php - 如何使用group by获取MySql中的特定部分？

Question

我在 SQL 查询方面遇到问题，我会尝试解释我想要做什么。这是我的查询，它返回一些结果行。

SELECT 
sck.sckid,
sck.prid,
sck.paid,
sck.sckcen,
sum(scd.scdkiek) as count_of_goods,
min(scn.scndat) as `date`
FROM sck 
INNER JOIN scd ON scd.sckid = sck.sckid
INNER JOIN scn ON scn.scnid = scd.scnid
INNER JOIN sandeliai ON sandeliai.paid = sck.paid
WHERE sck.prid = 1376 GROUP BY sckid

此查询返回结果“表 1”:

表 1:

sckid    |    prid    |    sckcen    |    count_of_goods    |      date     |
123      |    1376    |    10009     |          0           |   2012-12-31  |
124      |    1376    |    10007     |          15          |   2013-01-25  |
125      |    1376    |    10005     |          0           |   2013-02-13  |
126      |    1376    |    10000     |          18          |   2013-03-15  |

但我只需要一行，其中所有数据均按 prid 分组，我编写了以下查询:

SELECT 
sck.sckid,
sck.prid,
sck.paid,
sck.sckcen,
sum(scd.scdkiek) as count_of_goods,
min(scn.scndat) as `date`
FROM sck 
INNER JOIN scd ON scd.sckid = sck.sckid
INNER JOIN scn ON scn.scnid = scd.scnid
INNER JOIN sandeliai ON sandeliai.paid = sck.paid
WHERE sck.prid = 1376 GROUP BY prid

然后我在表中得到一行包含此数据的数据:

表 2:

sckid    |    prid    |    sckcen    |    count_of_goods    |      date     |
123      |    1376    |    10009     |          23          |   2012-12-31  |

这似乎都是正确的，但在 date 字段中，我需要从表一返回最早的日期，其中 count_of_goods > 0，所以对我来说需要这个结果:

表3:

sckid    |    prid    |    sckcen    |    count_of_goods    |      date     |
123      |    1376    |    10009     |          23          |   2013-01-25  |

那么我有什么想法可以得到如表 3 所示的结果吗？

Answer 1

我认为我最初得到的答案可能过于复杂。假设 scd.scdkiek 是数字而不是 id，那么您可以通过简单地更改 INNER JOIN 标准来获得您想要的内容，即

SELECT 
    sck.sckid,
    sck.prid,
    sck.paid,
    sck.sckcen,
    scn.scndat,
    SUM(scd.scdkiek) AS count_of_goods
FROM sck 
INNER JOIN scd 
    ON scd.sckid = sck.sckid
    AND scd.scdkiek > 0
INNER JOIN sandeliai 
    ON sandeliai.paid = sck.paid
INNER JOIN scn 
    ON scn.scnid = scd.scnid
WHERE sck.prid = 1376 
GROUP BY 
    sck.prid

如果做不到这一点，我会考虑使用子查询。子查询将计算每行的 count_of_goods。然后，外部查询将使用它来决定是否将该行的日期设置为 NULL。 MIN(...somedate...,NULL) 将返回日期。这反过来意味着您应该获得该行上的 count_of_goods 不为 NULL 的最小日期。

当您查找 count_of_goods > 0 的日期时，子查询按日期分组。

当然这只是一种选择，可能还有其他更有效的方法，但我相信这会达到您的目的

SELECT
    sck.sckid,
    sck.prid,
    sck.paid,
    sck.sckcen,
    SUM(d.count_of_goods) AS count_of_goods,
    MIN(IF(d.count_of_goods>0,d.scndat,NULL)) AS `date`
FROM (
    -- Get the data and group it by date
    SELECT 
        sck.sckid,
        sck.prid,
        sck.paid,
        sck.sckcen,
        scn.scndat,
        SUM(scd.scdkiek) AS count_of_goods
    FROM sck 
    INNER JOIN scd ON scd.sckid = sck.sckid
    INNER JOIN sandeliai ON sandeliai.paid = sck.paid
    INNER JOIN scn ON scn.scnid = scd.scnid
    WHERE sck.prid = 1376 
    GROUP BY 
        sck.prid, -- may not really be needed
        scn.scndat
) AS d
GROUP BY d.prid