子查询是一种嵌套在其他 SELECT 语句中的 SELECT 语句。嵌套子查询通常称为查询内语句，而包含查询通常称为查询语句或外查询块。子查询返回外查询用作条件的数据，以确定需要检索哪些数据。您可以创建的嵌套子查询的数量没有限制。

与任何查询一样，子查询返回（单列单记录、单列多记录或多列多记录）表中的记录。

Where 子句中的子查询举例

SELECT * FROM sub_query_correlated_subquery1 WHERE k1 > (SELECT AVG(k1) FROM sub_query_correlated_subquery3) OR k1 < 10 order by k1, k2;
select * from sub_query_correlated_subquery1 where sub_query_correlated_subquery1.k1 not in (select sub_query_correlated_subquery3.k3 from sub_query_correlated_subquery3 where sub_query_correlated_subquery3.v2 = sub_query_correlated_subquery1.k2) or k1 < 10 order by k1, k2

Join 子句中的子查询举例

select t1.* from t1 left join t2 on t1.k2 = t2.k3 and t1.k1 in ( select t3.k1 from t3 where t1.k2 = t3.k2 ) or t1.k1 < 10 order by t1.k1, t1.k2;
select t1.* from t1 left join t2 on t1.k2 = t2.k3 and exists ( select t3.k1 from t3 where t1.k2 = t3.k2 ) or t1.k1 < 10 order by t1.k1, t1.k2;