF
C) CASE
D) SWITCH SQL

66. Which expression returns the first non-NULL value from its arguments?

A) COALESCE()
B) FIRST_VALUE_NULL()
C) FIRST()
D) NVL_FIRST()

67. Which SQL function is commonly used to count non-NULL values in a specific column?

A) COUNTALL(column)
B) ROWCOUNT(column)
C) SUM(column)
D) COUNT(column)

68. What does COUNT(*) generally count?

A) Only non-NULL values in the first column
B) Rows returned by the query
C) Only distinct rows
D) Number of columns

69. Which SQL function is commonly used to determine the number of characters in a string?

A) SIZE()
B) CHARCOUNT()
C) LENGTH()
D) TEXTSIZE()

70. Which function is commonly used to convert text to uppercase?

A) UPPER()
B) CAPITAL()
C) TOUPPER()
D) UCASEONLY()

71. Which function is commonly used to convert text to lowercase?

A) LOWERCASE()
B) TOLOWER()
C) SMALL()
D) LOWER()

72. Which SQL function is commonly used to remove leading and trailing spaces?

A) CLEAN()
B) TRIM()
C) STRIPALL()
D) SPACE_REMOVE()

73. Which function is commonly used to concatenate strings in SQL?

A) COMBINE_TEXT()
B) MERGE_STRING()
C) CONCAT()
D) JOIN_TEXT()

74. Which SQL function is commonly used to extract part of a string?

A) SUBSTRING()
B) PART()
C) SLICE_TEXT()
D) EXTRACT_TEXT()

75. Which SQL function is commonly used to round a numeric value?

A) APPROX()
B) ROUNDUPONLY()
C) FORMATNUM()
D) ROUND()

76. Which SQL function returns the absolute value of a number?

A) POSITIVE()
B) ABS()
C) MAGNITUDE()
D) NONNEGATIVE()

77. Which SQL function can return the current date in many database systems?

A) TODAY()
B) CURRENT()
C) CURRENT_DATE
D) NOW_DATE()

78. Which SQL data type is intended to store calendar dates without time?

A) DATE
B) DAY
C) CALENDAR
D) DATETIMEONLY

79. Which SQL data type is designed to represent both date and time in systems that support it?

A) DATEONLY
B) CLOCK
C) TIMESTAMP_ONLY
D) TIMESTAMP

80. Which SQL clause is logically evaluated after GROUP BY for filtering grouped rows?

A) WHERE
B) HAVING
C) FROM
D) SELECT

81. Which clause is logically evaluated before GROUP BY to filter individual rows?

A) HAVING
B) ORDER BY
C) WHERE
D) LIMIT

82. Which clause specifies the source tables for a SELECT statement?

A) FROM
B) SOURCE
C) TABLES
D) USING

83. Which clause specifies the columns or expressions returned by a SELECT query?

A) FROM
B) WHERE
C) OUTPUT
D) SELECT

84. What does SELECT * generally mean?

A) Select only the primary key
B) Select all columns from the applicable row source
C) Select all databases
D) Select only numeric columns

85. Which SQL statement is commonly used to change a table by adding a column?

A) UPDATE TABLE ... ADD
B) MODIFY TABLE ... INSERT
C) ALTER TABLE ... ADD
D) INSERT COLUMN

86. Which SQL command can rename a table in database systems that support the standard-style syntax?

A) ALTER TABLE ... RENAME TO
B) CHANGE TABLE NAME
C) UPDATE TABLE NAME
D) RENAME ROW

87. Which SQL category primarily contains CREATE, ALTER, and DROP?

A) DML
B) DQL
C) DCL
D) DDL

88. Which category generally includes INSERT, UPDATE, and DELETE?

A) DDL
B) DML
C) DCL
D) TCL

89. Which SQL category is associated with COMMIT and ROLLBACK?

A) DDL
B) DML
C) TCL
D) DCL

90. Which SQL category includes GRANT and REVOKE?

A) DCL
B) DML
C) DDL
D) TCL

91. What does COMMIT do in a transaction?

A) Deletes the transaction
B) Creates an index
C) Starts a new database
D) Makes transaction changes permanent

