MySQL在JOIN语法中将子查询相关联

I would like to provide a WHERE condition on an inner query by specifying innertable.id = outertable.id. However, MySQL (5.0.45) reports "Unknown column 'outertable.id' in 'where clause'". Is this type of query possible?

我想通过指定innertable.id = outertable.id为内部查询提供WHERE条件。但是，MySQL（5.0.45）在'where子句'中报告“未知列'outertable.id'”。这种查询是否可行？

The inner query is pivoting rows to columns using a GROUP BY. This could be entirely be performed in the outer query, but would possibly incur additional overhead due to the extra joins.

内部查询使用GROUP BY将行旋转到列。这可以完全在外部查询中执行，但由于额外的连接可能会产生额外的开销。

Alternatively, I can leave off the WHERE condition in the inner query and instead specify an ON outertable.id = innerquery.id, but it would then fetch the entire inner query rowset to join again the outer, which is inefficient.

或者，我可以在内部查询中省略WHERE条件，而是指定一个ON outertable.id = innerquery.id，但它会获取整个内部查询行集再次连接外部，这是低效的。

The actual SQL appears below:

实际的SQL如下所示：

select t.ticketid, u.userid, t.fullname, u.loginapi_userid, t.email, tp.subject, tp.contents, a.PhoneNumber, a.Location, a.Extension, a.BusinessUnit, a.Department
from swtickets t
inner join swticketposts tp on t.ticketid = tp.ticketid
inner join swusers u on t.userid = u.userid
left join
  (
  select
  cfv.typeid,
  min(case cfv.customfieldid when 1 then cfv.fieldvalue end) as 'PhoneNumber',
  min(case cfv.customfieldid when 3 then cfv.fieldvalue end) as 'Location',
  min(case cfv.customfieldid when 5 then cfv.fieldvalue end) as 'Extension',
  min(case cfv.customfieldid when 8 then cfv.fieldvalue end) as 'BusinessUnit',
  min(case cfv.customfieldid when 9 then cfv.fieldvalue end) as 'Department'
  from swcustomfieldvalues cfv
  where cfv.typeid = t.ticketid
  group by cfv.typeid
  ) as a on 1 = 1
where t.ticketid = 2458;

5 个解决方案

#1

The answer to your question is no, it is not possible to reference correlation names as you are doing. The derived table is produced by your inner query before the outer query starts evaluating joins. So the correlation names like t, tp, and u are not available to the inner query.

您的问题的答案是否定的，不可能像您一样引用相关名称。派生表是在外部查询开始评估联接之前由内部查询生成的。因此，内部查询无法使用t，tp和u等相关名称。

To solve this, I'd recommend using the same constant integer value in the inner query, and then join the derived table in the outer query using a real condition instead of 1=1.

为了解决这个问题，我建议在内部查询中使用相同的常量整数值，然后使用实际条件而不是1 = 1将外部查询中的派生表连接起来。

SELECT t.ticketid, u.userid, t.fullname, u.loginapi_userid, t.email,
  tp.subject, tp.contents, a.PhoneNumber, a.Location, a.Extension,
  a.BusinessUnit, a.Department
FROM swtickets t
 INNER JOIN swticketposts tp ON (t.ticketid = tp.ticketid)
 INNER JOIN swusers u ON (t.userid = u.userid)
 LEFT OUTER JOIN (
  SELECT cfv.typeid,
    MIN(CASE cfv.customfieldid WHEN 1 THEN cfv.fieldvalue END) AS 'PhoneNumber',
    MIN(CASE cfv.customfieldid WHEN 3 THEN cfv.fieldvalue END) AS 'Location',
    MIN(CASE cfv.customfieldid WHEN 5 THEN cfv.fieldvalue END) AS 'Extension',
    MIN(CASE cfv.customfieldid WHEN 8 THEN cfv.fieldvalue END) AS 'BusinessUnit',
    MIN(CASE cfv.customfieldid WHEN 9 THEN cfv.fieldvalue END) AS 'Department'
  FROM swcustomfieldvalues cfv
  WHERE cfv.typeid = 2458
  GROUP BY cfv.typeid
  ) AS a ON (a.typeid = t.ticketid)
WHERE t.ticketid = 2458;

#2

You're using the Entity-Attribute-Value design, and there's ultimately no way to make this scalable if you try to generate conventional result sets. Don't try to do this in one query.

