Standard comparison operators in PostgreSQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of a simple equality comparison

What is the output of this SQL query?

SELECT 5 = 5 AS result;

PostgreSQL

SELECT 5 = 5 AS result;

A 5

B true

C Syntax error

D false

💡 Hint

The '=' operator checks if two values are equal and returns a boolean.

The '=' operator compares two values. Since 5 equals 5, the result is true.

❓ query_result

intermediate

Result of greater than comparison in WHERE clause

Given a table numbers with a column value, what rows will this query return?

SELECT value FROM numbers WHERE value > 10;

Assuming the table has values: 5, 10, 15, 20.

PostgreSQL

SELECT value FROM numbers WHERE value > 10;

A 10, 15, 20

B 5, 10

C 15, 20

D 5, 15, 20

💡 Hint

The '>' operator selects values strictly greater than 10.

Only values greater than 10 are 15 and 20, so those rows are returned.

📝 Syntax

advanced

Identify the syntax error in comparison

Which option contains a syntax error in using comparison operators in PostgreSQL?

A SELECT * FROM users WHERE age =! 30;

B SELECT * FROM users WHERE age != 30;

C SELECT * FROM users WHERE age > 30;

D SELECT * FROM users WHERE age <> 30;

💡 Hint

Check the order of symbols in the comparison operator.

The operator '=!' is invalid. The correct not equal operators are '<>' or '!='.

❓ query_result

advanced

Output of combined comparison with AND

What is the output of this query?

SELECT (5 > 3) AND (2 = 2) AS result;

PostgreSQL

SELECT (5 > 3) AND (2 = 2) AS result;

A true

B false

C Syntax error

D NULL

💡 Hint

Both comparisons must be true for AND to return true.

5 > 3 is true and 2 = 2 is true, so true AND true is true.

❓ query_result

expert

Understanding NULL in comparison operators

What is the result of this query?

SELECT NULL = NULL AS result;

PostgreSQL

SELECT NULL = NULL AS result;

A true

B false

C Syntax error

D NULL

💡 Hint

In SQL, NULL means unknown, so comparing NULL to NULL is not true or false.

Comparing NULL to NULL returns NULL because NULL represents an unknown value, so equality is unknown.