mysql - 在子查询中动态设置列名

Question

我有两张 table 。一个名为购买，其中

(id, month, purchase, storeid)

另一个名为purchases_targets，带有

(id, storeid, target1,target2, target3, target4, target5, target6, target7, target8, target9, target10, target11, target12)

purchasing_targets 中的列名称对应于月份，即第 1 个月的目标 = target1。 正常化这对我来说不是一个选择! 我想做的事情如下。

SELECT DISTINCT

    purchases.month,
    purchases.purchase, 
    CONCAT('target',purchases.month) AS column_name,
    (SELECT 
      column_name 
    FROM purchases_targets
    WHERE storeid = 1) AS 'target'

FROM purchases 
WHERE purchases.storeid = 1

注意上面column_name的使用。我已经尝试过了，结果总是返回目标列的目标+月份。我想知道是否有办法可以在 mysql 中完成这项工作。如果需要更多信息，请告诉我。谢谢

Answer 1

您可以使用 CASE 表达式根据月份选取不同的列:

SELECT DISTINCT
    p.month,
    p.purchase,
    CASE p.month
        WHEN 1 THEN pt.target1
        WHEN 2 THEN pt.target2
        ...
        WHEN 12 THEN pt.target12
    END AS target
FROM purchases AS p
JOIN purchases_targets AS pt ON p.storeid = pt.storeid
WHERE p.storeid = 1