Question

I wish to create a constraint that state as below

Code.CodeTable ( CodeID smallint, CategoryID smallint,....)  --> Parent Table
Admin.Document( DocumentTypeID smallint,.....)  --> Child Table

The FK will be Admin.Document.DocumentTypeID map with Code.CodeTable.CodeID

I wish to have the constraint that only check Code.CodeTable.CodeID which the Code.CodeTable.CategoryID = 15 only.

Answer 1

As OMG Ponies already said – you cannot create fk constraints across databases, but if those are just odd table names with dots in them (highly discouraged! since SQL Server already uses a dotted schema: (database).(schema).(object name) and thus having dots in your table names is just asking for trouble at some point….), then you should be able to create your constraint like this:

ALTER TABLE [Admin.Document]
  ADD CONSTRAINT FK_AdminDocument_CodeTableCodeID
  FOREIGN KEY(DocumentTypeID) REFERENCES [Code.CodeTable](CodeID)

Since you have dots in your table names, you need to enclose those names in square brackets [].

Basically, you need to modify the child table and tell SQL Server which column in that child table refers to what parent table and column in the parent table.