SQL如何根据另一个表构造一个表中两列的查询？

Question

Please take a look at my table structure as below:

The data in Area_PrimaryTable and Area_SecondaryTable will be mapped to Table_TableID . I'm wondering how to get the status ID for both table 100 and 111 when I'm querying AreaID = A101?

Select * from Area where AreaID = A101

UPDATES: I've found my way to build such query but not sure if there is any better way?

Please advise as below:

Select * from Table where TableID in
(
    Select PrimaryTable from Area where AreaID = 'A101'
    union
    Select SecondaryTable from Area where AreaID = 'A101'      
)

Answer 1

That is the kind of example where JOIN s are especially useful. Look that up, since JOIN s are something that ends up being used very, very often when working with SQL. They basically allow you to get information from several related tables as part of the same record.

So there you want to get information from two rows of Table, their relationship being that they are linked to the same row in Area. And I will consider you just want to get both status. That can be solved with the following code :

SELECT t1.Status, t2.Status
FROM   Area AS a
  JOIN Table AS t1 ON t1.TableID = a.PrimaryTable
  JOIN Table AS t2 ON t2.TableId = a.SecondaryTable
WHERE AreaID = 'A101'

Note that while using SELECT * is OK in my book when one experiments, I believe it should be avoided in production code in favour of explicitely naming the columns you want to get information from.

Edit: despite the apparent differences, note that my code proposal has the same behaviour as Xtoxico's. Both proposals are equivalent as far as I know.

Answer 2

Try with this....

SELECT
    t1.status as StatusPrimary, t2.status as StatusSecondary 
FROM
    Area as a, 
    table as t1, 
    table as t2 
WHERE 
    a.primaryTable=t1.TableID and 
    a.secondaryTable=t2.TableId and 
    a.AreaID = A101;

Your Sample return two rows, and my code return in the same row the information

Answer 3

I suggest this one:

select t.* from table t, area a
where a.AreaID = 'A101' 
and t.TableID in (a.PrimaryTable, a.SecondaryTable)

Answer 4

Try this one

SELECT a.AreaID, t.TableID, t.TableName, t.Status FROM table1 t
INNER JOIN (
    Select PrimaryTable, areaId from Area
    union
    Select SecondaryTable, areaID from Area
) a ON t.TableID = a.PrimaryTable order by a.AreaID

Output will look like this (base on your table's structure)

I hope this is the answer what you looking for, if you only need find Area A101 , you can simply add WHERE AreaID = 'A101' after a ON t.TableID = a.PrimaryTable .

EDIT : Sorry missed the point, maybe the query should like this :

SELECT a.AreaID,
   MAX(IF(a.PrimaryTable = t.TableID, t.Status, null)) as T1,
   MAX(IF(a.SecondaryTable = t.TableID, t.status,null)) as T2
FROM Table1 t INNER JOIN Area a ON a.PrimaryTable = t.TableID OR a.SecondaryTable = t.TableID
GROUP BY AreaID