Question

In MySQL, how can you select data where every row meets a certain condition? For example lets say I have a table showing when employees arrived at work, it has three fields:

CREATE TABLE ArrivalTimes (UserID INT ,Day DATE  ,ArrivalTime TIME ); 

I want to select all UserIDs of employees who have never been late (arrived 9am or earlier), what’s the best way to do this?

Answer 1

The answers from @jjclarkson and @davethegr8 are close, but you can’t put aggregate functions in the WHERE clause. The WHERE clause is evaluated for each row.

You need to evaluate the MAX() expression for each group, so you need to use a HAVING clause.

Try this:

SELECT UserID  FROM ArrivalTimes GROUP BY UserID HAVING MAX(ArrivalTime) <= '09:00:00'; 

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@MBCook comments that HAVING can be slow. You’re right, it might not be the absolute quickest way to produce the desired result. But the HAVING solution is the most clear. There are situations where performance has lower priority than clarity and maintainability.

I looked at the EXPLAIN output (on MySQL 5.1.30) for the HAVING solution: no indexes were used, and the extra notes said ‘Using temporary; Using filesort,’ which usually means performance will be poor.

Consider the following query:

SELECT DISTINCT a1.UserID FROM ArrivalTimes a1   LEFT OUTER JOIN ArrivalTimes a2    ON (a1.UserID = a2.UserID AND a2.ArrivalTime > '09:00:00') WHERE a2.UserID IS NULL; 

This generates an optimization plan that uses an index on UserID and says:

• a1: ‘Using index; Using temporary‘
• a2: ‘Using where; Distinct‘

Finally, the following query generates an optimization plan that appears to use indexes most effectively, and no temp tables or filesort.

SELECT DISTINCT a1.UserID FROM ArrivalTimes a1 WHERE NOT EXISTS (SELECT * FROM ArrivalTimes a2                    WHERE a1.UserID = a2.UserID                      AND a2.ArrivalTime > '09:00:00');  

• a1: ‘Using where; Using index‘
• a2: ‘Using where‘

This appears most likely to have the best performance. Admittedly, I only have four rows in my test table, so this isn’t a representative test.