I have 2 tables, 1 containing categories (parent) and another one containing sub-categories (children).
The sub-categories table has a reference to the parent in fldPID field:
CREATE TABLE [tbl_m_cate] (
[ID] [int] IDENTITY (1, 1) NOT NULL ,
[fldNAME] [varchar] (150) NULL ,
) ON [PRIMARY]...