mysql> select count(*) from t1 where t1.id = (select t2.id from t2);
ERROR 1242 (21000): Subquery returns more than 1 row

解决方案，当然，两者的含义也不一样，需要自行判断。

mysql> select count(*) from t1 where t1.id = any(select t2.id from t2);
+----------+
| count(*) |
+----------+
|   959368 |
+----------+
1 row in set (2.12 sec)

mysql> select count(*) from t1 where t1.id = (select t2.id from t2 limit 1);
+----------+
| count(*) |
+----------+
|        2 |
+----------+
1 row in set (0.01 sec)

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MYSQL