92. What does ROLLBACK generally do?

A) Permanently saves changes
B) Undoes uncommitted transaction changes
C) Creates a new table
D) Rebuilds all indexes

93. What is a database transaction?

A) A database user
B) A table definition
C) A logical unit of database work
D) A database index

94. In ACID properties, what does Atomicity mean?

A) A transaction is treated as an all-or-nothing unit
B) Data is always sorted
C) Every table must have an index
D) Queries execute in alphabetical order

95. In ACID properties, what does Consistency ensure?

A) Queries always return duplicate rows
B) Transactions never use locks
C) Every query runs at the same speed
D) Transactions preserve defined database rules and integrity

96. In ACID properties, what does Isolation address?

A) Data compression
B) Interaction between concurrent transactions
C) Table naming
D) Index storage

97. In ACID properties, what does Durability mean?

A) Queries cannot fail
B) Data is never duplicated
C) Committed changes survive subsequent failures
D) All tables are automatically indexed

98. Which isolation level generally permits dirty reads?

A) READ UNCOMMITTED
B) READ COMMITTED
C) REPEATABLE READ
D) SERIALIZABLE

99. What is a dirty read?

A) Reading a deleted table
B) Reading duplicate values
C) Reading only indexed values
D) Reading data modified by an uncommitted transaction

100. What is a non-repeatable read?

A) Reading the same row twice and getting the same result
B) Reading the same row twice and getting different committed values
C) Reading a row that does not exist
D) Reading a duplicate table

101. What is a phantom read?

A) A query returning no columns
B) A transaction being automatically deleted
C) New or removed rows appearing when the same range is queried again
D) A duplicated database

102. Which isolation level provides the strongest standard isolation?

A) SERIALIZABLE
B) READ UNCOMMITTED
C) READ COMMITTED
D) DEFAULT

103. What is a database deadlock?

A) A missing index
B) An invalid table name
C) A failed SELECT statement
D) A situation where transactions wait for resources held by each other

104. Which database mechanism is commonly used to control concurrent access to data?

A) Aliases
B) Locks
C) Views only
D) Comments

105. What is normalization primarily intended to reduce?

A) Query syntax
B) Number of users
C) Data redundancy and update anomalies
D) Database security

106. Which normal form requires atomic values and no repeating groups?

A) First Normal Form (1NF)
B) Second Normal Form (2NF)
C) Third Normal Form (3NF)
D) Fourth Normal Form (4NF)

107. What additional dependency does 2NF eliminate compared with 1NF?

A) Transitive dependency
B) Multivalued dependency
C) Join dependency
D) Partial dependency

108. What dependency does 3NF primarily eliminate?

A) Partial dependency
B) Transitive dependency
C) Multivalued dependency
D) Functional dependency

109. What is denormalization?

A) Deleting all redundant data
B) Converting SQL to NoSQL
C) Intentionally introducing redundancy to improve certain read workloads
D) Removing every index

110. What is a functional dependency?

A) A relationship where one attribute determines another
B) A relationship between databases
C) A type of SQL function
D) A query execution plan

111. Which key is a minimal set of attributes that uniquely identifies a row?

A) Foreign key
B) Secondary key
C) Non-key
D) Candidate key

112. What is a superkey?

A) A key that can never contain multiple columns
B) A set of attributes that uniquely identifies a row
C) A foreign key only
D) An index with no columns

113. What is a composite key?

A) A key generated by an index
B) A key containing only text
C) A key made from multiple columns
D) A key shared across databases

114. What is referential integrity intended to maintain?

A) Valid relationships between related tables
B) Query formatting
C) Column sorting
D) Index compression

115. Which foreign key action can automatically remove related child rows when a parent row is deleted?

A) DELETE CHILD
B) REMOVE RELATED
C) CASCADE DELETE ROWS
D) ON DELETE CASCADE

116. What is a stored procedure?

A) A physical database file
B) A stored program that can contain SQL logic
C) A type of index
D) A table constraint

