Question

I have a table which stores in each row a meeting with start date/time and end date/time.

meetingID int
meetingStart datetime
meetingEnd datetime

Desired output: For each pair of overlapping rows I would like to output
meetingID, meetingStart, meetingID, meetingEnd

What’s the most efficient way to perform such a query in MySQL?

Answer 1

SELECT  m1.meetingID, m1.meetingStart, m1.meetingEnd, m2.meetingID FROM    t_meeting m1, t_meeting m2 WHERE   (m2.meetingStart BETWEEN m1.meetingStart AND m1.meetingEnd         OR m2.meetingEnd BETWEEN m1.meetingStart AND m1.meetingEnd)         AND m1.meetingID <> m2.meetingID 

This will select each pair twice.

If you want each pair to be selected just once, use:

SELECT  m1.meetingID, m1.meetingStart, m1.meetingEnd, m2.meetingID FROM    t_meeting m1, t_meeting m2 WHERE   (m2.meetingStart BETWEEN m1.meetingStart AND m1.meetingEnd         OR m2.meetingEnd BETWEEN m1.meetingStart AND m1.meetingEnd)         AND m2.meetingID > m1.meetingID 

Make sure you have indexes on meetingStart and meetingEnd for the query to work efficiently.

MySQL, however, will probably use INDEX MERGE to run this query, which is not very efficient in current implementation.

You also may try to use:

SELECT  m1.*, m2.* FROM    (         SELECT  m1.meetingID AS mid1, m2.meetingID AS mid2         FROM    t_meeting m1, t_meeting m2         WHERE   m2.meetingStart BETWEEN m1.meetingStart AND m1.meetingEnd                 AND m2.meetingID <> m1.meetingID         UNION         SELECT  m1.meetingID, m2.meetingID         FROM    t_meeting m1, t_meeting m2         WHERE   m2.meetingEnd BETWEEN m1.meetingStart AND m1.meetingEnd                 AND m2.meetingID <> m1.meetingID         ) mo, t_meeting m1, t_meeting m2 WHERE   m1.meetingID = mid1         AND m2.meetingID = mid2 

, which is more complex but will most probably run a little bit faster.