RyanNehring RyanNehring - 1 year ago 35
SQL Question

SELECTING with multiple WHERE conditions on same column

Ok, I think I might be overlooking something obvious/simple here... but I need to write a query that returns only records that match multiple criteria on the same column...

My table is a very simple linking setup for applying flags to a user ...

ID contactid flag flag_type
118 99 Volunteer 1
119 99 Uploaded 2
120 100 Via Import 3
121 100 Volunteer 1
122 100 Uploaded 2

etc... in this case you'll see both contact 99 and 100 are flagged as both "Volunteer" and "Uploaded"...

What I need to be able to do is return those contactid's ONLY that match multiple criteria entered via a search form...the contactid's have to match ALL chosen flags... in my head the SQL should look something like:

SELECT contactid
WHERE flag = 'Volunteer'
AND flag = 'Uploaded'...

but... that returns nothing... What am I doing wrong here?

Answer Source

You can either use GROUP BY and HAVING COUNT(*) = _:

SELECT contact_id
FROM your_table
WHERE flag IN ('Volunteer', 'Uploaded', ...)
GROUP BY contact_id
HAVING COUNT(*) = 2 -- // must match number in the WHERE flag IN (...) list

(assuming contact_id, flag is unique).

Or use joins:

SELECT T1.contact_id
FROM your_table T1
JOIN your_table T2 ON T1.contact_id = T2.contact_id AND T2.flag = 'Uploaded'
-- // more joins if necessary
WHERE T1.flag = 'Volunteer'

If the list of flags is very long and there are lots of matches the first is probably faster. If the list of flags is short and there are few matches, you will probably find that the second is faster. If performance is a concern try testing both on your data to see which works best.