Data definition language

A data definition language or data description language (DDL) is a syntax similar to a computer programming language for defining data structures, especially database schemas.

Data Definition Language (DDL) statements are used to define the database structure or schema. Put simply, DDL is used for creating (and/or destroying) database objects such as tables. Some examples of DDL statements are:

  • CREATE - to create objects in the database
  • ALTER - alters the structure of the database
  • DROP - delete objects from the database
  • TRUNCATE - remove all records from a table, including all spaces allocated for the records are removed
  • COMMENT - add comments to the data dictionary
  • RENAME - rename an object

History

The data definition language concept and name was first introduced in relation to the Codasyl database model, where the schema of the database was written in a language syntax describing the records, fields, and sets of the user data model. Later it was used to refer to a subset of Structured Query Language (SQL) for creating tables and constraints. SQL-92 introduced a schema manipulation language and schema information tables to query schemas. These information tables were specified as SQL/Schemata in SQL:2003. The term DDL is also used in a generic sense to refer to any formal language for describing data or information structures.

SQL

Many data description languages use a declarative syntax to define fields and data types. SQL, however, uses a collection of imperative verbs whose effect is to modify the schema of the database by adding, changing, or deleting definitions of tables or other objects. These statements can be freely mixed with other SQL statements, so the DDL is not truly a separate language.

CREATE statements

Create - To make a new database, table, index, or stored procedure.

A CREATEstatement in SQL creates an object in a relational database management system (RDBMS). In the SQL 1992 specification, the types of objects that can be created are schemas, tables, views, domains, character sets, collations, translations, and assertions. Many implementations extend the syntax to allow creation of additional objects, such as indexes and user profiles. Some systems (such as PostgreSQL) allow CREATE, and other DDL commands, inside a transaction and thus they may be rolled back.

CREATE TABLE statement

A commonly used CREATEcommand is the CREATE TABLEcommand. The typical usage is:

CREATE 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:

CREATE TABLE employees (
    id            INTEGER      PRIMARY KEY,
    first_name    VARCHAR(50)  NULL,
    last_name     VARCHAR(75)  NOT NULL,
    dateofbirth   DATE         NULL
);

Note that some forms of CREATE TABLEDDL may incorporate DML (data manipulation language)-like constructs as well, such as the CREATE TABLE AS SELECT(CTAS) syntax of SQL.

DROP statements

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

A DROPstatement 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 DROPand 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:

DROP TABLE employees;

The DROPstatement is distinct from the DELETEand TRUNCATEstatements, in that DELETEand TRUNCATEdo not remove the table itself. For example, a DELETEstatement might delete some (or all) data from a table while leaving the table itself in the database, whereas a DROPstatement would remove the entire table from the database.

ALTER statements

Alter - To modify an existing database object.

An ALTERstatement 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 (then remove) a column named bubbles for an existing table named sink would be:

ALTER TABLE sink ADD bubbles INTEGER;
ALTER TABLE sink DROP COLUMN bubbles;

Referential integrity statements

Finally, another kind of DDL sentence in SQL is one used to define referential integrity relationships, usually implemented as primary key and foreign key tags in some columns of the tables. These two statements can be included inside a CREATE TABLEor an ALTER TABLEsentence.

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