结合多选和更新

Question

我有一个查询拖累了我的应用程序。必须有一种方法可以通过选择加入更新来改善这一点。任何帮助表示赞赏。这是非常慢的查询：

Select t1.id, t2.tracker from Table1 t1, Table2 t2 where t1.id=t2.id and t2.tracker is not null

上面返回一个我可以处理的行集。对于返回的每一行，我在 第三个表，看看是否只有 1 行存在：

select tracker from Table3 where tracker="from above query". 

如果跟踪器在表 3 中的计数为 1 行，则我对表 1 执行更新。

Update Table 1, set some field where id=t1.id

我该如何组合呢？列出的答案很棒，但我想这个问题还不够清楚。因此，我编辑了这个问题。

表 1 返回要更新的 ID 的可能列表。 表 2 返回我需要搜索表 3 的跟踪器。 表 3 告诉我跟踪器是否只存在一次，所以我可以使用它返回表 1 并 更新它。

Answer 1

通过使用HAVING 子句和额外的LEFT JOIN，您可以将前两个查询组合成一个查询，仅返回tracker，仅针对在Table3 中有一条记录且表达式为HAVING COUNT(t3.tracker) = 1 的那些查询

SELECT 部分看起来像：

SELECT
  t1.id,
  t2.tracker
FROM 
  -- Your original comma-separated FROM clause (implicit inner join)
  -- has been replaced with a more modern explicit INNER JOIN, which
  -- works more clearly with the LEFT JOIN we need to do.
  Table1 t1
  INNER JOIN Table2 t2 ON t1.id = t2.id
  -- left join table 3 by tracker value
  LEFT JOIN Table3 t3 ON t2.tracker = t3.tracker
WHERE t2.tracker IS NOT NULL
GROUP BY
  t1.id,
  t2.tracker
-- limit the group to only those with 1 tracker in t3
HAVING COUNT(t3.tracker) = 1

现在，您应该可以使用JOIN 将其填充到UPDATE 查询中。 MySQL 的 update-join 语法如下所示：

UPDATE 
  Table1 t_up
  -- join Table1 in its normal form against the query from above
  -- MySQL won't allow an IN () subquery for update in most versions
  -- so it has to be done as a join instead.
  JOIN (
    -- the subquery only needs to return t1.id
    SELECT t1.id
    FROM 
      Table1 t1
      INNER JOIN Table2 t2 ON t1.id = t2.id
      LEFT JOIN Table3 t3 ON t2.tracker = t3.tracker
    -- Filter to those with non-null t2.tracker
    WHERE t2.tracker IS NOT NULL
    GROUP BY
      -- Since only id was in SELECT, only id needs to be in GROUP BY
      t1.id
    HAVING COUNT(t3.tracker) = 1
  ) t_set ON t_up.id = t_set.id
SET t_up.some_field = 'SOME NEW VALUE'

Here is a demonstration of the concept in action...