使用子查询更新列，过滤掉空值

Question

我正在尝试使用子查询更新列。但它给出了一个错误：

cannot update PRODUCT.AVERAGE_COST to NULL

子查询结果为null时不知道如何不更新。

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) 是 0 或者子查询结果是 null.
我根本不想更新查询应在 Oracle、MySQL、MSSQL、PostgreSQL 上运行。

Answer 1

如果您使用的是 MSSQL，请试试这个：

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

您应该在查询中添加 WHERE 子句。所以它应该看起来像：

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

中查看

使用子查询更新列，过滤掉空值

Update column with subquery, filter out null values

sql

sql-update