DROP TABLE IF EXISTS t1;
CREATE TABLE t1 (c1 VARCHAR(255), c2 VARCHAR(255), c3 INT);
INSERT INTO t1 VALUES (REPEAT('x', 255), REPEAT('y', 255), 1);
INSERT INTO t1 VALUES (REPEAT('x', 255), REPEAT('y', 255), 2);
INSERT INTO t1 VALUES (REPEAT('x', 255), REPEAT('y', 255), 3);
SELECT COUNT(*) FROM t1;
COUNT(*)
3
drop table if exists t1;
create table t1 (a int, unique index indexa (a));
insert into t1 values (-1), (-4), (-2), (NULL);
select -10 IN (select a from t1 FORCE INDEX (indexa));
-10 IN (select a from t1 FORCE INDEX (indexa))
NULL