117. What is a database trigger?

A) A manually executed SELECT query
B) A database backup
C) A stored action that executes automatically in response to specified events
D) A table alias

118. What is a common concern with excessive trigger usage?

A) Hidden side effects and harder debugging
B) It prevents all INSERT statements
C) It automatically removes indexes
D) It disables transactions

119. What is a user-defined function in SQL?

A) A database user account
B) An automatically generated index
C) A table constraint
D) A function created by the developer for reusable logic

120. What is a common advantage of a stored procedure?

A) It eliminates all database locks
B) It centralizes reusable database-side logic
C) It guarantees every query is faster
D) It removes the need for indexes

121. What is a Common Table Expression (CTE)?

A) A permanent physical table
B) A database user role
C) A named temporary result defined within a statement
D) A mandatory index

122. Which keyword introduces a standard CTE?

A) WITH
B) CTE
C) TEMP
D) DEFINE

123. What is a recursive CTE useful for?

A) Encrypting columns
B) Creating indexes
C) Changing database credentials
D) Querying hierarchical or recursive relationships

124. What is a recursive CTE composed of conceptually?

A) Only an ORDER BY clause
B) An anchor member and a recursive member
C) Two indexes
D) A trigger and a procedure

125. What is a temporary table primarily used for?

A) Permanent archival storage only
B) Database authentication
C) Intermediate data that is needed temporarily
D) Replacing all indexes

126. Which SQL concept represents a query result as a virtual table without necessarily storing the result data separately?

A) View
B) Index
C) Trigger
D) Cursor

127. Which type of view stores the query result physically in systems that support materialized views?

A) Dynamic view
B) Temporary view
C) Logical view
D) Materialized view

128. What is a major benefit of a materialized view?

A) It always reflects source changes instantly without maintenance
B) It can speed up repeated expensive queries by storing results
C) It eliminates all indexes
D) It prevents all writes

129. What is query optimization?

A) Renaming every table
B) Removing all database constraints
C) Finding an efficient execution strategy for a query
D) Converting SQL to HTML

130. What does a database optimizer typically use to choose an execution plan?

A) Statistics, available indexes, and estimated costs
B) Table names only
C) User passwords
D) SQL comments only

131. Why can SELECT * be undesirable in production queries?

A) It always causes syntax errors
B) It cannot read any indexes
C) It automatically deletes unused columns
D) It can retrieve unnecessary columns and increase data transfer

132. What is a sargable predicate?

A) A predicate that must use a trigger
B) A predicate that can potentially use an index efficiently
C) A predicate containing only strings
D) A predicate that always scans every row

133. Which condition can make an indexed column less likely to be searched efficiently in many databases?

A) A simple equality comparison
B) A direct range predicate
C) Applying a function to the indexed column in the predicate
D) Filtering on the indexed column

134. What is a full table scan?

A) Reading rows across the table to evaluate a query
B) Reading only the primary key
C) Reading one index entry
D) Scanning database users

135. When can a full table scan be a reasonable execution strategy?

A) Only when indexes are forbidden
B) Never
C) Only for DELETE operations
D) When a large portion of the table must be read

136. What is a covering index?

A) An index covering every table in a database
B) An index containing enough information for a query without needing the base table for required columns
C) An index that prevents all writes
D) An index used only for passwords

137. What is index selectivity?

A) Number of indexes in a database
B) Number of tables in a schema
C) How effectively an index distinguishes among rows
D) Number of database users

138. Which column is generally a strong candidate for indexing?

A) A frequently filtered column with useful selectivity
B) A column never used in queries
C) A constant value repeated in every row
D) A column that is never read or filtered

139. What is a database schema?

A) A backup file
B) A SQL password
C) A query result
D) A logical organization and definition of database objects

140. What is a database catalog or data dictionary?

A) A list of application passwords
B) Metadata describing database objects and structures
C) A user-created backup
D) A query optimizer

