XPost: microsoft.public.sqlserver.programming   
   From: esquel@sommarskog.se   
      
   Gene Wirchenko (genew@ocis.net) writes:   
   >      How do I force a condition based on multiple rows?   
      
   Generally, you need to use a trigger.   
      
      
   > I want the necessary rows to be entered in a batch.  As a simple   
   > example, what would be the coding of the constraint in this case   
   > following?   
   >   
   >           create table Symmetric   
   >            (   
   >            a int not null,   
   >            b int not null   
   >            )   
   >   
   >      Constraint: If (x,y) is in Symmetric, then (y,x) is also in   
   > Symmetric.   
      
   Assuming that (a, b) is the primary key, this can be enforced with a   
   constraint:   
      
   CREATE TABLE Symmetric (   
      a int NOT NULL,   
      b int NOT NULL,   
      CONSTRAINT pk PRIMARY KEY (a, b),   
      CONSTRAINT fk FOREIGN KEY (b, a) REFERENCES Symmetric (a, b)   
   )   
      
      
      
   --   
   Erland Sommarskog, SQL Server MVP, esquel@sommarskog.se   
      
   Links for SQL Server Books Online:   
   SQL 2008: http://msdn.microsoft.com/en-us/sqlserver/cc514207.aspx   
   SQL 2005: http://msdn.microsoft.com/en-us/sqlserver/bb895970.aspx   
      
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