Question

我有这个冒犯：

SELECT A, B, C
FROM (  SELECT 
        lla.id as A,
        max(decode(lla.attrid, 2, lla.valstr, null)) as B, 
        max(decode(lla.attrid, 3, lla.valstr, null)) as C
    FROM 
        llattrdata lla, 
        llattrdata lla2
    WHERE 
        lla.id = lla2.id 
    GROUP BY lla.id)
WHERE C = "Yes"

有更好的方法吗？我试图使用HAVING子句，但无法使其对C列=“是”起作用。提前谢谢。

Answer 1

我会告诉你一个秘密。你拥有什么都没有错。所以不要重写它。

SELECT A, B, C
FROM (  SELECT 
        lla.id as A,
        max(decode(lla.attrid, 2, lla.valstr, null)) as B, 
        max(decode(lla.attrid, 3, lla.valstr, null)) as C
    FROM 
        llattrdata lla, 
        llattrdata lla2
    WHERE 
        lla.id = lla2.id 
    GROUP BY lla.id)
WHERE C = 'Yes'

的表现与：

相同

SELECT 
    lla.id as A,
    max(decode(lla.attrid, 2, lla.valstr, null)) as B, 
    max(decode(lla.attrid, 3, lla.valstr, null)) as C
FROM 
    llattrdata lla, 
    llattrdata lla2
WHERE 
    lla.id = lla2.id 
GROUP BY lla.id
HAVING max(decode(lla.attrid, 3, lla.valstr, null)) = 'Yes';

通过避免此处的内嵌视图，您将无法获得任何性能。无论哪个更具可读性，请继续使用。

Answer 2

怎么样

SELECT 
        lla.id as A,
        max(decode(lla.attrid, 2, lla.valstr, null)) as B, 
        'Yes' as C
    FROM 
        llattrdata lla, 
        llattrdata lla2
    WHERE 
        lla.id = lla2.id 
    GROUP BY lla.id
    HAVING max(decode(lla.attrid, 3, lla.valstr, null)) = 'Yes'

您不能使用在同一查询中创建的别名。你必须再写一次

Answer 3

SELECT 
        lla.id as A,
        max(decode(lla.attrid, 2, lla.valstr, null)) as B, 
        max(decode(lla.attrid, 3, lla.valstr, null)) as C
    FROM 
        llattrdata lla, 
        llattrdata lla2
    WHERE 
        lla.id = lla2.id 
    GROUP BY lla.id
    HAVING max(decode(lla.attrid, 3, lla.valstr, null)) = "Yes"

请阅读：Why can't I use alias in a count(*) "column" and reference it in a having clause?

Answer 4

使用HAVING clause：

SELECT 
    lla.id as A,
    max(decode(lla.attrid, 2, lla.valstr, null)) as B, 
    max(decode(lla.attrid, 3, lla.valstr, null)) as C
FROM 
    llattrdata lla, 
    llattrdata lla2
WHERE 
    lla.id = lla2.id 
GROUP BY lla.id
HAVING max(decode(lla.attrid, 3, lla.valstr, null)) = 'Yes'

此子句在GROUP BY之后进行评估，用于过滤分组结果。

不幸的是，你不能在having子句中使用别名。在某些情况下，子查询的意图可能比有条款更清晰。

编写此SQL SELECT语句是否有比使用子查询更好的方法？

4 个答案: