HAVING clause in MySQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the HAVING clause in SQL?

The HAVING clause is used to filter groups of rows created by GROUP BY based on a condition. It works like WHERE but for grouped data.

beginner

How does HAVING differ from WHERE in SQL?

WHERE filters rows before grouping, while HAVING filters groups after aggregation.

intermediate

Write a simple SQL query using HAVING to find departments with more than 5 employees.

SELECT department, COUNT(*) AS employee_count FROM employees GROUP BY department HAVING COUNT(*) > 5;

intermediate

Can HAVING be used without GROUP BY? Explain.

Yes, HAVING can be used without GROUP BY to filter aggregated results over the entire table, like HAVING COUNT(*) > 10.

advanced

Why might you get an error if you use a column in HAVING that is not in GROUP BY or an aggregate function?

Because HAVING filters groups, any column used must be part of the grouping or an aggregate. Otherwise, SQL doesn't know how to apply the condition.

What does the HAVING clause filter in an SQL query?

A Only NULL values

B Individual rows before grouping

C Groups after aggregation

D Table schema

HAVING filters groups created by GROUP BY after aggregation, unlike WHERE which filters rows before grouping.

Which clause is used to filter rows before grouping in SQL?

A WHERE

B HAVING

C GROUP BY

D ORDER BY

WHERE filters rows before grouping, HAVING filters groups after aggregation.

Can HAVING be used without GROUP BY in MySQL?

A Yes, to filter aggregated results over the whole table

B Only with ORDER BY

C No, it always requires GROUP BY

D Only with JOIN

HAVING can filter aggregated results without GROUP BY, applying conditions on the entire result.

Which of these is a valid HAVING clause condition?

A HAVING salary > 50000

B HAVING COUNT(*) > 10

C HAVING department = 'Sales'

D HAVING employee_id = 100

HAVING conditions must use aggregate functions or grouped columns. COUNT(*) > 10 is valid.

What happens if you use a non-aggregated column in HAVING that is not in GROUP BY?

A Results are sorted

B Query runs fine

C Column is ignored

D SQL error occurs

SQL throws an error because it cannot apply HAVING condition on columns not grouped or aggregated.

Explain the difference between WHERE and HAVING clauses in SQL.

Think about when filtering happens in the query process.

Describe a scenario where you would use HAVING without GROUP BY.

Consider aggregate functions applied to the whole table.