Question

This query displays the correct result but when doing an EXPLAIN, it lists it as a “Dependant SubQuery” which I’m led to believe is bad?

SELECT Competition.CompetitionID, Competition.CompetitionName,     Competition.CompetitionStartDate  
FROM Competition  
WHERE CompetitionID NOT   
IN (  
SELECT CompetitionID  
FROM PicksPoints  
WHERE UserID =1    
)

I tried changing the query to this:

SELECT Competition.CompetitionID, Competition.CompetitionName,   Competition.CompetitionStartDate  
FROM Competition  
LEFT JOIN PicksPoints ON Competition.CompetitionID = PicksPoints.CompetitionID  
WHERE UserID =1  
and PicksPoints.PicksPointsID is null  

but it displays 0 rows. What is wrong with the above compared to the first query that actually does work?

Answer 1

The seconds query cannot produce rows: it claims:

WHERE UserID =1  
and PicksPoints.PicksPointsID is null 

But to clarify, I rewrite as follows:

WHERE PicksPoints.UserID =1  
and PicksPoints.PicksPointsID is null 

So, on one hand, you are asking for rows on PicksPoints where UserId = 1, but then again you expect the row to not exist in the first place. Can you see the fail?

External joins are so tricky at that! Usually you filter using columns from the “outer” table, for example Competition. But you do not wish to do so; you wish to filter on the left-joined table. Try and rewrite as follows:

SELECT Competition.CompetitionID, Competition.CompetitionName,   Competition.CompetitionStartDate  
FROM Competition  
LEFT JOIN PicksPoints ON (Competition.CompetitionID = PicksPoints.CompetitionID AND UserID = 1)
WHERE 
PicksPoints.PicksPointsID is null  

For more on this, read this nice post.

But, as an additional note, performance-wise you’re in some trouble, using either subquery or the left join.

With subquery you’re in trouble because up to 5.6 (where some good work has been done), MySQL is very bad with optimizing inner queries, and your subquery is expected to execute multiple times.

With the LEFT JOIN you are in trouble since a LEFT JOIN dictates the order of join from left to right. Yet your filtering is on the right table, which means you will not be able to use an index for filtering the USerID = 1 condition (or you would, and lose the index for the join).