如果條件滿足，SQL會進行內連接

Question

我想要一個改進我的sql代碼的好方法，當條件滿足時我必須使用內連接。我目前正在複製代碼：

@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 

，我想做到在這樣一個辦法：

@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 

編輯： 解決方案（感謝@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

這應該（近似）做同樣的事情：

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) 

當然，這也意味着必須編寫systemTable中對列的任何其他引用，以期望這些列爲NULL。

Answer 2

動態SQL如何？

declare @sel varchar(max) 

set @sel = ' SELECT 
     ....... 
     FROM myTable 
     INNER JOIN table ON table.param1=myTable.param1 
     ' 

if (@SystemMerge=1) 
    begin 
    set @sel = @sel+'INNER JOIN systemTable on systemTable.param2=myTable.param2' 
    end 

exec(@sel)