Sunday, July 15, 2018

Describe the purpose of Data Definition Language (DDL)


Data Definition Language (DDL) is a standard for commands that define the different structures in a database. DDL statements create, modify, and remove database objects such as tables, indexes, and users.



Commonly used DDL in SQL querying are:

CREATE: This command builds a new table and has a predefined syntax. The CREATE statement syntax is CREATE TABLE [table name] ([column definitions]) [table parameters]. CREATE TABLE Employee (Employee Id INTEGER PRIMARY KEY, First name CHAR (50) NULL, Last name CHAR (75) NOT NULL).


ALTER: An alter command modifies an existing database table. This command can add up additional column, drop existing columns and even change the data type of columns involved in a database table. An alter command syntax is ALTER object type object name parameters. ALTER TABLE Employee ADD DOB Date.


DROP: A drop command deletes a table, index or view. Drop statement syntax is DROP object type object name. DROP TABLE Employee.

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