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Welcome to the World of SQL

Oct 4, 2002

📄 Contents

␡

SQL Definition and History
SQL Sessions
Types of SQL Commands
An Introduction to the Database Used in This Book
Summary
Q&A
Workshop

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SQL Sessions

A SQL session is an occurrence of a user interacting with a relational database through the use of SQL commands. When a user initially connects to the database, a session is established. Within the scope of a SQL session, valid SQL commands can be entered to query the database, manipulate data in the database, and define database structures, such as tables. A session may be invoked by either direct connection to the database or through a front-end application. In both cases, sessions are normally established by a user at a terminal or workstation that communicates through a network with the computer that hosts the database.

`CONNECT`

When a user connects to a database, the SQL session is initialized. The CONNECT command is used to establish a database connection. With the CONNECT command, you can either invoke a connection or change connections to the database. For example, if you are connected as USER1, you can use the CONNECT command to connect to the database as USER2. When this happens, the SQL session for USER1 is implicitly disconnected.

CONNECT user@database

When you attempt to connect to a database, you are automatically prompted for a password that is associated with your current username. The username is used to authenticate yourself to the database, and the password is the key that allows entrance.

`DISCONNECT` and `EXIT`

When a user disconnects from a database, the SQL session is terminated. The DISCONNECT command is used to disconnect a user from the database. When you disconnect from the database, the software you are using may still appear to be communicating with the database, but you have lost your connection. When you use EXIT to leave the database, your SQL session is terminated and the software that you are using to access the database is normally closed.

DISCONNECT

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