The most common use of a subquery is in the form:子查询最常用的形式是：

non_subquery_operand comparison_operator (subquery)

Where comparison_operator is one of these operators:

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

For example:例如：

... WHERE 'a' = (SELECT column1 FROM t1)

MySQL also permits this construct:MySQL也允许这种构造：

non_subquery_operand LIKE (subquery)

At one time the only legal place for a subquery was on the right side of a comparison, and you might still find some old DBMSs that insist on this.曾经，子查询的唯一合法位置是在比较的右侧，您可能仍然会发现一些旧的DBMS坚持这样做。

Here is an example of a common-form subquery comparison that you cannot do with a join. 下面是一个公共窗体子查询比较的示例，您不能使用联接进行比较。It finds all the rows in table t1 for which the column1 value is equal to a maximum value in table t2:它查找表t1中column1值等于表t2中最大值的所有行：

SELECT * FROM t1
  WHERE column1 = (SELECT MAX(column2) FROM t2);

Here is another example, which again is impossible with a join because it involves aggregating for one of the tables. 这里是另一个例子，这同样是不可能的，因为它涉及到一个表的聚合。It finds all rows in table t1 containing a value that occurs twice in a given column:它查找表t1中包含给定列中出现两次的值的所有行：

SELECT * FROM t1 AS t
  WHERE 2 = (SELECT COUNT(*) FROM t1 WHERE t1.id = t.id);

For a comparison of the subquery to a scalar, the subquery must return a scalar. 为了将子查询与标量进行比较，子查询必须返回标量。For a comparison of the subquery to a row constructor, the subquery must be a row subquery that returns a row with the same number of values as the row constructor. 要将子查询与行构造函数进行比较，子查询必须是返回与行构造函数具有相同值数目的行的行子查询。See Section 13.2.11.5, “Row Subqueries”.请参阅第13.2.11.5节，“行子查询”。