CASE in SELECT for computed columns in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the CASE statement do in a SELECT query?

It allows you to create new columns with values that depend on conditions, like an IF-THEN-ELSE in your query.

beginner

Write a simple CASE expression to show 'Adult' if age >= 18, else 'Minor'.

CASE WHEN age >= 18 THEN 'Adult' ELSE 'Minor' END

intermediate

Can CASE be used to create multiple computed columns in one SELECT?

Yes, you can use multiple CASE expressions in the same SELECT to create several computed columns.

intermediate

What happens if no WHEN condition matches and there is no ELSE in CASE?

The CASE expression returns NULL for that row.

beginner

Why use CASE in SELECT instead of filtering rows with WHERE?

CASE lets you keep all rows but change or add column values based on conditions, while WHERE filters out rows.

What keyword starts a conditional expression inside SELECT to create computed columns?

A CASE

B IF

C WHEN

D SELECT

The CASE keyword starts the conditional expression in SQL SELECT statements.

What does ELSE do in a CASE expression?

A Specifies the default value if no WHEN matches

B Ends the CASE expression

C Starts a new condition

D Filters rows

ELSE provides a default value when none of the WHEN conditions are true.

If you want to label ages >= 65 as 'Senior', which WHEN clause is correct?

A WHEN age <= 65 THEN 'Senior'

B WHEN age < 65 THEN 'Senior'

C WHEN age = 65 THEN 'Senior'

D WHEN age >= 65 THEN 'Senior'

The condition 'age >= 65' correctly selects seniors.

What will this CASE return if age = 20? CASE WHEN age < 18 THEN 'Child' ELSE 'Adult' END

A 'Child'

B 'Adult'

C NULL

D Error

Since 20 is not less than 18, ELSE 'Adult' applies.

Can CASE expressions be nested inside each other in SELECT?

A Only in WHERE clause

B No

C Yes

D Only in UPDATE statements

CASE expressions can be nested to handle complex conditions.

Explain how to use CASE in a SELECT statement to create a new column based on conditions.

Think of CASE as an IF-THEN-ELSE inside your query.

Describe what happens if no WHEN condition matches and there is no ELSE clause in a CASE expression.

What does SQL do when no condition fits and no default is given?