GROUP BY multiple columns in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the SQL clause GROUP BY do when used with multiple columns?

It groups rows that have the same values in all the specified columns, so you can perform aggregate functions on each unique combination of those columns.

beginner

Write a simple example of a GROUP BY clause using two columns: city and department.

Example: <br>SELECT city, department, COUNT(*) FROM employees GROUP BY city, department;<br>This counts employees grouped by each city and department combination.

intermediate

Why is it important to include all non-aggregated columns in the GROUP BY clause?

Because SQL requires that every column in the SELECT list that is not inside an aggregate function must be listed in the GROUP BY clause to know how to group the data.

intermediate

What happens if you use GROUP BY on multiple columns but forget one column that appears in SELECT without aggregation?

The query will cause an error because SQL does not know how to group by the missing column, violating grouping rules.

beginner

Can GROUP BY multiple columns be used to find unique combinations of those columns?

Yes, grouping by multiple columns returns one row per unique combination of those columns, which helps identify distinct groups.

What does GROUP BY city, department do in a SQL query?

A Groups rows by city only

B Groups rows by department only

C Groups rows by unique pairs of city and department

D Sorts rows by city and department

GROUP BY with multiple columns groups rows by each unique combination of those columns.

Which columns must appear in the GROUP BY clause?

A Only columns with numeric data

B All columns in SELECT that are not aggregated

C Only the first column in SELECT

D No columns are required

All non-aggregated columns in SELECT must be in GROUP BY to define grouping.

What will happen if you omit a non-aggregated column from GROUP BY?

A Query will return an error

B Query will run but ignore that column

C Query will group by all columns automatically

D Query will return duplicate rows

SQL requires all non-aggregated columns in SELECT to be in GROUP BY; otherwise, it errors.

Which aggregate function can be used with GROUP BY?

A NOW()

B CONCAT()

C SUBSTRING()

D COUNT()

COUNT() is an aggregate function that works with GROUP BY to count rows per group.

If you want to count employees per city and department, which query is correct?

A SELECT city, department, COUNT(*) FROM employees GROUP BY city, department;

B SELECT city, department, COUNT(*) FROM employees;

C SELECT city, COUNT(*) FROM employees GROUP BY department;

D SELECT COUNT(*), city, department FROM employees GROUP BY city;

Option A correctly groups by both city and department to count employees per group.

Explain how GROUP BY with multiple columns works and why it is useful.

Think about grouping by city and department together.

Describe what happens if you include a column in SELECT that is not aggregated and not in GROUP BY.

Remember SQL grouping rules.