What kind of MySQL join do I need?

Question

Simple MySQL tables with matching ID values across tables:

table 1;
pid, firstname

table 2;
id, pid, property, value

Lets say there is one person entry in table 1:

pid: 1
firstname: fred

For each person there are multiple table 2 entries:

pid: 1
property: likes cats?
value: no

pid: 1
property: eye colour
value: orange

pid: 1
property: favourite food
value: sox

I want to select just two of the many table two entries, say the eye colour and favour food entries for a given person ID. What kind of outer join can achieve this?

SELECT `t1`.name
FROM `table1` AS t1
LEFT JOIN `table2` t2 ON `t1`.pid = `t2`.pid
WHERE `t1`.pid = 1
AND `t2`.property =  'eye colour'

I'm stuck here, how to get two rows from table2 and include favour food also? (Before anyone says it, no I can't change the structure of this database).

Thanks for reading.

Answer 1

You need to left join twice:

SELECT 
  t1.name
  ,t2a.value as eye_color
  ,t2b.value as fav_food
FROM table1 t1
LEFT JOIN table2 t2a ON (t1.pid = t2a.pid AND t2a.property = 'eye colour')
LEFT JOIN table2 t2b ON (t1.pid = t2b.pid AND t2b.property = 'fav food')
WHERE t1.pid = 1

Answer 2

How about something like:

 SELECT t1.name, t2.property, t2.value
 FROM table2 t2
 INNER JOIN table1 t1 ON t1.pid = t2.pid
 WHERE t2.pid = 1
 AND t2.property IN ('eye colour','favourite food')

Or you just wanted the first two however MySQL indexed it:

 SELECT t1.name, t2.property, t2.value
 FROM table2 t2
 INNER JOIN table1 t1 ON t1.pid = t2.pid
 WHERE t2.pid = 1
 LIMIT 2

There's no real reason to use a LEFT JOIN here, since there should always be an associated pid defined in table1 .

Answer 3

How about this?

SELECT t.name, t.value, t3.value
FROM
(SELECT t1.name, t2.value, t1.pid
FROM table1 t1
LEFT JOIN table2 t2 ON t1.pid = t2.pid
WHERE t1.pid = 1
AND t2.property =  'eye colour') t
LEFT JOIN table2 t3 ON t.pid = t3.pid
WHERE t.pid = 1
AND t3.property =  'favourite food'

This will also work for people who don't have an 'eye colour' or 'favourite food' entry

Answer 4

If you want to do this with one query then you should use INNER JOIN but i'm not recommending this. If i were you i would select first the person then i would create another query to recieve the necessary table2 entries. Try both of my solutions you could choose the right one for you.