您正在使用实体 - 属性 - 值设计，如果您尝试生成传统结果集，最终无法实现此可伸缩性。不要尝试在一个查询中执行此操作。

Instead, query your normalized tables first:

而是首先查询规范化表：

SELECT t.ticketid, u.userid, t.fullname, u.loginapi_userid, t.email, 
  tp.subject, tp.contents
FROM swtickets t
 INNER JOIN swticketposts tp ON (t.ticketid = tp.ticketid)
 INNER JOIN swusers u ON (t.userid = u.userid)
WHERE t.ticketid = 2458;

Then query your custom fields, with the result on multiple rows of the result set:

然后查询自定义字段，结果集在结果集的多行上：

SELECT cfv.customfieldid, cfv.fieldvalue
FROM swcustomfieldvalues cfv
WHERE cfv.typeid = 2458;

You'll get multiple rows in the result set, one row for each custom field:

您将在结果集中获得多行，每个自定义字段占一行：

+---------------+--------------+
| customfieldid | fieldvalue   |
+---------------+--------------+
|             1 | 415-555-1234 |
|             3 | Third office |
|             5 | 123          |
|             8 | Support      |
|             9 | Engineering  |
+---------------+--------------+

You then need to write application code to map the result-set fields to the application object fields, in a loop.

然后，您需要编写应用程序代码，以便在循环中将结果集字段映射到应用程序对象字段。

Using an Entity-Attribute-Value table in this way is more scalable both in terms of performance and code maintenance.

以这种方式使用Entity-Attribute-Value表在性能和代码维护方面都更具可伸缩性。

#3

I would write it with multiple joins. When you say that it "would possibly incur additional overhead" that tells me that you haven't tested it to be sure. If you have decent indexes the joins should be pretty trivial.

我会用多个连接来编写它。当你说它“可能会产生额外的开销”时，它会告诉我你没有对它进行测试以确定。如果你有合适的索引，那么连接应该是非常简单的。

This also shows just one of the pitfalls of the generic "hold everything" table design pattern.

这也只显示了通用“保持一切”表设计模式的一个缺陷。

#4

My suggestion was going to be what you ruled out on the grounds of efficiency. E.g. leaving out the where clause and using a join (as per t.ticketid = a.ticketid)

我的建议是你在效率方面排除的。例如。省略where子句并使用连接（根据t.ticketid = a.ticketid）

Have you been able to prove your thoughts on inefficiency by some concrete examples? I know what you are saying but whatever method you use every row in the outer query is being joined to every row in the inner query so depending on the execution plan it may not be as inefficient as you suspect?

您是否能够通过一些具体的例子证明您对低效率的看法？我知道你在说什么，但无论你使用什么方法，外部查询中的每一行都被连接到内部查询中的每一行，所以根据执行计划，它可能不像你怀疑那样低效？

#5

I imagine the problem is 'cfv.typeid = t.ticketid' then? My thinking about that would be that, while MySQL supports correlated subqueries, what you're trying to do seems like it could fail in a join because the 'inner' query isn't really 'inside' the rest of the query like it is in a WHERE clause. But it looks like you could just take the where clause out of the subquery and make your join condition on a.typeid = t.ticketid.

我想问题是'cfv.typeid = t.ticketid'呢？我的想法是，虽然MySQL支持相关子查询，但你想要做的事情看起来似乎可能在连接中失败，因为'内部'查询实际上不在查询的其余部分内部，就像它是在WHERE子句中。但看起来您可以从子查询中取出where子句，并在a.typeid = t.ticketid上创建连接条件。

#1