141. What does SQL injection exploit?

A) Database indexes
B) Backup compression
C) Unsafe incorporation of untrusted input into SQL commands
D) Table normalization

142. Which technique is a primary defense against SQL injection?

A) Parameterized queries
B) String concatenation
C) Removing all indexes
D) Disabling transactions

143. Why should application code avoid building SQL with untrusted string concatenation?

A) It always makes queries slower
B) It prevents database backups
C) It disables foreign keys
D) It can allow user input to alter SQL semantics

144. What is the principle of least privilege in database security?

A) Every user gets administrator access
B) Users receive only the permissions necessary for their tasks
C) All users share one account
D) Permissions are never reviewed

145. Which SQL command is commonly used to grant privileges?

A) PERMIT
B) ALLOW
C) GRANT
D) AUTHORIZE

146. Which SQL command removes previously granted privileges?

A) REVOKE
B) DENYALL
C) REMOVE ACCESS
D) DELETE GRANT

147. What is a database role?

A) A query result
B) An index type
C) A transaction isolation level
D) A named collection of permissions that can be assigned to users or other principals

148. What is data masking used for?

A) Increasing query speed
B) Hiding sensitive data from unauthorized viewing
C) Removing primary keys
D) Creating duplicate tables

149. What is database encryption primarily intended to protect?

A) Query formatting
B) Table names
C) Data from unauthorized disclosure
D) SQL syntax

150. What is a database backup?

A) A copy of data and/or database state used for recovery
B) A query execution plan
C) An index
D) A temporary alias

151. What is a point-in-time recovery capability designed to do?

A) Optimize SELECT queries
B) Rename tables
C) Remove indexes
D) Restore a database to a chosen time using appropriate backups and logs

152. What is a transaction log commonly used for?

A) Storing application CSS
B) Recording changes to support recovery and transaction management
C) Storing database passwords in plain text
D) Creating table aliases

153. What is database replication?

A) Normalizing one table
B) Compressing SQL text
C) Maintaining copies of data across multiple database instances or nodes
D) Removing duplicate columns

154. What is read replication commonly used for?

A) Scaling read workloads
B) Eliminating all writes
C) Removing database constraints
D) Replacing authentication

155. What is replication lag?

A) Delay in query parsing
B) Delay in creating a primary key
C) Delay in loading SQL syntax
D) Delay before changes on a source are visible on a replica

156. What is sharding?

A) Combining all tables into one
B) Horizontally partitioning data across multiple database nodes
C) Creating duplicate indexes on one column
D) Encrypting a database

157. What is horizontal partitioning?

A) Splitting a table's columns into different schemas
B) Splitting SQL queries into functions
C) Splitting rows into partitions
D) Splitting indexes into users

158. What is vertical partitioning?

A) Splitting columns into separate structures or tables
B) Splitting users into groups
C) Splitting rows randomly
D) Splitting transactions by time only

159. What is partition pruning?

A) Deleting all partitions
B) Rebuilding every index
C) Compressing query results
D) Skipping partitions that cannot contain matching rows

160. Which partitioning strategy divides data according to ranges of values?

A) Hash partitioning
B) Range partitioning
C) Random partitioning
D) Index partitioning

161. Which partitioning strategy uses a hash function to distribute rows?

A) Range partitioning
B) List partitioning
C) Hash partitioning
D) Date partitioning

162. Which partitioning strategy assigns rows based on specified discrete values?

A) List partitioning
B) Hash partitioning
C) Range partitioning
D) Random partitioning

163. What is a surrogate key?

A) A foreign key copied from another database
B) A descriptive business attribute
C) A composite index
D) An artificial identifier generated for a row

164. What is a natural key?

A) A random database-generated number
B) An identifier derived from meaningful business data
C) An index name
D) A temporary table

165. What does cardinality describe in a database relationship?

A) The number of database users
B) The size of a SQL statement
C) The relationship count between entities, such as one-to-many
D) The number of indexes only

