COUNT(*)
counts all rows in the result set (or group if using GROUP BY).COUNT(column_name)
only counts those rows wherecolumn_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 asCOUNT(*)
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