Question

I have a composite primary key table (Maintenance Items). I would like to create another table which maps a parent child relationship between items within this table.

I am not sure how to create the Parent Child Relationship table see below either option 1 or option 2.

NOTE: Our business rules stipulate that a parent child relationship can only exits between maintenance items which share the same maintenance program. This will never change!

Table Maintenance Programs
ProgramCode (Primary Key)
ProgramDescription
Other Columns …

Table Maintenance Items (Composite Primary key Table)
ProgramCode (Composite Primary Key)
MaintenanceCode (Composite Primary Key)
MaintenanceDescription
Other Columns …

Table Parent/Child Maintenance Items (Option 1)
ProgramCode
ParentMaintenanceCode
ChildMaintenanceCode

Table Parent/Child Maintenance Items (Option 2)
ParentProgramCode (Same value as ChildProgramCode)
ParentMaintenanceCode
ChildProgramCode (Same value as ParentProgramCode)
ChildMaintenanceCode

There will be no other columns in the Parent/Child tables it is a relationship mapping table only.

Which is the better option? Option 2 seems to suit best practises but given our buisness rule means we essentially have two columns with exactly the same data (ProgramCode).

Answer 1

The only reason for using an associative entity like you’re suggesting is if you want a many-to-many relationship between maintenance items. If that is what you’re after, then option 1 is what you do. The schema would look like something like

create table MaintenanceProgram
(
  ProgramCode int not null ,

  ... -- other non-key attributes here

  primary key ( ProgramCode ) ,

)
create table MaintenanceItem
(
  ProgramCode     int not null ,
  MaintenanceCode int not null ,

  ... -- other non-key attributes here

  primary key ( ProgramCode , MaintenanceCode ) ,

  foreign key                     ( ProgramCode )
    references MaintenanceProgram ( ProgramCode ) ,

)
create table MaintenanceItemMap
(
  ProgramCode           int not null , 
  ParentMaintenanceCode int not null ,
  ChildMaintenanceCode  int not null ,

  primary key ( ProgramCode , ParentMaintenanceCode , ChildMaintenanceCode ) ,

  foreign key                  ( ProgramCode , ParentMaintenanceCode )
    references MaintenanceItem ( ProgramCode , MaintenanceCode       ) ,
  foreign key                  ( ProgramCode , ChildMaintenanceCode  )
    references MaintenanceItem ( ProgramCode , MaintenanceCode       ) ,

  check ( ParentMaintenanceCode != ChildMaintenanceCode ) ,

)

This ensure that all related maintenance items share the same ProgramCode and that a maintenance item cannot map to itself (the check constraint).

However, your problem statement refers to a parent/child relationship, which sounds more like a hierarchy/tree. In this case, the schema you would want would look something like this:

create table MaintenanceProgram
(
  ProgramCode int not null ,

  ... -- other non-key attributes here

  primary key ( ProgramCode ) ,

)
create table MaintenanceItem
(
  ProgramCode           int not null ,
  MaintenanceCode       int not null ,
  ParentMaintenanceCode int     null ,

  ... -- other non-key attributes here

  primary key ( ProgramCode , MaintenanceCode ) ,

  foreign key                     ( ProgramCode  )
    references MaintenanceProgram (  ProgramCode ) ,
  foreign key                     ( ProgramCode , ParentMaintenanceCode )
    references MaintenanceItem    ( ProgramCode , MaintenanceCode       ) ,

  check ( MaintenanceCode != ParentMaintenanceCode or ParentMaintenanceCode is null ) ,

)

The above says that each maintenance item is related to a single maintenance program; conversely, that each maintenance program has zero or more maintenance items.

Further, that each maintenance item has to zero or 1 parent maintenance items, which must be related to the same maintenance program.

The check constraint says that a given maintenance item may not be its own parent.