Question

I asked this question on SO. However, I wish to extend it further. I would like to find the max value of the ‘Reading’ column only where the ‘state’ is of value ‘XX’ for example.

So if I join the two tables, how do I get the row with max(Reading) value from the result set. Eg.

SELECT s.*, g1.*
FROM Schools AS s
JOIN Grades AS g1 ON g1.id_schools = s.id
WHERE s.state = 'SA' // how do I get row with max(Reading) column from this result set

The table details are:

Table1 = Schools
    Columns: id(PK), state(nvchar(100)), schoolname

Table2 = Grades
    Columns: id(PK), id_schools(FK), Year, Reading, Writing...

Answer 1

I’d think about using a common table expression:

WITH SchoolsInState (id, state, schoolname)
AS (
    SELECT id, state, schoolname
    FROM Schools
    WHERE state = 'XX'
)
SELECT *
FROM SchoolsInState AS s
JOIN Grades AS g
ON s.id = g.id_schools
WHERE g.Reading = max(g.Reading)

The nice thing about this is that it creates this SchoolsInState pseudo-table which wraps all the logic about filtering by state, leaving you free to write the rest of your query without having to think about it.