SQL COUNT function

Here’s a clean and practical guide to the SQL COUNT function, including behavior with NULLs, best practices, and real-world examples.

 What SQL COUNT function Does

COUNT() returns the number of rows that match a condition.

 Types of COUNT

1. COUNT(*) — Count all rows

Counts every row, including those with NULL values.

2. COUNT(column) — Count non-NULL values

Ignores NULLs in the specified column.

3. COUNT(DISTINCT column) — Count unique non-NULL values

Useful for determining how many unique values exist.

 COUNT with WHERE (filter first)

COUNT with GROUP BY (per category)

 Important Behavior Notes

• COUNT(*) counts all rows
• COUNT(column) ignores NULLs
• COUNT(DISTINCT ...) ignores NULLs and removes duplicates
•  Use COUNT with GROUP BY for per-group counts
•  Use WHERE to filter before counting

 Real-World Examples

Number of completed orders