SQL as a Data Definition Language

Introduction

• SQL contains a Data Definition Language (DDL) which is used for defining and maintaining the structure of the database.  It contains various versions of 
  • create 
  • drop and
  • alter
  statements.

   

• Create statements are used for defining new elements. An element may be a user account, a role (user group), a schema, a table, a domain, a constraint, a stored procedure, etc. All elements must be provided with a name. SQL-99 standard defines the maximum length of names to be 128 characters. In practice they are shorter.

• Drop statements delete definitions of elements. As a consequence the defined element  and  other things dependent on  it may also be deleted. For example, 

  drop table Customer

  deletes the data in table Customer, the definition of table Customer, and the elements that are dependent on table Customer. There may be restrictions on the use of the drop statements. For example, the above example statement might not be allowed to be executed if there were elements dependent on table Customer.

   

• Alter statements modify the definition and thus the structure of an element, for example, the statement

alter table Customer add newColumn int 

adds a new column named newColumn in table Customer.