JBone JBone - 4 months ago 8
MySQL Question

Nested Select statement in MYSQL join

SELECT * FROM A
JOIN B
ON B.ID = A.ID
AND B.Time = (SELECT max(Time)
FROM B B2
WHERE B2.ID = B.ID)


I am trying to join these two tables in MYSQL. Don't pay attention to that if the ID is unique then I wouldn't be trying to do this. I condensed the real solution to paint a simplified picture. I am trying to grab and join the table B on the max date for a certain record. This procedure is getting run by an SSIS package and is saying B2.ID is an unknown column. I do things like this frequently in MSSQL and am new to MYSQL. Anyone have any pointers or ideas?

Answer

I do this type of query differently, with an exclusion join instead of a subquery. You want to find the rows of B which have the max Time for a given ID; in other words, where no other row has a greater Time and the same ID.

SELECT A.*, B.*
FROM A JOIN B ON B.ID = A.ID
LEFT OUTER JOIN B AS B2 ON B.ID = B2.ID AND B.Time < B2.Time
WHERE B2.ID IS NULL

You can also use a derived table, which should perform better than using a correlated subquery.

SELECT A.*, B.*
FROM A JOIN B ON B.ID = A.ID
JOIN (SELECT ID, MAX(Time) AS Time FROM B GROUP BY ID) AS B2
  ON (B.ID, B.Time) = (B2.ID, B2.Time)

P.S.: I've added the greatest-n-per-group tag. This type of SQL question comes up every week on Stack Overflow, so you can follow that tag to see dozens of similar questions and their answers.