166. Which relationship means one record in one table can relate to many records in another?

A) One-to-many
B) One-to-one
C) Many-to-zero
D) Zero-to-zero

167. Which relationship allows records in both tables to relate to multiple records in the other table?

A) One-to-one
B) One-to-many
C) Zero-to-one
D) Many-to-many

168. How is a many-to-many relationship commonly implemented in a relational database?

A) By adding an index to every column
B) By using a junction or associative table
C) By removing primary keys
D) By using only NULL values

169. What is an entity in relational database design?

A) A SQL keyword
B) A database backup
C) A distinct object or concept represented by data
D) An index scan

170. What does an ER diagram primarily represent?

A) Entities, attributes, and relationships
B) SQL execution time only
C) User passwords
D) Backup schedules

171. Which SQL query finds rows whose value appears more than once in a column?

A) SELECT value FROM table WHERE COUNT(value) > 1
B) SELECT DISTINCT value FROM table
C) SELECT value FROM table ORDER BY value
D) SELECT value FROM table GROUP BY value HAVING COUNT(*) > 1

172. Which technique is commonly used to remove duplicate rows while retaining one row per group?

A) CROSS JOIN
B) ROW_NUMBER() with a partition and appropriate filtering
C) COUNT(*) without GROUP BY
D) ORDER BY only

173. Which query pattern is commonly used to find the second-highest distinct salary?

A) SELECT MAX(salary) FROM employees
B) SELECT MIN(salary) FROM employees
C) SELECT MAX(salary) FROM employees WHERE salary < (SELECT MAX(salary) FROM employees)
D) SELECT COUNT(salary) FROM employees

174. Which window function is particularly useful for finding the top N rows per group?

A) ROW_NUMBER() with PARTITION BY
B) COUNT(*) without grouping
C) SUM() without OVER()
D) DISTINCT without ordering

175. Why is DENSE_RANK() often useful for top-N-per-group queries when ties should share a rank without gaps?

A) It removes all rows
B) It sorts without any expression
C) It ignores partitioning
D) It assigns the same rank to ties and does not leave gaps

176. Which clause can be used with FETCH FIRST in databases that support the standard syntax to limit returned rows?

A) GROUP BY
B) ORDER BY
C) HAVING ONLY
D) JOIN BY

177. What is the purpose of LIMIT in SQL dialects that support it?

A) Limit column definitions
B) Limit transaction size
C) Limit the number of rows returned
D) Limit database users

178. Why should a row limit usually be combined with ORDER BY when retrieving a top-N result?

A) To define which rows are considered the top rows
B) To create a primary key
C) To enable foreign keys
D) To prevent NULL values

179. What is keyset pagination?

A) Pagination using random rows
B) Pagination that requires a full database export
C) Pagination based only on table names
D) Pagination that uses a stable ordered key as the position marker

180. What is a common disadvantage of deep OFFSET pagination on large tables?

A) It always deletes previous pages
B) The database may need to process or skip many earlier rows
C) It prevents sorting
D) It disables indexes

181. Which join condition is most important for preventing an accidental Cartesian product in a typical relational join?

A) ORDER BY
B) GROUP BY
C) An appropriate join predicate
D) DISTINCT only

182. What is an anti-join pattern commonly used for?

A) Finding rows in one table with no matching row in another
B) Finding every duplicate row
C) Updating every row
D) Creating indexes

183. Which SQL construct is commonly used to implement an anti-join?

A) CROSS JOIN
B) UNION ALL
C) GROUP BY only
D) NOT EXISTS

184. Which construct is often used to test whether a matching row exists without needing columns from the subquery?

A) UNION
B) EXISTS
C) DISTINCT
D) ORDER BY

185. Why can NOT EXISTS be preferable to NOT IN when the subquery can contain NULL values?

A) NOT EXISTS always creates an index
B) NOT EXISTS disables NULL handling
C) NOT IN can produce UNKNOWN results because of NULL semantics
D) NOT EXISTS cannot use subqueries

