mysql filter rows by field value in linked table

Question

Scenario:

I have a main table, say " items ". Each item can be of many types, and I use a table " types " and a linked table to keep the relations between the two, say " items_types " in a typical one to many situation. Now, each type has a supertype, with a straight one to one relation.

items: i_id, i_name

types: t_id, t_name, t_st_id

items_types: it_id, it_i_id, it_t_id

supertypes: st_id, st_name

Now I have to filter all the items that are related to at least one supertype in a given set, say supertypes with ids 1,2,3

I am thinking to use a group_concat on the ids of the related supertypes and then filtering using multiple FIND_IN_SET in or between each other, using a WHERE clause.

However this, if working, would slow down the query and I don't like comparing ids as if they were strings.

Any idea?

Answer 1

You can do like this.

select i.*
from items_types it
join items i on i.i_id = it.it_i_id 
join types t on t.t_id = it.it_t_id
join supertypes st on st.st_id = t.t_st_id and st_id in (1,2,3)

Answer 2

I found a better solution: instead using related records to keep track of all the types the item belongs to, I am giving each type a numeric code which can be used in binary comparison, eg:

Type 1: code 1
Type 2: code 2
Type 3: code 4
Type 4: code 8
Type 5: code 10

I think I gave the idea.

Each item will have a field to keep track of all its types, named 'coded_type'. For example if a field has coded_type 7 it means that it belongs to Type 1, Type 2, Type 3.

For instance, if a user then wants to look for all items that are of type 2 or 3 the query will look for all items whose coded_type field binary ANDed against 6 gives a value greater than zero.

That is:

select * from items where (coded_type & 6)>0

I think this way the query will much simpler and run faster. Sorry I thought of this just now.