如何选择不同条件的同一列？

Question

我想从两个表中选择数据。
- 仓库有两列：warehouseId，warehouseName
- 交通有三列：转运，warehouseId1，warehouseId2
我想从两个表中选择并从warehouseId1和warehouseId2 表A中得到warehouseName这里是我的代码，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

您必须添加warehouse两次到FROM（只记得使用两个不同的别名为他们）：

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

这可能是更清晰使用子查询：

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的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。