Question

我想知道从MySQL数据库获取数据的哪种方式具有更好的性能特征。

在一个主查询中使用子查询：

SELECT
(SELECT SUM(`number`) FROM `table`) as `number_sum`,
(SELECT MAX(`number`) FROM `table`) as `number_max`,
(SELECT MIN(`number`) FROM `table`) as `number_min`

或者，3个不同的SELECT语句检索相同的数据。

提前致谢！

Answer 1

由于这三个聚合来自具有相同WHERE条件的同一个表，因此您不需要子选择。所有三个聚合都在同一行分组上运行（指定了GROUP BY，因此整个表都有一行），因此它们都可以直接存在于SELECT列表中。

SELECT
  SUM(number) AS number_sum,
  MAX(number) AS number_max,
  MIN(number) AS number_min
FROM `table`

如果任何聚合需要基于不同的条件，您将在WHERE子句中进行过滤，那么您将需要使用子选择来表示不同的条件，或者进行笛卡尔连接。对于仅返回一行的聚合，此子选择和以下LEFT JOIN方法应该是等效的，性能方面：

SELECT
  /* Unique filtering condition - must be done in a subselect */
  (SELECT SUM(number) FROM `table` WHERE `somecolumn` = `somevalue`) AS number_sum,
  MAX(number) AS number_max,
  MIN(number) AS number_min
FROM `table`

或者等同于上面的查询，您可以LEFT JOIN针对没有ON子句的子查询。只有当您知道子查询只返回一行时才应该这样做。否则，您将得到一个笛卡尔积 - 由连接的一侧返回的行数乘以另一侧返回的行数。

如果您需要返回包含一组WHERE子句条件的几列和一些具有不同WHERE条件的列，但只有一个 JOIN每一行的行。在这种情况下，JOIN应该比使用相同WHERE子句的两个子选择更快。

这应该更快......

SELECT /* this one has two aggregates sharing a WHERE condition */ subq.number_sum_filtered, subq.number_max_filtered, /* ...and two aggregates on the main table with no WHERE clause filtering */ MAX(`table`.number) AS number_max, MIN(`table`.number) AS number_min FROM `table` LEFT JOIN ( SELECT SUM(number) AS number_sum_filtered, MAX(number) AS number_max_filtered FROM `table` WHERE `somecolumn = `somevalue` ) subq /* No ON clause here since there's no common column to join on... */

比......

SELECT /* Two different subselects each over the same filtered set */ (SELECT SUM(number) FROM `table` WHERE `somecolumn` = `somevalue`) AS number_sum_filtered, (SELECT MAX(number) FROM `table` WHERE `somecolumn` = `somevalue`) AS number_max_filtered, MAX(`table`.number) AS number_max, MIN(`table`.number) AS number_min FROM `table`

MySQL多子查询与整个查询

1 个答案: