Question

I have a question regarding enforce a constraint to my table.
I have a table called workson and a table called staff, each staff has specific title(supervisor,authorizer,manager,…). I need to ensure that the supervisor and the authorizer cannot be the same staff on workson table.The cardinality between them is many to many. I am not sure how to do it.
Can you please advise me to solve this problem?

Thanks

Answer 1

Consider the following query

SELECT AssignmentNo, StaffNo
  FROM worksOnStaff
 WHERE StaffType = 'supervisor'
INTERSECT 
SELECT AssignmentNo, StaffNo
  FROM worksOnStaff
 WHERE StaffType = 'authorizer'

When the data in the table satisifes your constraint the above query will be the empty set i.e.

CHECK ( NOT EXISTS ( SELECT AssignmentNo, StaffNo
                       FROM worksOnStaff
                      WHERE StaffType = 'supervisor'
                     INTERSECT 
                     SELECT AssignmentNo, StaffNo
                       FROM worksOnStaff
                      WHERE StaffType = 'authorizer' ) );

The problem is, few SQL products allow a subquery in a CHECK constraint.

The usual workaround is to implment the same logic in procedural code e.g. using a trigger or forcing users to update data via a stored procedures (by removing update privileges on the base tables) that ensure the constraint is never violated. You have to take care to serialize updates properly without making the whole thing run like glue.

Happily, there is an excellent book on the subject:

Applied Mathematics for Database Professionals By Lex de Haan, Toon Koppelaars

For implementing constraints that cannot be declared to the DBMS, we prefer to follow the triggered procedural strategy… Like declared constraints, the triggered procedural strategy cannot be subverted; …is likely to create a more manageable code architecture …[however] implementing efficient data integrity code for these constraints through triggers is far from being a trivial task…
However, you’ll experience that by implementing table constraints regularly and becoming
proficient in doing so, implementing table constraints procedurally is in general quite
doable…

execution model EM6: On-Transition-Effect-Property Plus Optimized-Query

1. Translate the formal specification into a constraint validation query.
2. Develop code to maintain transition effects.
3. Devise transition effect (TE) queries that ensure the constraint validation query is only run when
   necessary.
4. Discover a means to optimize the constraint validation query by having the TE query
   provide values that can be used in the validation query.
5. Devise and add a serialization strategy to the data integrity (DI) code.

They give full details on how to implement such a strategy in Oracle which can be ported to other SQL products.