mysql - count(*) and count(column_name), what's the diff?

Question

count(*) and count(column_name), what's the difference in mysql.

Answer 1

• COUNT(*) counts all rows in the result set (or group if using GROUP BY).
• COUNT(column_name) only counts those rows where column_name is NOT NULL. This may be slower in some situations even if there are no NULL values because the value has to be checked (unless the column is not nullable).
• COUNT(1) is the same as COUNT(*) since 1 can never be NULL.

To see the difference in the results you can try this little experiment:

CREATE TABLE table1 (x INT NULL);
INSERT INTO table1 (x) VALUES (1), (2), (NULL);
SELECT
    COUNT(*) AS a,
    COUNT(x) AS b,
    COUNT(1) AS c
FROM table1;

Result:

a   b   c
3   2   3