186. What is three-valued logic in SQL?

A) TRUE, FALSE, and UNKNOWN
B) TRUE, FALSE, and ERROR
C) YES, NO, and NULL
D) 1, 2, and 3

187. Why is comparing a column directly with NULL using = problematic?

A) NULL is always converted to zero
B) SQL converts NULL to an empty string
C) Equality comparisons cannot use strings
D) NULL represents an unknown or missing value and comparisons produce UNKNOWN

188. Which operator checks whether an expression is not NULL?

A) NOT NULL()
B) IS NOT NULL
C) != NULL
D) EXISTS NULL

189. Which function can return the first non-NULL value among several expressions?

A) FIRST_NON_NULL_ONLY()
B) NULLFIRST()
C) COALESCE()
D) SELECT_FIRST()

190. Which SQL expression is useful for returning one value when a condition is true and another when it is false?

A) CASE
B) CHOOSE COLUMN
C) SWITCH TABLE
D) CONDITIONAL ROW

191. Which clause is generally evaluated last in the logical processing order of a SELECT query?

A) FROM
B) WHERE
C) GROUP BY
D) ORDER BY

192. Why can't a SELECT-list alias always be referenced in the WHERE clause?

A) Aliases are invalid in SQL
B) WHERE is logically processed before the SELECT list is assigned
C) WHERE accepts only numbers
D) SELECT aliases are only for tables

193. Which clause can often reference a SELECT-list alias because it is processed later?

A) WHERE
B) FROM
C) ORDER BY
D) JOIN condition only

194. What is the purpose of DISTINCT?

A) Remove duplicate result rows
B) Delete duplicate records from the table
C) Create a unique index automatically
D) Prevent INSERT operations

195. What is the difference between DELETE and TRUNCATE at a high level?

A) DELETE removes the table while TRUNCATE changes its name
B) They are identical in every database
C) TRUNCATE deletes only selected rows
D) DELETE can filter rows, while TRUNCATE typically removes all rows more directly

196. Which statement is used to delete selected rows based on a condition?

A) TRUNCATE TABLE ... WHERE
B) DELETE FROM ... WHERE
C) DROP TABLE ... WHERE
D) REMOVE ... FILTER

197. What is an upsert?

A) A query that only reads data
B) A command that always deletes rows
C) An operation that inserts a row or updates it when a conflict/match occurs
D) A database backup

198. Which SQL feature is commonly associated with upsert behavior in PostgreSQL?

A) INSERT ... ON CONFLICT
B) UPDATE ... ON SELECT
C) MERGE ONLY TABLE
D) INSERT ... WHEN NOT EXISTS ONLY

199. What is the purpose of the SQL MERGE statement in systems that support it?

A) Create a database backup
B) Rename multiple tables
C) Perform only SELECT operations
D) Perform conditional INSERT, UPDATE, or related actions based on matches

200. What is an identity column commonly used for?

A) Storing encrypted passwords
B) Automatically generating numeric identifiers
C) Sorting text alphabetically
D) Creating foreign keys automatically

201. What is a sequence object commonly used for?

A) Storing query plans
B) Compressing tables
C) Generating a sequence of numeric values
D) Defining a foreign key relationship

202. What is an auto-incrementing key commonly used for?

A) Generating unique row identifiers automatically
B) Encrypting table data
C) Preventing SELECT queries
D) Creating views

203. What is a generated column?

A) A column manually populated by every user
B) A column that can never be queried
C) A column that must contain a foreign key
D) A column whose value is derived from an expression or other columns

204. What is a check constraint useful for?

A) Creating a database backup
B) Enforcing valid values based on a Boolean condition
C) Sorting query results
D) Creating a user role

205. What is a unique constraint designed to enforce?

A) Every row must have the same value
B) Every column must contain NULL
C) No two constrained values can violate the uniqueness rule
D) Every value must be numeric

