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.

Practice

1. What does the COUNT(*) function do in MySQL?

easy

A. Counts all rows in the table, including those with NULL values

B. Counts only rows where all columns are NOT NULL

C. Counts only rows with NULL values in any column

D. Counts the number of columns in the table

Solution

1. Step 1: Understand COUNT(*) behavior

   The COUNT(*) function counts every row in the table regardless of NULL values.

2. Step 2: Compare with other COUNT variants

   Unlike COUNT(column), which counts only non-NULL values in that column, COUNT(*) counts all rows.

3. Final Answer:

   Counts all rows in the table, including those with NULL values -> Option A

4. Quick Check:

   COUNT(*) counts all rows [OK]

Hint: COUNT(*) counts every row, NULL or not [OK]

Common Mistakes:

• Thinking COUNT(*) ignores rows with NULL
• Confusing COUNT(*) with COUNT(column)
• Assuming COUNT(*) counts columns

2. Which of the following is the correct syntax to count non-NULL values in the column age from the table users?

easy

A. SELECT COUNT(*) FROM users;

B. SELECT COUNT(age) FROM users WHERE age != NULL;

C. SELECT COUNT(age) FROM users WHERE age = NULL;

D. SELECT COUNT(age) FROM users;

Solution

1. Step 1: Identify correct COUNT syntax for non-NULL values

   COUNT(column) counts only non-NULL values in that column, so COUNT(age) counts non-NULL ages.

2. Step 2: Check WHERE clause correctness

   SELECT COUNT(*) FROM users; uses COUNT(*) which counts all rows. Using WHERE age IS NOT NULL is valid but unnecessary with COUNT(age). Options B and C use incorrect NULL comparisons.

3. Final Answer:

   SELECT COUNT(age) FROM users; -> Option D

4. Quick Check:

   COUNT(column) counts non-NULL values [OK]

Hint: Use COUNT(column) to count non-NULL values directly [OK]

Common Mistakes:

• Using WHERE age = NULL instead of IS NOT NULL
• Using COUNT(*) to count non-NULL values
• Using != NULL which is invalid in SQL

3. Given the table orders with 5 rows where the discount column has 3 NULL values and 2 non-NULL values, what will this query return?

SELECT COUNT(discount) FROM orders;

medium

A. 5

B. 3

C. 2

D. 0

Solution

1. Step 1: Understand COUNT(column) counts non-NULL values

   The query counts only rows where discount is NOT NULL.

2. Step 2: Apply to given data

   There are 2 non-NULL discount values, so the count is 2.

3. Final Answer:

   2 -> Option C

4. Quick Check:

   COUNT(discount) = 2 [OK]

Hint: COUNT(column) ignores NULLs, counts only real values [OK]

Common Mistakes:

• Counting total rows instead of non-NULL values
• Confusing NULL count with non-NULL count
• Assuming COUNT(column) counts NULLs

4. Identify the error in this query:

SELECT COUNT(name) FROM employees WHERE name = NULL;

medium

A. Using COUNT(name) instead of COUNT(*)

B. Incorrect NULL comparison with '=' instead of IS NULL

C. Missing GROUP BY clause

D. COUNT cannot be used with WHERE clause

Solution

1. Step 1: Check WHERE clause NULL comparison

   In SQL, NULL cannot be compared with '='; use IS NULL instead.

2. Step 2: Validate COUNT usage

   COUNT(name) is valid and WHERE clause is allowed; no GROUP BY needed here.

3. Final Answer:

   Incorrect NULL comparison with '=' instead of IS NULL -> Option B

4. Quick Check:

   Use IS NULL for NULL checks [OK]

Hint: Use IS NULL, not = NULL, to check for NULL values [OK]

Common Mistakes:

• Using '=' to compare NULL values
• Thinking COUNT can't be combined with WHERE
• Adding unnecessary GROUP BY

5. You have a table sales with columns region and amount. You want to find how many sales records exist for each region, including regions with zero sales. Which query will correctly show all regions with their sales count, including zero counts?

hard

A. SELECT r.region, COUNT(s.amount) FROM regions r LEFT JOIN sales s ON r.region = s.region GROUP BY r.region;

B. SELECT region, COUNT(*) FROM sales GROUP BY region;

C. SELECT region, COUNT(amount) FROM sales GROUP BY region;

D. SELECT region, COUNT(amount) FROM sales WHERE amount IS NOT NULL GROUP BY region;

Solution

1. Step 1: Understand requirement to include regions with zero sales

   To include regions with zero sales, we must use a LEFT JOIN from all regions to sales.

2. Step 2: Analyze query options

   SELECT r.region, COUNT(s.amount) FROM regions r LEFT JOIN sales s ON r.region = s.region GROUP BY r.region; uses LEFT JOIN from regions table to sales, counting sales per region including zero counts. Options A, B, D only count existing sales records, missing zero counts.

3. Final Answer:

   SELECT r.region, COUNT(s.amount) FROM regions r LEFT JOIN sales s ON r.region = s.region GROUP BY r.region; -> Option A

4. Quick Check:

   LEFT JOIN + COUNT(column) includes zero counts [OK]

Hint: Use LEFT JOIN with COUNT(column) to include zero counts [OK]

Common Mistakes:

• Using only sales table, missing regions with zero sales
• Using COUNT(*) which counts joined rows, not zero
• Not using LEFT JOIN to include all regions