MySQL, How do I only select rows based on other rows data? - mysql

Question

Ok, here's the deal, I have a table for stats where there are different types and I only want to select a row if it has a previous (not directly previous) row that has some matching data (hash) and is unique based on another column value.
In the following example we need to only get SALE rows
This example should help:
id link_id member_id stat_type hash
----------------------------------------------
108 41 82 SALE fffff
107 41 82 CLICK fffff
106 41 82 CLICK eeeee
105 41 67 SALE ddddd
104 41 67 CLICK ddddd
103 41 35 SALE ccccc
102 41 35 CLICK bbbbb
101 41 35 CLICK aaaaa
The only row I want to get back here is member_id = 67 because that member's only previous CLICK to link_id 41 has the same hash. Members 82 and 35 do not get selected because they both have a previous click to link_id 41 with a mismatched hash.
Query result should be:
id link_id member_id stat_type hash
----------------------------------------------
105 41 67 SALE ddddd
TIA

Answer 1

If I've understood your problem correctly, then you first need to group the table by link_id and member_id, filter those groups for ones that contain only one distinct hash, and then join the results back to your table to obtain all the matching SALE records:
SELECT * FROM my_table NATURAL JOIN (
SELECT link_id, member_id
FROM my_table
GROUP BY link_id, member_id
HAVING COUNT(DISTINCT hash) = 1
) t WHERE stat_type = 'SALE'
See it on sqlfiddle.

Answer 2

One way to get this result is to use an anti-join pattern.
SELECT r.*
FROM mytable r
LEFT
JOIN ( SELECT t.member_id
, t.link_id
, t.hash
FROM mytable t
GROUP
BY t.member_id
, t.link_id
, t.hash
) s
ON s.member_id = r.member_id
AND s.link_id = r.link_id
AND NOT (s.hash = r.hash)
WHERE r.stat_type = 'SALE'
AND s.member_id IS NULL
The inline view (derived table) aliased as s gets all of the distinct hash values for each member_id and link_id. That is left joined to all of the rows in the table that have stat_type='SALE', with matching member_id and link_id, and with a NON-matching hash.
The "trick" is the s.member_id IS NULL predicate. Any rows that found a match (i.e. a non-matching HASH will have a non-null value for s.member_id. Only rows that did not have a match will return a NULL (due to the LEFT JOIN).