206. Which constraint is generally used to ensure a column cannot be left without a value?

A) NOT NULL
B) UNIQUE
C) CHECK NULL
D) REQUIRED KEY

207. Which SQL statement is used to grant SELECT permission on a table?

A) ALLOW SELECT table TO user
B) PERMIT SELECT table
C) AUTHORIZE SELECT ON table
D) GRANT SELECT ON table TO user

208. What does CASCADE generally mean in a referential action?

A) Ignore all related rows
B) Propagate the specified action to related rows
C) Disable foreign keys permanently
D) Create a backup

209. What does ON DELETE SET NULL do when supported and applicable?

A) Deletes the parent row only
B) Deletes all child rows
C) Sets matching foreign key values in child rows to NULL
D) Creates a new foreign key

210. Which requirement must generally be satisfied before a foreign key can use SET NULL?

A) The foreign key column must allow NULL values
B) The column must be a primary key
C) The column must be UNIQUE only
D) The table must have no indexes

211. What is an aggregate query?

A) A query that only creates tables
B) A query that only uses joins
C) A query that modifies schema
D) A query that summarizes multiple rows using functions such as SUM or COUNT

212. Which function can count distinct customer IDs?

A) COUNT(*) ONLY
B) COUNT(DISTINCT customer_id)
C) DISTINCT(COUNT(customer_id))
D) UNIQUE(COUNT(*))

213. Why might COUNT(column) return fewer rows than COUNT(*)?

A) COUNT(column) counts only indexes
B) COUNT(*) always removes duplicates
C) COUNT(column) generally ignores NULL values
D) COUNT(column) cannot process numbers

214. Which aggregate function can calculate the total number of non-NULL numeric values?

A) COUNT()
B) SUM()
C) MAX()
D) AVG()

215. Which clause is required when you want separate aggregate results for each department?

A) DISTINCT BY
B) ORDER BY ONLY
C) PARTITION ONLY
D) GROUP BY

216. Which query correctly filters departments whose average salary exceeds 50000?

A) WHERE AVG(salary) > 50000
B) HAVING AVG(salary) > 50000
C) ORDER BY AVG(salary) > 50000
D) GROUP BY AVG(salary) > 50000

217. Which clause should normally be used to filter rows before calculating an aggregate?

A) HAVING
B) ORDER BY
C) WHERE
D) LIMIT

218. What is the purpose of FILTER in aggregate expressions in SQL dialects that support it?

A) Apply a condition to rows included in a particular aggregate
B) Filter database users
C) Filter table definitions
D) Filter indexes only

219. What is a window aggregate?

A) An aggregate that always deletes rows
B) An aggregate used only in table creation
C) An aggregate that requires GROUP BY and removes detail rows
D) An aggregate used with OVER() while retaining individual rows

220. Which query pattern can calculate each employee's salary as a percentage of total salary?

A) salary / COUNT(*)
B) salary / SUM(salary) OVER()
C) salary * GROUP BY salary
D) salary / DISTINCT salary

221. What is a lateral join used for in SQL dialects that support it?

A) Encrypting joins
B) Sorting joined tables alphabetically
C) Allowing a table function or subquery on the right side to reference preceding FROM items
D) Preventing all joins

222. What does NATURAL JOIN do in systems that support it?

A) Joins tables using columns with matching names
B) Joins every row regardless of columns
C) Joins only primary keys
D) Automatically creates a foreign key

223. Why can NATURAL JOIN be risky in production SQL?

A) It cannot return rows
B) It always causes a Cartesian product
C) It disables all indexes
D) Schema changes can unintentionally change which columns participate in the join

224. What is a semi-join conceptually used to find?

A) All rows from both tables
B) Rows from one table that have at least one match in another
C) Rows that have no matches only
D) Every duplicate row

225. Which SQL construct commonly expresses a semi-join?

A) DROP
B) UNION ALL
C) EXISTS
D) TRUNCATE

226. What is the main difference between WHERE and HAVING?

A) WHERE filters rows, while HAVING filters groups or aggregate results
B) HAVING filters rows, while WHERE creates tables
C) They always perform exactly the same operation
D) WHERE can only be used with DELETE

