SQL基于列值的选择查询

Question

I have a Table1 like this:

ApplicableTo IdApplicable
---------------------------
Dept              1
Grade             3
section           1
Designation       2

There other tables like:

tblDept :

ID     Name
1       dept1
2       baking
3       other

tblGrade :

ID      Name
1       Grd1
2       Manager
3       gr3

tblSection :

id      Name
1       Sec1
2       sec2
3       sec3

tblDesignation :

id      Name
1       Executive
2       Developer
3       desig3

What I need is a query for table1 in such a way that gives me

ApplicableTo (table1)
Name (from the relevant table based on the value in `ApplicableTo` column)

Is this possible?

Desired Result:

eg: ApplicableTo IdApplicable Name
      Dept            1       dept1
      grade           3       gr3
      Section         1       sec1
      Designation     2       Developer.

This is the result I desire.

Answer 1

You could do something like the following so the applicable to becomes part of the JOIN predicate:

SELECT  t1.ApplicableTo, t1.IdApplicable, n.Name
FROM    Table1 AS t1
        INNER JOIN
        (   SELECT  ID, Name, 'Dept' AS ApplicableTo
            FROM    tblDept
            UNION ALL
            SELECT  ID, Name, 'Grade' AS ApplicableTo
            FROM    tblGrade
            UNION ALL
            SELECT  ID, Name, 'section' AS ApplicableTo
            FROM    tblSection
            UNION ALL
            SELECT  ID, Name, 'Designation' AS ApplicableTo
            FROM    tblDesignation
        ) AS n
            ON n.ID = t1.IdApplicable 
            AND n.ApplicableTo = t1.ApplicableTo

I would generally advise against this approach, although it may seem like a more consice approach, you would be better having 4 separate nullable columns in your table:

ApplicableTo | IdDept | IdGrade | IdSection | IdDesignation
-------------+--------+---------+-----------+---------------
Dept         |    1   |  NULL   |    NULL   |     NULL
Grade        |  NULL  |    3    |    NULL   |     NULL
section      |  NULL  |   NULL  |      1    |     NULL
Designation  |  NULL  |   NULL  |    NULL   |       2

This allows you to use foreign keys to manage your referential integrity properly.

Answer 2

The easiest way to achieve that is to add an extra column in table1 to keep the table where id is refferred to. Otherwise you can't know in which table the applicable id is reffered to.

Or you can create the applicable id in a way that you can extract the table afterwords from it for example a1 for id 1 in tblDept. And then use [case] ( http://dev.mysql.com/doc/refman/5.0/en/case.html ) (for mysql) in order to make the correct Join.

Answer 3

You can use CASE here,

SELECT ApplicableTo,
       IdApplicable,
       CASE 
         WHEN ApplicableTo = 'Dept' THEN (SELECT Name FROM tblDept WHERE tblDept.ID = IdApplicable) 
         WHEN ApplicableTo = 'Grade' THEN (SELECT Name FROM tblGrade WHERE tblGrade.ID = IdApplicable)
         WHEN ApplicableTo = 'Section' THEN (SELECT Name FROM tblSection WHERE tblSection.ID = IdApplicable)
         WHEN ApplicableTo = 'Designation' THEN (SELECT Name FROM tblDesignation WHERE tblDesignation.ID = IdApplicable)
       END AS 'Name'
FROM Table1