内部联接以查找相同修订号中的项目数

Question

我有以下表格

tblMainequipment
asset_id   rev    equipment_name
123        0      box
123        1      box 
124        0      box 
125        0      bottle  

tblmainswablocation
asset_id   rev   swab_location
123        0     cover
123        0     base
123        1     cover
123        1     base
123        1     lock
124        0     cover
124        0     base
125        0     tube
125        0     cover

我确实想获取特定资产 ID 的最大转速的拭子位置总数。例如，asset_id 123 rev 0 的拭子位置总数为 2，但 rev 1 为 3。过去几个小时我一直在试图弄清楚，但似乎找不到解决方案。我对连接很不好。以下显示了我想要得到的，

query
asset_id   maxrev    #swablocation  equipment_name
123        1         3              box
124        0         2              cover
125        0         2              bottle

我使用下面的sql

SELECT MEQ.*
FROM tblMainEquipment AS MEQ 
INNER JOIN (Select asset_id, max(rev) as maxrev     
            From tblmainequipment  GROUP By asset_id)  AS groupmeq ON 
            (MEQ.asset_id = groupmeq.asset_id) AND (MEQ.rev = groupmeq.maxrev)

我不确定如何将#swablocation 添加到我的查询中。

Answer 1

这是一种使用相关子查询的方法：

select me.*,
       (select count(*)
        from tblmainswablocation as sl
        where sl.asset_id = me.asset_id
       ) as numSwabs
from tblMainEquipment as me
where me.rev = (select max(me2.rev) from tblMainEquipment as me2 where me2.asset_id = me.asset_id);

优点是外层查询不需要聚合。

Answer 2

你需要一个计数和一个分组

SELECT MEQ.asset_id maxrev, MEQ.maxrev, 
    count(blmainswablocations.swab_location), MEQ.equipment_name
FROM tblMainEquipment AS MEQ 
INNER JOIN (Select asset_id, max(rev) as maxrev     
        From tblmainequipment  GROUP By asset_id)  AS groupmeq ON 
        (MEQ.asset_id = groupmeq.asset_id) AND (MEQ.rev = groupmeq.maxrev)
Inner join blmainswablocations on MEQ.asset_id =  blmainswablocations.asset_id
Group by MEQ.asset_id maxrev