MySQL 8.0-13.2.11.2 Comparisons Using Subqueries(比较使用子查询)

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原文链接：https://dev.mysql.com/doc/refman/8.0/en/comparisons-using-subqueries.html

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本文介绍了子查询在SQL中的常见应用，包括用于比较操作符的子查询以及与标量和行构造函数的比较。子查询可以用于找到特定条件下的行，如查找某列值等于另一表最大值的行，或者找出在某一列中重复出现的值。虽然某些子查询操作过去只能放在比较的右侧，但现在许多数据库系统已经放宽了这一限制。子查询在某些场景下是连接操作无法替代的，例如涉及聚合或寻找特定条件的最大值。

The most common use of a subquery is in the form:子查询最常见的用法是:

non_subquery_operand comparison_operator (subquery)

Where comparison_operator is one of these operators: 其中comparison_operator是以下操作符之一:

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

For example:

... 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. It finds all the rows in table t1 for which the column1 value is equal to a maximum value in table t2:

下面是一个使用连接无法实现的公共形式子查询比较的示例。它查找表t1中列n1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. It finds all rows in table t1 containing a value that occurs twice in a given column:

下面是另一个例子，使用连接也是不可能的，因为它涉及到一个表的聚合。它查找表t1中包含在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. 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. See Section 13.2.11.5, “Row Subqueries”.

对于子查询与标量的比较，子查询必须返回标量。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数量值的行子查询。