227. Which clause is used to specify ascending or descending order?

A) GROUP BY
B) SORT
C) ORDER
D) ORDER BY

228. Which keyword explicitly requests descending sort order?

A) DOWN
B) DESC
C) REVERSE
D) DOWNWARD

229. Which SQL operator combines multiple conditions where all specified conditions must be true?

A) OR
B) XOR
C) AND
D) ALL

230. Which SQL operator is true when at least one of its conditions is true?

A) OR
B) AND
C) ONLY
D) BOTH

231. Which operator reverses a Boolean condition?

A) REVERSE
B) NEGATE
C) NOTIFY
D) NOT

232. Which comparison operator means "not equal" in standard SQL?

A) ==
B) <>
C) =>
D) !==

233. Which operator means "greater than or equal to"?

A) =>
B) >=
C) >=
D) >>

234. Which operator checks whether a value falls inclusively between two boundaries?

A) BETWEEN
B) RANGE
C) WITHIN ONLY
D) INSIDE

235. Which statement about BETWEEN is generally correct?

A) It excludes both boundaries
B) It works only with text
C) It requires exactly one boundary
D) It normally includes both boundary values

236. Which operator can test whether a value belongs to a specified list?

A) WITH
B) IN
C) HAS
D) MEMBER

237. What does NOT IN return when the list contains NULL under SQL's three-valued logic?

A) Always TRUE
B) Always FALSE
C) It can evaluate to UNKNOWN
D) It always throws a syntax error

238. Which SQL feature is useful for recursive hierarchical queries?

A) Recursive CTE
B) DISTINCT
C) CHECK constraint
D) ORDER BY

239. Which example is best suited for a recursive SQL query?

A) Converting text to uppercase
B) Calculating a single average
C) Removing spaces from a string
D) Traversing an employee-manager hierarchy

240. What is a recursive query's base or anchor member?

A) The final query result only
B) The initial rows from which recursion starts
C) The database index
D) The transaction log

241. What is an execution plan operator such as an index seek or scan?

A) A database user
B) A table constraint
C) A step used by the database engine to execute a query
D) A stored password

242. Why are database statistics important to query optimization?

A) They help estimate data distribution and row counts
B) They store application passwords
C) They replace indexes completely
D) They define table relationships automatically

243. What can happen when optimizer statistics are significantly outdated?

A) SQL syntax changes automatically
B) Primary keys disappear
C) All queries are automatically cached
D) The optimizer may choose a less efficient execution plan

244. What is a parameterized SQL query?

A) A query with no WHERE clause
B) A query where values are supplied separately from the SQL statement structure
C) A query stored only in a view
D) A query containing only numeric columns

245. Besides improving security, what can parameterized queries help with?

A) Removing primary keys
B) Automatically normalizing tables
C) Reusing query structure and potentially improving statement handling
D) Eliminating all database locks

246. What is SQL prepared statement execution?

A) Preparing a SQL statement structure and executing it with supplied parameters
B) Creating a permanent backup
C) Removing all query parameters
D) Creating a database schema

247. Which SQL concept is most useful for returning a value from a previous row without joining the table to itself?

A) GROUP BY
B) UNION
C) DISTINCT
D) LAG()

248. Which SQL concept is useful for comparing the current row with the next row?

A) LAG()
B) LEAD()
C) RANK()
D) COUNT()

249. Which window function returns the first value within an ordered window frame?

A) START_VALUE()
B) FIRST()
C) FIRST_VALUE()
D) BEGIN_VALUE()

250. Which window function returns the last value within an ordered window frame?

A) LAST_VALUE()
B) FINAL_VALUE()
C) END_VALUE()
D) LAST()

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About the Author

Jaya

Jaya has spent much of her career building large-scale data pipelines, helping teams move from spreadsheet reporting to distributed systems like Hadoop and Spark. She's handled the operational headaches of big data projects firsthand, from schema drift to cluster tuning. She tests new tools before recommending them, helping engineers build systems that hold up under real data volumes.

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