Question

对于某些类别，我的查询会返回每个月的销售列总计和每月的购买总额。

SELECT theMonth, 
       sum(Sales) as sumSales, 
       sum(Saleswotax) as sumSaleswotax, 
       sum(Purchases) as sumPurchases, 
       sum(Purchaseswotax) as sumPurchaseswotax 
 FROM ( SELECT date_format(saledate, '%Y-%m') AS theMonth, 
               sales.cost as Sales, 
               ROUND(sales.cost*0.85, 2) AS Saleswotax, 
               0 AS Purchases, 
               0 AS Purchaseswotax 
          FROM sales, products 
         WHERE sales.product = products.name 
           AND category='Food'
        UNION ALL
        SELECT date_format(purchasedate, '%Y-%m') AS theMonth, 
               0 as Sales, 
               0 AS Saleswotax, 
               purchases.cost as Purchases, 
               ROUND(purchases.cost*0.85, 2) AS Purchaseswotax, 
          FROM purchases) AS all_costs
    group by theMonth

我试图在我的查询中返回一个列（实际上不存在于表中），这只是对现有表的计算。即saleswotax和purchaseswotax列。

我正在使用一个函数，并将其作为名称返回...为什么它不起作用？

Answer 1

在联盟中，您使用0作为销售和购买列，但是也没有使用-wotax列。他们需要匹配联盟才能正常工作（我想你知道，因为你是为销售和购买而做的）。

Answer 2

上次我看到，MySQL中的计算字段没有声明性支持。您必须向表中添加计算列并使用UPDATE / INSERT触发器填充它们。或者使用其他计算列创建视图。

Answer 3

你需要删除UNION后半部分AS Purchasewotax之后的逗号：

SELECT theMonth, 
       sum(Sales) as sumSales, 
       sum(Saleswotax) as sumSaleswotax, 
       sum(Purchases) as sumPurchases, 
       sum(Purchaseswotax) as sumPurchaseswotax 
 FROM ( SELECT date_format(saledate, '%Y-%m') AS theMonth, 
               sales.cost as Sales, 
               ROUND(sales.cost*0.85, 2) AS Saleswotax, 
               0 AS Purchases, 
               0 AS Purchaseswotax 
          FROM sales, products 
         WHERE sales.product = products.name 
           AND category='Food'
        UNION ALL
        SELECT date_format(purchasedate, '%Y-%m') AS theMonth, 
               0 as Sales, 
               0 AS Saleswotax, 
               purchases.cost as Purchases, 
               ROUND(purchases.cost*0.85, 2) AS Purchaseswotax
          FROM purchases) AS all_costs
 GROUP BY theMonth

将计算/虚拟列添加到现有查询

3 个答案: