如何在与子查询的连接中使用连接表中的列

Question

What I'm trying to do is this:

SELECT *
FROM MainTable m
INNER JOIN JoinedTable j on j.ForeignID = m.ID
INNER JOIN (SELECT TOP 1 *
            FROM SubQueryTable sq
            WHERE sq.ForeignID = j.ID
            ORDER BY VersionColumn DESC)

So basically, from SubQueryTable, I only want to retrieve a single row which has the maximum value for VersionColumn for all rows with a certain ID that I can get from JoinedTable.

T-SQL doesn't let me do this, what's a good way to solve this problem?

What I'm trying to prevent is loading the entire SubQueryTable and doing the filtering when it's too late as in....

SELECT *
FROM MainTable m
INNER JOIN JoinedTable j on j.ForeignID = m.ID
INNER JOIN (SELECT TOP 1 *
            FROM SubQueryTable sq
            ORDER BY VersionColumn DESC) sj ON sj.ForeignID = j.ID

I fear this second version performs the very slow subquery first and only filters it when it has loaded all the rows, but I want to filter sooner.

Any thoughts?

Answer 1

This will perform well if you have index on VersionColumn

SELECT *
FROM MainTable m
INNER JOIN JoinedTable j on j.ForeignID = m.ID
CROSS APPLY (SELECT TOP 1 *
            FROM SubQueryTable sq
            WHERE  sq.ForeignID = j.ID
            ORDER BY VersionColumn DESC) sj

Answer 2

Answer :

Hi,

Below query I have created as per your requirement using Country, State and City tables.

 SELECT * FROM ( SELECT m.countryName, j.StateName,c.CityName , ROW_NUMBER() OVER(PARTITION BY c.stateid ORDER BY c.cityid desc) AS 'x' FROM CountryMaster m INNER JOIN StateMaster j on j.CountryID = m.CountryID INNER JOIN dbo.CityMaster c ON j.StateID = c.StateID ) AS numbered WHERE x = 1 

Below is your solution and above is only for your reference.

 SELECT * FROM ( SELECT m.MainTablecolumnNm, j.JoinedTablecolumnNm,c.SubQueryTableColumnName , ROW_NUMBER() OVER(PARTITION BY sj.ForeignID ORDER BY c.sjID desc) AS 'abc' FROM MainTable m INNER JOIN JoinedTable j on j.ForeignID = m.ID INNER JOIN SubQueryTable sj ON sj.ForeignID = j.ID ) AS numbered WHERE abc = 1 

Thank you, Vishal Patel