SQL COUNT Function

What is COUNT?

COUNT is an aggregate function that returns the number of rows matching a condition. It is one of the most frequently used SQL functions.

Variants

• COUNT(*) — Counts all rows, including those with NULLs.
• COUNT(column) — Counts non-NULL values in the specified column.
• COUNT(DISTINCT column) — Counts unique non-NULL values.

Syntax

SELECT COUNT(*) FROM table_name;
SELECT COUNT(column_name) FROM table_name;
SELECT COUNT(DISTINCT column_name) FROM table_name;

When to Use

• Getting total record counts
• Counting records per group with GROUP BY
• Checking if related records exist

Key Points

1. **COUNT(*) vs COUNT(column)** — COUNT(*) includes NULLs; COUNT(column) excludes them.
2. With GROUP BY — Use COUNT with GROUP BY to count records per category.
3. With WHERE — Apply WHERE before counting to filter the source rows.
4. DISTINCT — COUNT(DISTINCT grade) counts the number of unique grades.
5. Performance — COUNT(*) on large tables can be slow. Consider approximate counts for large datasets.