Question

我有2个表：Equipment和Equipment_Type：

+-------------------------------+
| EqId [PK] | ETId [FK] | EqNum |
+-------------------------------+
| 1         | 1         | ABC   |
| 2         | 1         | DEF   |
| 3         | 3         | GHI   |

+-------------------------------+
| ETId [PK] | Code | Discipline |
+-------------------------------+
| 1         | MOT  | ELEC       |
| 2         | MOT  | MECH       |
| 3         | SW   | ELEC       |

因此，从这个例子中，我们可以看到我们的两台设备都是电动机。

然而，由于对初始人群的误解，所有设备类型都被确定为ELEC学科。从那时起，MECH设备已被识别，我必须找到Equipment_Type表中已复制的所有设备，并将其更改为引用MECH设备类型而不是

我试过了：

SELECT * FROM Equipment EQ 
INNER JOIN Equipment_Type ET on ET.ETId = EQ.ETId
WHERE ET.Discipline = 'MECH';

与所有其他JOIN查询一样，显然不会返回任何结果。

我想要实现的是一种搜索，它将选择仅具有ELEC设备类型 a MECH的设备设备类型。我意识到这需要一个嵌套查询，但我不确定在哪里放置它。

所以搜索应该返回：

+---------------------------+
| EqNum | ETId | Discipline |
+---------------------------+
| DEF   | 1    | ELEC       |

因为该条目需要更改为MECH学科（即ETId = 2而不是1）

Answer 1

以下是一种聚合类型的方法，以获取具有这两个原则的代码：

select e.*
from equipment e join
     equipment_type et
     on e.etid = et.etid join
     (select et.code
      from equipment_type et
      group by et.code
      having sum(discipline = 'MECH') > 0 and sum(discipline = 'ELEC') > 0
     ) ett
     on ett.code = et.code;

另一种方法是使用两个连接：

select e.*
from equipment e join
     equipment_type ete
     on e.etid = ete.etid and ete.discipline = 'ELEC' join
     equipement_type etm
     on ete.code = etm.code and etm.discipline = 'MECH';

使用正确的索引，此版本可能会更快。

Answer 2

这样做：

select eq_id, eq_name,et_id,description from
(select eq_id,eq_name from equipment) as a

left join

(select et_id,description from equipment_type) as b
on a.eq_id = b.et_id where description = 'ELEC';

如果你想包括'MECH';

select eq_id, eq_name,et_id,description from
(select eq_id,eq_name from equipment) as a

left join

(select et_id,description from equipment_type) as b
on a.eq_id = b.et_id where description = 'ELEC' or description = 'MECH';

将'ELEC'改为'MECH'：

select eq_id, eq_name,et_id,

case when description = 'ELEC' then 'MECH' else description end  as description,

'MECH' as MECH_FIELD from
(select eq_id,eq_name from equipment) as a

left join

(select et_id,description from equipment_type) as b
on a.eq_id = b.et_id where description = 'ELEC' or description = 'MECH';

基于外键的SELECT，更改引用

2 个答案: