如何选择不同条件的同一列？

Question

我正在尝试从两个表格中选择数据 -warehouse有两列：warehouseId，warehouseName
-transportation有三列：transporter，warehouseId1，warehouseId2
我想从两个表中选择并从表A获取warehouseId1和warehouseId2的warehouseName 这是我的代码，但它不起作用。

select a.transporter, b.warehouseName as warehouse1, b.warehouseName as warehouse2
from transportation a, warehouse b
where a.warehouseId1 = b.warehouseId and a.warehouseId2 = b.warehouseId

Answer 1

您必须将warehouse两次添加到FROM中（只记得为它们使用两个不同的别名）：

SELECT
    a.transporter,
    b1.warehouseName as warehouse1,
    b2.warehouseName as warehouse2
FROM
    transportation a,
    warehouse b1,
    warehouse b2
WHERE
        a.warehouseId1 = b1.warehouseId
    AND
        a.warehouseId2 = b2.warehouseId

或使用JOIN语法：

SELECT
    a.transporter,
    b1.warehouseName AS warehouse1,
    b2.warehouseName AS warehouse2
FROM
    transportation a
JOIN
    warehouse b1 ON a.warehouseId1 = b1.warehouseId
JOIN
    warehouse b2 ON a.warehouseId2 = b2.warehouseId

Answer 2

使用subselect可能更清楚：

SELECT 
    a.transporter,
    (SELECT warehouseName FROM warehouse WHERE warehouseId=a.warehouseId1) AS warehouse1,
    (SELECT warehouseName FROM warehouse WHERE warehouseId=a.warehouseId2) AS warehouse2
FROM
    transportation a

这通常会使用与MarcinJuraszek解决方案完全相同的查询计划，但对于发生的事情可能会更清楚一些。