SQL MCQs

Question 1

What is the difference between DELETE and TRUNCATE in SQL?

Accepted Answer

DELETE removes rows one at a time and can be rolled back, while TRUNCATE removes all rows at once and is generally faster

Answer

DELETE removes the entire table structure, while TRUNCATE only removes rows

Answer

TRUNCATE can be used with a WHERE clause, while DELETE cannot

Answer

Both commands behave identically in every database system

Question 2

What does the UNION operator do in SQL?

Accepted Answer

Combines the results of two or more SELECT queries and removes duplicate rows

Answer

Joins two tables based on a matching column

Answer

Filters rows based on a condition

Answer

Sorts the combined result set automatically by every column

Question 3

What is the difference between UNION and UNION ALL?

Accepted Answer

UNION removes duplicate rows from the combined result, while UNION ALL keeps every row including duplicates

Answer

UNION ALL removes duplicates, while UNION keeps them

Answer

UNION can only combine two queries, while UNION ALL can combine more

Answer

There is no difference between the two

Question 4

What does the SQL EXISTS operator check for?

Accepted Answer

Whether a subquery returns at least one row

Answer

Whether a column contains a NULL value

Answer

Whether a table has any indexes defined

Answer

Whether two tables have matching column names

Question 5

What is the purpose of a composite primary key in a table?

Accepted Answer

To uniquely identify a row using a combination of two or more columns together

Answer

To allow a column to store multiple values at once

Answer

To automatically generate unique numeric identifiers

Answer

To link a table to itself

Question 6

What does the SQL constraint UNIQUE ensure?

Accepted Answer

That all values in a column, or set of columns, are different from one another across rows

Answer

That a column cannot contain any NULL values

Answer

That a column only stores numeric values

Answer

That a row cannot be deleted once inserted

Question 7

What is a subquery in SQL?

Accepted Answer

A query nested inside another SQL query, often used within a WHERE or FROM clause

Answer

A query that runs automatically every night

Answer

A backup copy of a query result

Answer

A query that only works with numeric columns

Question 8

What is the purpose of the SQL ALTER TABLE statement?

Accepted Answer

To modify the structure of an existing table, such as adding or removing a column

Answer

To change the values stored in existing rows

Answer

To create a new table from scratch

Answer

To permanently delete a table

Question 9

What does a RIGHT JOIN return?

Accepted Answer

All rows from the right table, along with matching rows from the left table, filling with NULL where there is no match

Answer

Only rows that match in both tables

Answer

All rows from the left table only

Answer

All rows from both tables regardless of a match

Question 10

What is the purpose of the SQL DISTINCT keyword?

Accepted Answer

To remove duplicate rows from the result set of a query

Answer

To sort the result set in descending order

Answer

To count the total number of rows returned

Answer

To combine two columns into one

Question 11

What is a trigger in SQL?

Accepted Answer

A stored procedure that automatically executes in response to a specific event on a table, such as an insert or update

Answer

A type of index used to speed up searches

Answer

A constraint that prevents duplicate rows

Answer

A scheduled backup of the entire database

Question 12

What is the difference between the WHERE clause and the HAVING clause in SQL?

Accepted Answer

WHERE filters individual rows before grouping, while HAVING filters groups after GROUP BY has been applied

Answer

WHERE filters groups, while HAVING filters individual rows

Answer

Both clauses do exactly the same thing

Answer

HAVING can only be used with numeric columns

Question 13

What does the SQL LIKE operator allow you to do?

Accepted Answer

Search for a specified pattern within a column using wildcard characters

Answer

Compare two columns for exact numeric equality

Answer

Join two tables without a matching key

Answer

Sort results based on string length

Question 14

What is the purpose of the SQL COALESCE function?

Accepted Answer

To return the first non-NULL value from a list of expressions

Answer

To count the number of NULL values in a column

Answer

To convert a NULL value into a zero permanently in the table

Answer

To combine two tables into one

Question 15

What is the effect of adding a NOT NULL constraint to a column?

Accepted Answer

It prevents that column from storing NULL values in any row

Answer

It automatically fills empty values with zero

Answer

It makes the column a primary key

Answer

It prevents the column from being indexed

Question 16

What does the SQL BETWEEN operator do?

Accepted Answer

Selects values within a given inclusive range

Answer

Selects values that match a text pattern

Answer

Removes duplicate rows from a result

Answer

Joins two tables based on a range of dates only

Question 17

What is a natural join in SQL?

Accepted Answer

A join that automatically matches columns with the same name and compatible data type across both tables

Answer

A join that requires no matching condition at all

Answer

A join that only works on numeric columns

Answer

A join that always returns every possible row combination

Question 18

What is the purpose of the SQL CASE expression?

Accepted Answer

To perform conditional logic within a query, returning different values based on specified conditions

Answer

To define a new table structure

Answer

To create a loop within a stored procedure

Answer

To permanently rename a column

Question 19

What does an outer join guarantee that an inner join does not?

Accepted Answer

It includes rows from one or both tables even when there is no matching row in the other table

Answer

It always runs faster than an inner join

Answer

It removes all NULL values from the result

Answer

It only works with a single table

Question 20

What is the purpose of the SQL AS keyword?

Accepted Answer

To assign a temporary alias to a column or table for the duration of a query

Answer

To create a permanent copy of a table

Answer

To enforce a constraint on a column

Answer

To sort query results in ascending order

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