Question

I have two tables ticket and attr. Table ticket has ticked_id field and several other fields. Table attr has 3 fields:

ticket_id - numeric
attr_type - numeric
attr_val - string

attr_type is a fixed enum of values. For example, it can be 1, 2 or 3.

I need to make a query, the result of which will be 4 columns:

ticket_id, attr_val for attr_type=1, attr_val for attr_type=2, attr_val for attr_type=3

If there is no corresponding value for attr_type in attr table then NULL value should be shown in corresponding column.

ticket table :

ticket_id
1
2
3

attr table :

ticket_id	attr_type	attr_val
1	1	Foo
1	2	Bar
1	3	Egg
2	2	Spam

Expected output :

ticket_id	attr_val1	attr_val2	attr_val3
1	Foo	Bar	Egg
2	null	Spam	null
3	null	null	null

I tried left joining attr table 3 times, but cannot figure out how to arrange output by attr_type

Answer 1

You need to use multiple LEFT JOINs:

SELECT 
    ticket.ticket_id,  
    a1.attr_val AS attr_val1,
    a2.attr_val AS attr_val2,
    a3.attr_val AS attr_val3
FROM ticket
    LEFT JOIN attr a1 ON ticket.ticket_id=a1.ticket_id AND a1.attr_type=1
    LEFT JOIN attr a2 ON ticket.ticket_id=a2.ticket_id AND a2.attr_type=2
    LEFT JOIN attr a3 ON ticket.ticket_id=a3.ticket_id AND a3.attr_type=3

Here is an example: SQL Fiddle.