使用子查詢更新列，過濾出空值

Question

我正在嘗試使用子查詢更新列。 但是它給出了一個錯誤：

無法將PRODUCT.AVERAGE_COST更新為NULL

我不知道當子查詢結果為null時如何不更新。

UPDATE PRODUCT P SET P.AVERAGE_COST = 
    (SELECT 
        SUM(C.COST)/SUM(C.QTY)
    FROM PRODUCT_COSTS C
    WHERE C.PRODUCT_CODE = P.PRODUCT_CODE 
    GROUP BY C.PRODUCT_CODE )

如果SUM(C.QTY)為0或子查詢結果為null我根本就不想更新。
查詢應在Oracle，MySQL，MSSQL，PostgreSQL上運行。

Answer 1

如果您使用的是MSSQL，請嘗試以下一項：

UPDATE PRODUCT P SET P.AVERAGE_COST = 
(SELECT 
    CASE WHEN ISNULL(SUM(C.QTY),0)='0' THEN '0' ELSE ISNULL(SUM(C.COST),0)/ISNULL(SUM(C.QTY),0) END
FROM PRODUCT_COSTS C
WHERE C.PRODUCT_CODE = P.PRODUCT_CODE 
GROUP BY C.PRODUCT_CODE )

Answer 2

您應該在查詢中添加WHERE子句。 所以它應該看起來像：

UPDATE PRODUCT P SET P.AVERAGE_COST = 
    (SELECT SUM(C.COST)/SUM(C.QTY)
     FROM PRODUCT_COSTS C
     WHERE C.PRODUCT_CODE = P.PRODUCT_CODE)
WHERE
    (SELECT COUNT(1)
     FROM PRODUCT_COSTS C
     WHERE C.PRODUCT_CODE = P.PRODUCT_CODE ) > 0
AND
    (SELECT SUM(C.QTY)
     FROM PRODUCT_COSTS C
     WHERE C.PRODUCT_CODE = P.PRODUCT_CODE ) <> 0

您可以在SQLFiddle中檢查它