MySQL连接与子查询从连接表中仅选择一行

Question

Suppose there are two tables:

Table Parts:

Location PartNum Descrip  
-------- ------- --------
Whse1    abc     Frobbitz
Whse2    abc     Frobbitz
Whse3    def     Widget
Whse3    def     Widget

Table Status:

Location  PartNum Status
--------- ------- --------------
*Default* abc     Ready To Ship
*Default* def     Ready To Ship
Whse1     abc     Backordered

In most cases, the default status should be used.

To get the status, the query might look like:

SELECT p.Location,p.PartNum,p.Descrip,s.Status FROM Parts p
LEFT JOIN Status s ON s.PartNum=p.PartNum AND
s.Location = (SELECT MAX(Location) FROM Status s1
WHERE s1.PartNum=p.PartNum AND s1.Location
IN('*Default*',p.Location))

The question is, is this the most efficient (or at least reasonably efficient) way of doing this? My actual application will have up to 7 tables joined in a single query.

Answer 1

I think you can use Limit 1 with Order by as below:

 SELECT p.Location,p.PartNum,p.Descrip,s.Status FROM Parts p
  LEFT JOIN Status s ON s.PartNum=p.PartNum AND
  s.Location = (SELECT Location FROM Status s1
        WHERE s1.PartNum=p.PartNum ORDER BY s1.Location ASC LIMIT 1);

Assuming *Default* will come at the top in order by retrieval.

Answer 2

SELECT p.Location,p.PartNum,p.Descrip,s.Status FROM Parts p
LEFT outer JOIN Status s ON s.PartNum=p.PartNum 
where
s.Location = (SELECT MAX(Location) FROM Status s1
WHERE s1.PartNum=p.PartNum AND s1.Location
IN('*Default*',p.Location))

Answer 3

It appears like what you are trying to do is use a fallback default. You could try something like joining the table twice instead of using a subquery:

SELECT p.Location,p.PartNum,p.Descrip,s.Status 
FROM Parts p 
LEFT JOIN Status s1 ON s1.PartNum=p.PartNum AND p.Location = s1.Location 
LEFT JOIN Status s ON p.PartNum = s.PartNum 
   AND s.Location = IFNULL(s1.Location, '*Default*');

Essentially, s1 represents the non-default information. By checking whether s1.Location is NULL or not, we can tell if such a row exists and fallback to *Default* if it doesn't.

Hope that helps!