如果条件满足，SQL 执行内部联接

问题描述：

我想要一种改进我的 sql 代码的好方法，我必须在满足条件时使用内部联接.我目前正在复制代码:

i want a good way to improve my sql code, i have to use inner join when condition is met. I am currently replicates the code:

@SystemMerge bit

if (@SystemMerge=1)  
BEGIN
   SELECT
         .......
      FROM myTable
      INNER JOIN table ON table.param1=myTable.param1
      INNER JOIN systemTable on systemTable.param2=myTable.param2
   END
ELSE
   BEGIN
      SELECT
         .......
      FROM myTable
      INNER JOIN table ON table.param1=myTable.param1
   END

我想以这样的方式做到这一点:

and i would like to do it in a way like this:

@SystemMerge bit
BEGIN
   SELECT
      .......
   FROM myTable
   INNER JOIN table ON table.param1=myTable.param1
   ***//the next 4 lines is not working, but this pseudo of what i want:***
   if (@SystemMerge=1)  
   begin
      INNER JOIN systemTable on systemTable.param2=myTable.param2
   end

解决方案(感谢@Damien_The_Unbeliever):

edit: the solution (thanks to @Damien_The_Unbeliever):

LEFT JOIN systemTable ON systemTable.param2=myTable.param2
WHERE 
   ((@SystemMerge=1 AND systemTable.param2 is not null) 
   OR
   (@SystemMerge=0 OR @SystemMerge is null))

答

这应该(大约)做同样的事情:

This should (approxmately) do the same thing:

SELECT
     .......
  FROM myTable
  INNER JOIN table ON table.param1=myTable.param1
  LEFT JOIN systemTable on systemTable.param2=myTable.param2 and @SystemMerge = 1
  WHERE (@SystemMerge = 0 OR systemTable.NonNullableColumn IS NOT NULL)

当然，这也意味着必须写入对 systemTable 中列的任何其他引用，以期望这些列是 NULL.

Of course, this also means that any other references to columns within systemTable must be written to expect such columns to be NULL.

如果条件满足，SQL 执行内部联接

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