? COUNT DISTINCT in SQL

The COUNT(DISTINCT column) function counts the number of unique (distinct) non-NULL values in a column.

? Syntax

SELECT COUNT(DISTINCT column_name) FROM table_name;

? Example

Consider a sales table with a column customer_id. To count how many unique customers made purchases:

SELECT COUNT(DISTINCT customer_id) AS unique_customersFROM sales;

? Notes

• Counts only distinct, non-null values.

• If you use COUNT(column_name) without DISTINCT, it counts all non-null values (including duplicates).

• You can also use COUNT(DISTINCT col1, col2) in some databases to count unique combinations of columns (supported in MySQL, PostgreSQL 9.4+).

If you want, I can show you examples with multiple columns or in specific SQL dialects!