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Usage of subquery and comparison operators

Grammar Description

The most common usages of subquery and comparison operators are as follows:

non_subquery_operand comparison_operator (subquery)

Where comparison_operator refers to the following operator:

= > < >= <= <> != <=>

Grammar Structure

> SELECT column_name(s) FROM table_name WHERE 'a' = (SELECT column1 FROM t1)

Example

create table t1 (a int);
create table t2 (a int, b int);
create table t3 (a int);
create table t4 (a int not null, b int not null);
insert into t1 values ​​(2);
insert into t2 values ​​(1,7),(2,7);
insert into t4 values ​​(4,8),(3,8),(5,9);
insert into t3 values ​​(6),(7),(3);

mysql> select * from t3 where a = (select b from t2);
ERROR 1105 (HY000): scalar subquery returns more than 1 row
mysql> select * from t3 where a = (select distinct b from t2);
+------+
| a |
+------+
| 7 |
+------+
1 rows in set (0.01 sec)

mysql> select a,b from t4 where a > ( select a ,b from t2 where a>1);
ERROR 1105 (HY000): Internal error: Unknown type TUPLE