Types of SQL Commands

 These functions include building database objects, manipulating objects, populating database tables with data, updating existing data in tables, deleting data, performing database queries, controlling database access, and overall database administration.

The main categories are

• DDL (Data Definition Language)
• DML (Data Manipulation Language)
• DQL (Data Query Language)
• DCL (Data Control Language)
• Data administration commands
• Transactional control commands

Defining Database Structures

Data Definition Language, DDL, is the part of SQL that allows a database user to create and restructure database objects, such as the creation or the deletion of a table.

Some of the most fundamental DDL commands discussed during following hours include the following:

CREATE TABLE

ALTER TABLE

DROP TABLE

CREATE INDEX

ALTER INDEX

DROP INDEX

CREATE VIEW

DROP VIEW

These commands are discussed in detail during Hour 3, "Managing Database Objects," Hour 17, "Improving Database Performance," and Hour 20, "Creating and Using Views and Synonyms."

Manipulating Data

Data Manipulation Language, DML, is the part of SQL used to manipulate data within objects of a relational database.

There are three basic DML commands:

INSERT

UPDATE

DELETE

Selecting Data (Data Query Language)

Though comprised of only one command, Data Query Language (DQL) is the most concentrated focus of SQL for modern relational database users. The base command is as follows:

SELECT

This command, accompanied by many options and clauses, is used to compose queries against a relational database. Queries, from simple to complex, from vague to specific, can be easily created.

A query is an inquiry to the database for information. A query is usually issued to the database through an application interface or via a command line prompt.

Data Control Language

Data control commands in SQL allow you to control access to data within the database. These DCL commands are normally used to create objects related to user access and also control the distribution of privileges among users. Some data control commands are as follows:

ALTER PASSWORD

GRANT

REVOKE

CREATE SYNONYM

You will find that these commands are often grouped with other commands and may appear in a number of different lessons throughout this book.

Data Administration Commands

Data administration commands allow the user to perform audits and perform analyses on operations within the database. They can also be used to help analyze system performance. Two general data administration commands are as follows:

START AUDIT

STOP AUDIT

Do not get data administration confused with database administration. Database administration is the overall administration of a database, which envelops the use of all levels of commands. Database administration is much more specific to each SQL implementation than are those core commands of the SQL language.

Transactional Control Commands

In addition to the previously introduced categories of commands, there are commands that allow the user to manage database transactions.

• COMMIT Saves database transactions
• ROLLBACK Undoes database transactions
• SAVEPOINT Creates points within groups of transactions in which to ROLLBACK
• SET TRANSACTION Places a name on a transaction