SQL Server Cascade Delete

The CNAME foreign key in the T_student table is constrained to the CNAME in the T_class table

You cannot directly delete records in the T_class table:

1 Delete  from where cname='CR19'

Because the T_student table uses the data from the T_class table

So you need to set the Cascade Delete property

1 Alter Table Add Foreign Key References  on Delete Cascade

When deleting records in the T_class table, the associated data in the t_student is also deleted, so that the records in the T_class table can be deleted.

SQL Server Cascade Delete