Database Design of three categories
such as the type comprising 0> Type 1 Type 1 comprises> Type 2
Type 0 -> Type 1 -> 2 Type
Type of database fields 0 id = 1 parentId = 0
database field 1 of the type id = 2 parentId = 1
database field type 3 id = 3 parentId = 2
e.g.
Because type 0 ID = 1 of the next stage so that queries can be classified according to the type 1 the parentId a = 1 ( 1 here is the type 0 of the ID ) condition
Query the Type 1 result set then check the type 3 can be according parentId type 3 (= 2 where 2 is the type of a query result set classification of the third layer of the id).
To sum it up: The value of the id field of the current category, as parentId field under a category of values
The following case study
the first sort
sql statement is the SELECT * from tb_category WHERE the parent_id = ' 0 ' ;
The second category
SELECT * from tb_category where parent_id='1';
The third category
SELECT * from tb_category where parent_id='2';
The advantage of this is that newly added types can be added id the upper classification as the value of parent_id.