For more information on SQL syntax, click here.

SQL Parts

• SQL DDL: defining, deleting and modifying relation schemas
• SQL DML: query information from, insert tuples into, delete tuples from and modify tuples in the database
• integrity: SQL DDL includes commands for specifying integrity constraints
• View definition: SQL DDL includes commands for defining views
• Transaction control: SQL commands for specifying the beginning and ending of transactions
• Embedded SQL and dynamic SQL: define how SQL statements can be embedded within general-purpose programming languages
• Authorization: SQL DDL includes commands for specifying access rights to relations and views

Data Definition Language

The SQL data-definition language (DDL) allows the specification of relations and information about relations, including:

• the schema for each relation
• the type of values associated with each attribute
• the integrity constraints
• the set of indices to be maintained for each relation
• security and authorization information for each relation
• the physical storage structure of each relation on disk

Views

A view is defined as:

create view v as < query expression >

Once a view is defined, the view name can be used to refer to the virtual relation that the view generates. This is not the same as creating a new relation by evaluating the query expression.

You can use views in the expression defining another view.

Certain database systems allow view relations to be physically stored. A physical copy is created when the view is defined.

Transactions

A transaction consists of a sequence of query and/or update statements and is a "unit" of work. The SQL standard specifies that a transaction begins implicitly when an SQL statement is executed.

• the transaction must end with one of the following statements:
  • Commit work: the updates performed by the transaction become permanent in the database
  • Rollback work: All the updates performed by the SQL statements in the transaction are undone.
• Atomic transaction (indivisible)
  • either fully executed or rolled back as if never occurred
• Isolation from concurrent transactions