Question

I’m not the greatest SQL guy in the world and this problem has been driving me crazy for about a week now.

I have 3 tables. The first is an attributes table with the following data:

attr_group_table
attr_id   group    price  prefix   id
     4      2       20.00   +      5000
     1      2       30.00   +      5000

attr_table
attr_id   id    value_id   price   prefix
     1    5000        2     10.00    -
     4    5000        3     10.00    +
    13    5000        9     50.00    +

name_table
     id   name
     2    widget1
     3    widget2
     9    widget3

So basically, I need all the all the items from the attr_table that match the ID of 5000. HOWEVER, if those items exist in the attr_group_table with group=2 then use the price and prefix from the attr_group_table. Then just suck the names about of the name table. The only data I need returned is the attr_id, name, price, and prefix.

I’ve been messing around with joins, unions, if’s and just can’t get the data I want. Any help/suggestions would be great.

Answer 1

Use function COALESCE:

SELECT 
  attr_table.attr_id, attr_table.id, attr_table.value_id, 
  COALESCE(attr_group_table.prefix, attr_table.prefix) prefix, 
  COALESCE(attr_group_table.price, attr_table.price) price,
  name_table.name
FROM attr_table 
  LEFT JOIN attr_group_table ON attr_table.attr_id=attr_group_table.attr_id
  LEFT JOIN name_table ON attr_table.value_id=name_table.id

It will return the first non-null value of its arguments.