Do You Have Check Constraints On Your Start and End Dates?

Adam Machanic tweeted this advice last week:

Please do this. Thanks.

ALTER TABLE [your_table] ADD CONSTRAINT [start date must be less than end date] CHECK (start_date < end_date);

— Adam Machanic (@AdamMachanic) November 17, 2016

Are you missing any of these check constraints? Run this query to check.
This query looks for any columns in the same table that begin with “Start” and “End”. It then looks for check constraints that reference both these columns. If it doesn’t find them, it suggests a check constraint.

WITH StartColumnNames AS(SELECTobject_id,
           column_id,
           name AS column_name
      FROM sys.columnsWHERE name like 'start%'),
EndColumnNames AS(SELECTobject_id,
           column_id,
           name AS column_name
      FROM sys.columnsWHERE name like 'end%')SELECT t.object_id,            
       OBJECT_SCHEMA_NAME(t.object_id)AS[schema_name], 
       t.[name]AS table_name,
       s.column_nameAS start_column,
       e.column_nameAS end_column,
       N'ALTER TABLE '+QUOTENAME(OBJECT_SCHEMA_NAME(t.object_id))+ N'.'+QUOTENAME(t.name)+ 
           N' ADD CONSTRAINT '+QUOTENAME(N'CK_'+ t.name+ N'_'+ s.column_name+ N'_'+ e.column_name)+ 
           N' CHECK ('+QUOTENAME(s.column_name)+ N' <= '+QUOTENAME(e.column_name)+ N');'as check_suggestion
  FROM StartColumnNames s
  JOIN EndColumnNames e
       ON s.object_id= e.object_idAND s.column_id<> e.column_idANDREPLACE(s.column_name, 'start', 'end')= e.column_nameJOIN sys.tables t
       ON t.object_id= s.object_idWHERENOTEXISTS(SELECT*FROM sys.check_constraints c
             JOIN sys.sql_expression_dependencies start_dependency
                  ON start_dependency.referencing_id= c.object_idAND start_dependency.referenced_id= t.object_idAND start_dependency.referenced_minor_id= s.column_idJOIN sys.sql_expression_dependencies end_dependency
               ON end_dependency.referencing_id= c.object_idAND end_dependency.referenced_id= t.object_idAND end_dependency.referenced_minor_id= e.column_idWHERE c.parent_object_id= t.object_id)

Caveats

Don’t blindly run scripts that you got from some random guy’s blog. Even if that someone is me. That’s terribly irresponsible.

But this query may be useful if you do want to look for a very specific, simple kind of constraint to add that may match your business specs. But these constraints are just suggestions and may not match your business rules. For example, when I run this query on Adventureworks, I get one “missing” check constraint for HumanResources.Shift(StartTime, EndTime) and when I look at the contents of the Shift table, I get this data:

Notice that I can’t create a constraint on this table because of the night shift. The constraint doesn’t make sense here.

Creating constraints on existing tables may take time if the table is huge. Locks may be held on that table for an uncomfortably long time.

Of course if your table has data that would violate the constraint, you can’t create it. But now you have to make some other choices. You can correct or delete the offending data or you can add the constraint with NOCHECK.