Question

I have a simple requirement, but can’t get the SELECT statement right, so that it will deliver the desired result.

I have two tables: –

1. Person table with ID and other miscellaneous info
2. A Licences table with the ID to JOIN on and multiple records per person (10-15)

My aim is to get all people with a Blue licence and a White licence (for example).

Using:

SELECT distinctrow person.ID AS client_ID, postcode, locality 
FROM (person INNER JOIN licences ON person.ID = licences.client_ID)
WHERE licence LIKE '%Blue%' **OR** licence LIKE '%White%'

of course returns me people that have either, NOT both.

Using:

SELECT distinctrow person.ID AS client_ID, postcode, locality 
FROM (person INNER JOIN licences ON person.ID = licences.client_ID)
WHERE licence LIKE '%Blue%' **AND** licence LIKE '%White%'

of course returns no results (how could it?).

This is a really simplified example, as I am pulling 5 tables together in a nested JOIN, that all have criteria (coming in on 40-odd $_GET variables!) … but this is where I am stuck. I have used AND in the INNER JOIN, instead of the WHERE, without any change in the outcome. I’ve drunk lots of beer, still no change 🙂

I know I am revealing my lack of knowledge of even the basics, but surely I don’t need to put SELECTs within SELECTs? LEFT or RIGHT JOIN, perhaps?

I feel I am about to learn something elementary here, so bring it ON!

Answer 1

SELECT  *
FROM    person p
WHERE   (
        SELECT  COUNT(DISTINCT licence)
        FROM    licences l
        WHERE   l.client_id = p.id
                AND l.licence IN ('white', 'blue')
        ) = 2

or

SELECT  DISTINCT p.*
FROM    person p
JOIN    licences l
ON      l.client_id = p.id
        AND l.licence IN ('white', 'blue')
GROUP BY
        p.id
HAVING  COUNT(DISTINCT licence) = 2

Depending on data distribution (how many licenses per person do you have), one of the queries will be more efficient.