Data Definition Language

A Data Definition Language (DDL) is a computer language for defining data structures. The term was first introduced in relation to the Codasyl database model, where the schema of the database was written in a Data Definition Language describing the records, fields, and "sets" making up the user Data Model. It is now used in a generic sense to refer to any formal language for describing data or information structures.

XML Schema

XML Schema is an example of a pure DDL^{[citation needed]} (although only relevant in the context of XML).

SQL

A subset of SQL's instructions form another DDL. These SQL statements define the structure of a database, including rows, columns, tables, indexes, and database specifics such as file locations. DDL SQL statements are more part of the DBMS and have large differences between the SQL variations. DDL SQL commands include the following:

CREATE statements

Create - To make a new database, table, index, or stored query. A CREATE statement in SQL creates an object inside of a relational database management system (RDBMS). The types of objects that can be created depends on which RDBMS is being used, but most support the creation TABLEs, INDices, USERs, and DATABASEs. Some systems (such as PostgreSQL) allow CREATE, and other DDL commands, inside of a transaction and thus they may be rolled back.

CREATE TABLE

Perhaps the most common CREATE command is the CREATE TABLE command. The typical usage is:

CREATE [TEMP[ORARY]] TABLE [table name] ( [column definitions] ) [table parameters].

Column Definitions: A comma-separated list consisting of any of the following

• Column definition: [column name] [data type] {NULL | NOT NULL} {column options}
• Primary key definition: PRIMARY KEY ( [comma separated column list] )
• CONSTRAINTS: {CONSTRAINT} [constraint definition]
• RDBMS specific functionality

For example, the command to create a table named employees with a few sample columns would be: <source lang="sql"> CREATE TABLE employees (

   id            INTEGER   PRIMARY KEY,
   first_name    CHAR(50)  null,
   last_name     CHAR(75)  not null,
   date_of_birth DATE      null

); </source>

DROP statements

Drop - To destroy an existing database, table, index, or view.

A DROP statement in SQL removes an object from a relational database management system (RDBMS). The types of objects that can be dropped depends on which RDBMS is being used, but most support the dropping of tables, users, and databases. Some systems (such as PostgreSQL) allow DROP and other DDL commands to occur inside of a transaction and thus be rolled back.

The typical usage is simply DROP objecttype objectname. For example, the command to drop a table named employees would be:

<source lang="sql"> DROP TABLE employees; </source>

The DROP statement is distinct from the DELETE statement. For example, a DELETE statement might delete some (or all) data from a table, whereas a DROP statement might remove the entire table from the database.

ALTER statements

Alter - To modify an existing database object.

An ALTER statement in SQL changes the properties of an object inside of a relational database management system (RDBMS). The types of objects that can be altered depends on which RDBMS is being used.

The typical usage is ALTER objecttype objectname parameters. For example, the command to add a column named bubbles to an existing table named sink would be:

<source lang="sql"> ALTER TABLE sink ADD bubbles INTEGER; </source>

See also

• Data Manipulation Language
• Data Control Language

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