Data Definition

1. The SQL DDL (Data Definition Language) allows specification of a set of relations and the following information for each relation:
   • The scheme for each relation.
   • The domain of values for each attribute.
   • The set of indices for each relation.

   • Security and authorization information.
   • Integrity constraints.
   • Physical storage structure on disk.
2. An SQL relation is defined by:

   

   where is the relation name, is the name of an attribute, and is the domain of that attribute.

3. A newly loaded table is empty. The insert command can be used to load it.

4. To remove a relation from the database, we can use the drop table command:

   

   This is not the same as

   

   which retains the relation, but deletes all tuples in it.

5. The alter table command can be used to add attributes to an existing relation:

   

   where is the attribute and is the domain to be added. This command appears in some versions of SQL, but not in the SQL standard.