如果条件满足，SQL进行内部联接

Question

我想要一种改进我的sql代码的好方法，当条件满足时，我必须使用内部联接。我目前正在复制代码：

@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

我想以这种方式做到这一点：

@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

编辑： 解决方案（由于@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))

Answer 1

这应该（大约）执行相同的操作：

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，这还意味着systemTable必须编写对其中的列的任何其他引用，以期望此类列为NULL。