MySQL: Specific query to obtain info from 2 different tables based of a field being 0 or 1

Question

I suppose this is a very common question and possibly is duplicated, but I didn't know how to search it. This is my scenario: I have 3 tables: registers, foods and recipes. Tables structure simplified:

Registers

id | item_id | type (0 food|1 recipe)

(item_id can be whatever a food id or recipe id)

Foods

id | name | weight

Recipes

id | name | num_ingredients

What I want is to query the registers table and return the food or recipe name (and some specific fields from each table, like weight in foods and num_ingredients in recipes) based on the type field being 0 or 1.

If the tables structure is not the best, I would thank to help me find the best solution. Thanks a lot in advance.

Answer 1

You can do this like so:

select r.id, r.type, f.name
from
  Registers r
  inner join Foods f on f.id = r.item_id and r.type = 0
union all
select r.id, r.type, c.name
from
  Registers r
  inner join Recipes c on c.id = r.item_id and r.type = 1

But your table structure isn't ideal. First of all, since r.item_id can contain an id from two tables, it is impossible to add a constraint that enforces referential integrity. You'll need a trigger to check it, which is more complex and slower.

Instead, I'd choose to make the relation the other way around:

Add a register_id to Foods and Recipes. Then, you can write your query like this:

select r.id, r.type, f.name
from
  Registers r
  inner join Foods f on f.register_id = r.id
union all
select r.id, r.type, c.name
from
  Registers r
  inner join Recipes c on c.register_id = r.id

That's almost the same, but you don't need the type and it allows you to make proper foreign key constraints.

Answer 2

You need to combine the two detail tables with a UNION , and then JOIN them with the Registers table using the ID and type fields.

SELECT item_id, name, data
FROM Registers r
JOIN (SELECT 0 type, id, name, weight as data
      FROM Foods
      UNION
      SELECT 1 type, id, name, num_ingredients as data) x
ON r.item_id = x.id AND r.type = x.type

GolezTroi's answer where he does the UNION after JOIN is probably better, though, since it doesn't create a large temporary table before joining.