如何在MySQL更新语句中正确使用子查询以避免语法错误？

了解更新查询中的 SQL 子查询

在 MySQL 中更新数据时，通常需要使用子查询引用其他表中的值。但是，当子查询与外部更新语句没有正确关联时，可能会发生错误。

考虑以下查询：

Update Competition
Set Competition.NumberOfTeams =
(
SELECT count(*) as NumberOfTeams
FROM PicksPoints
where UserCompetitionID is not NULL
group by CompetitionID
) a
where a.CompetitionID =  Competition.CompetitionID

此查询失败并显示错误消息：

#1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use
near 'a where a.CompetitionID = Competition.CompetitionID' at line 8

解决错误

出现错误是因为内部子查询与外部更新语句上的 where 子句不相关。在执行子查询之前，where 条件适用于目标表（Competition）。为了解决这个问题，可以采用多表更新：

Update
  Competition as C
  inner join (
    select CompetitionId, count(*) as NumberOfTeams
    from PicksPoints as p
    where UserCompetitionID is not NULL
    group by CompetitionID
  ) as A on C.CompetitionID = A.CompetitionID
set C.NumberOfTeams = A.NumberOfTeams

这个多表更新正确地将竞争表（别名为C）与子查询（别名为A）连接起来，确保子查询的值可用于在外部更新语句中进行过滤。

有关更正查询的现场演示，请参阅以下 SQL Fiddle：http://www.sqlfiddle.com/#!2/a74f3/ 1

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