COUNT function in MySQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the COUNT function do in SQL?

The COUNT function counts the number of rows that match a specified condition or all rows if no condition is given.

beginner

How do you count all rows in a table named 'employees'?

Use: SELECT COUNT(*) FROM employees; This counts every row in the 'employees' table.

intermediate

What is the difference between COUNT(*) and COUNT(column_name)?

COUNT(*) counts all rows including those with NULLs. COUNT(column_name) counts only rows where the column is NOT NULL.

beginner

Can COUNT be used with a WHERE clause? Give an example.

Yes. Example: SELECT COUNT(*) FROM employees WHERE department = 'Sales'; counts only employees in Sales.

intermediate

What will SELECT COUNT(DISTINCT department) FROM employees; return?

It returns the number of unique departments in the employees table, ignoring duplicates.

What does COUNT(*) count in a table?

A Only rows with non-NULL values in the first column

B All rows including those with NULL values

C Only rows with NULL values

D Only distinct rows

COUNT(*) counts every row regardless of NULL values.

Which query counts only rows where the 'age' column is NOT NULL?

A SELECT COUNT(age) FROM table;

B SELECT COUNT(*) FROM table;

C SELECT COUNT(DISTINCT age) FROM table;

D SELECT COUNT(NULL) FROM table;

COUNT(age) counts rows where 'age' is not NULL.

How to count unique values in a column 'city'?

A SELECT COUNT(DISTINCT city) FROM table;

B SELECT COUNT(NULL) FROM table;

C SELECT COUNT(*) FROM table;

D SELECT COUNT(city) FROM table;

COUNT(DISTINCT city) counts unique city values.

What does this query do? SELECT COUNT(*) FROM employees WHERE salary > 50000;

A Counts all employees

B Counts employees with NULL salary

C Counts employees with salary less than 50000

D Counts employees with salary greater than 50000

The WHERE clause filters employees with salary > 50000 before counting.

If a column has NULL values, which COUNT counts them?

A COUNT(DISTINCT column_name)

B COUNT(column_name)

C COUNT(*)

D None count NULL values

COUNT(*) counts all rows including those with NULLs.

Explain how the COUNT function works and how it handles NULL values.

Think about counting all rows versus counting only non-NULL values in a column.

Describe how to count unique values in a column using COUNT.

Use DISTINCT keyword inside COUNT.