Using a SELECT statement within a WHERE clause

Question

SELECT * FROM ScoresTable WHERE Score = 
  (SELECT MAX(Score) FROM ScoresTable AS st WHERE st.Date = ScoresTable.Date)

Is there a name to describe

Answer 1

The principle of subqueries is not at all bad, but I don't think that you should use it in your example. If I understand correctly you want to get the maximum score for each date. In this case you should use a GROUP BY.

Answer 2

This is a correlated sub-query.

(It is a "nested" query - this is very non-technical term though)

The inner query takes values from the outer-query (WHERE st.Date = ScoresTable.Date) thus it is evaluated once for each row in the outer query.

There is also a non-correlated form in which the inner query is independent as as such is only executed once.

e.g.

 SELECT * FROM ScoresTable WHERE Score = 
   (SELECT MAX(Score) FROM Scores)

There is nothing wrong with using subqueries, except where they are not needed :)

Your statement may be rewritable as an aggregate function depending on what columns you require in your select statement.

SELECT Max(score), Date FROM ScoresTable 
Group By Date

Answer 3

Subquery is the name.

At times it's required, but good/bad depends on how it's applied.

Answer 4

It's called correlated subquery. It has it's uses.

Answer 5

It's not bad practice at all. They are usually referred as SUBQUERY, SUBSELECT or NESTED QUERY.

It's a relatively expensive operation, but it's quite common to encounter a lot of subqueries when dealing with databases since it's the only way to perform certain kind of operations on data.

Answer 6

In your case scenario, Why not use GROUP BY and HAVING clause instead of JOINING table to itself. You may also use other useful function. see this link