如何使用sum函数进行查询分组

Question

我在SQL Server中有inventoryTransaction表就像这样：

productID StockLocationID TransactionType   Quantity
1046          1                "in"           100
1046          1               "out"            20
1046          2                "in"            70
1046          2               "out"            65
...

如何进行查询，输出如下：

productID  StockLocationID     stock
1046          1                 80
1046          2                 5

Answer 1

您可以尝试：

SELECT 
    productID, 
    StockLocationID,
    SUM(CASE 
            WHEN TransactionType = 'out' THEN -1 
            ELSE 1 
        END) * Quantity) AS stock
GROUP BY 
    productID, 
    StockLocationID

Answer 2

同样但稍微缩短了消除不必要的乘法：

SELECT 
    productID, 
    StockLocationID,
    SUM(CASE TransactionType 
            WHEN 'out' THEN -Quantity 
            ELSE Quantity 
        END) AS stock
GROUP BY 
    productID, 
    StockLocationID