Many a times need arises to find all constraints on a table. A common use case is when dropping columns from a table we need to know the dependent objects so as to drop them first before dropping a column. Another use case can be of to maintain metadata/inventory of a database. A T-SQL find constraints on table is shown below
SELECT obj_table.NAME AS TableName, obj_Constraint.NAME AS ConstraintName, columns.NAME AS ColumnName FROM sys.objects obj_table JOIN sys.objects obj_Constraint ON obj_table.object_id = obj_Constraint.parent_object_id JOIN sys.sysconstraints constraints ON constraints.constid = obj_Constraint.object_id JOIN sys.columns columns ON columns.object_id = obj_table.object_id AND columns.column_id = constraints.colid ORDER BY tablename
The constraint information is in sysconstraints system object. The sysconstraints.constid is the object_id of the constraints object in sys.objects table and sysconstraints.id is the object_id of the table that the constraints belong too. Thus joining sys.objects with sysconstraints based on above information, lists out all the constraint for a table. The sysconstraints.colid maps the column to the constraint. Thus joining sys.columns.column_id with sysconstraints.colid gives information about the columns a constraint refers too. The output from above query is shown below.