从 sql 查询得到空结果
get empty result from sql query
我有这个查询:
select p1.cost as V1, p2.cost as V2 , p3.cost as V3
from product p1, product p2, product p3
where p1.id =1 and p2.id =2 and p3.id=3
在我的产品中 table 我只有一行:
id | name | cost
1 | product1 | 20
如您所见,我将得到空结果!
但是我该怎么做才能得到:
V1 | V2 | V3
20 | |
您可以使用左连接:
SELECT p1.cost AS V1, p2.cost AS V2 , p3.cost AS V3
FROM product p1
LEFT JOIN product p2 on p2.id = 2
LEFT JOIN product p3 on p3.id = 3
WHERE p1.id = 1
我有这个查询:
select p1.cost as V1, p2.cost as V2 , p3.cost as V3
from product p1, product p2, product p3
where p1.id =1 and p2.id =2 and p3.id=3
在我的产品中 table 我只有一行:
id | name | cost
1 | product1 | 20
如您所见,我将得到空结果! 但是我该怎么做才能得到:
V1 | V2 | V3
20 | |
您可以使用左连接:
SELECT p1.cost AS V1, p2.cost AS V2 , p3.cost AS V3
FROM product p1
LEFT JOIN product p2 on p2.id = 2
LEFT JOIN product p3 on p3.id = 3
WHERE p1.id = 1