COUNT

Return the number of non NULL entries of a column.

COUNT(*) counts the total number of entries of a table, including entries that only contain NULL values.

CREATE TABLE t (c0 INT, c1 INT);
INSERT INTO t VALUES (0, 0), (1, NULL), (NULL, NULL);
SELECT COUNT(c0) FROM t;
SELECT COUNT(c1) FROM t;
SELECT COUNT(*) FROM t;
DROP TABLE t;

Output:

COUNT(c0)
2

COUNT(c1)
1

COUNT(*)
3