Question

Can anyone help me optimize this query? It takes 15 seconds. If I move the WHERE clauses up to the join itself, still the same result.

SELECT
`users`.`employeeID` as `username`,
`users`.`firstName` as `fname`,
`users`.`lastName` as `lname`
FROM `enrollment`
INNER JOIN `users` ON `enrollment`.`employeeID` = `users`.`employeeID`
WHERE `enrollment`.`number` = [int]
AND
`enrollment`.`term` = [int]
AND
(
`enrollment`.`status` = 'E'
OR
`enrollment`.`status` = 'M'
)
ORDER BY    
`users`.`lastName` ASC,
`users`.`firstName` ASC;

Takes as long as this:

SELECT
`users`.`employeeID` as `username`,
`users`.`firstName` as `fname`,
`users`.`lastName` as `lname`
FROM `enrollment`
INNER JOIN
`users` ON `enrollment`.`employeeID` = `users`.`employeeID`
AND
`enrollment`.`number` = [int]
AND
`enrollment`.`term` = [int]
AND
(
`enrollment`.`status` = 'E'
OR
`enrollment`.`status` = 'M'
)
ORDER BY    
`users`.`lastName` ASC,
`users`.`firstName` ASC;

This is the EXPLAIN results

id  select_type table   type    possible_keys   key key_len ref rows    Extra
1   SIMPLE  users   ALL PRIMARY NULL    NULL    NULL    52925   Using filesort
1   SIMPLE  enrollment  ref employeeID,number_term  employeeID  9   ezlrn.users.employeeID  2   Using where

Answer 1

To answer exhaustively we would need to check out the query plan, which you obtain issuing an EXPLAIN SELECT...(your query), and the index structure already in place.

In the hypothesis that you have no indexes yet (except maybe the PRIMARY KEY ones), and that is the problem, let’s see how to whip up something (possibly further improveable):

SELECT
    `users`.`employeeID` as `username`,
    `users`.`firstName` as `fname`,
    `users`.`lastName` as `lname`
FROM `enrollment`
    INNER JOIN `users` ON `enrollment`.`employeeID` = `users`.`employeeID`
WHERE `enrollment`.`number` = [int]
    AND
    `enrollment`.`term` = [int]
    AND
    (
        `enrollment`.`status` = 'E'
    OR
    `enrollment`.`status` = 'M'
    )
    ORDER BY    
        `users`.`lastName` ASC,
        `users`.`firstName` ASC;

You have a JOIN clause beetween enrollment.employeeID and users.employeeID.

So the first couple of indexes you need is right there:

-- CREATE INDEX enrollment_ndx ON enrollment(employeeID); -- wait on this one
CREATE INDEX users_ndx ON users(employeeID);

Then the WHERE clause contains some more conditions that can be usefully indexed. So the final index for enrollment would be:

CREATE INDEX enrollment_ndx ON enrollment(employeeID, number, term, status);

Try creating the indexes on enrollment and users, and try again (posting the query plan).

Note: you can also use the syntax enrollment.status IN ('M', 'E') in your query to simplify it.