LEFT JOIN多功能 [英] LEFT JOIN multiple function

问题描述

问候，



如果我使用给定查询LEFT JOIN几个函数，其中每个函数的连接条件都不同于另一个。将执行所有这些函数或仅具有正确条件的函数将是执行的函数。我知道我的话可能令人困惑所以我会尝试通过一个例子来解释

Greetings,

If I LEFT JOIN several functions with a given query with each one of them having a join condition different from the other one. Will all those functions execute or the one with the correct condition only will be the one that is executed. I know that my words may be confusing so I will try to explain by an example

query LEFT JOIN function1(parameters) on categorycolumn = 1
      LEFT JOIN function2(parameters) on categorycolumn = 2

我尝试了什么：



尝试谷歌寻求答案并尝试调试，但SSMS调试器对我不起作用。

What I have tried:

Tried to google for an answer and trying to debug however the SSMS debugger is not working for me.

推荐答案

也许你可以使用 APPLY ，参见这里的例子： sql server - 如何使左外连接成为任何sql表的UD函数？ - 堆栈溢出 [ ^ ]

Maybe you can use APPLY, see example here: sql server - How to make left outer join a UD-function with any sql table? - Stack Overflow[^]

将功能放在 FROM <中通常是一个坏主意/ code>子句。

看看你是否可以将它们移动到 SELECT 子句，然后你也可以使用<$过滤它们c $ c> CASE 。

像这样：

It's performance wise generally a bad idea to put functions in the FROM clause.
See if you can move them to the SELECT clause, then you can also filter them using a CASE.
Like this:

SELECT  CASE 
            WHEN categorycolumn = 1 THEN function1(parameters) 
            WHEN categorycolumn = 2 THEN function2(parameters)
        END AS MyResult
FROM...

如果结果不兼容，很容易分成几个CASE。

If the results arent compatible it's easy enough to split into several CASEs.

这篇关于LEFT JOIN多功能的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！