SQL - 和一些行，減和其他行

Question

我在mysql用下列的表中：

CUSTOMER_CODE | TRANS_TYPE | TRANS_VALUE 

TRANS_TYPE可能是因爲存在「DRINV」（銷售）或「DRCDT」（一信用）。

我想每個客戶的總銷售額，所以我的查詢到目前爲止是：

SELECT CUSTOMER_CODE, SUM(TRANS_VALUE) as SALES FROM DR_TRANS GROUP BY CUSTOMER_CODE 

問題是這樣的總額，而不是給人是銷售額減去信用銷售和學分。我想要的結果是

SUM(TRANS_VALUE) where TRANS_TYPE = "DRINV" - SUM(TRANS_VALUE) where TRANS_TYPE = "DRCDT". 

是否有可能在SQL查詢中這樣做？

Answer 1

select customer_code, 
     sum(case 
      when trans_type = 'DRINV' then 
       trans_value 
      else 
       -trans_value 
      end) as net_sales 
    from dr_trans 
group by customer_code 

Answer 2

是的，這是可能的。使用CASE caluse：

SELECT CUSTOMER_CODE, 
SUM(CASE WHEN TRANS_TYPE = 'DRINV' THEN TRANS_VALUE ELSE (- TRANS_VALUE) END) as SALES 
FROM DR_TRANS GROUP BY CUSTOMER_CODE