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Introduction to SQL




SQL is an ANSI standard language for accessing databases.


What is SQL?

SQL stands for Structured Query Language
SQL allows you to access a database
SQL is an ANSI standard language
SQL can execute queries against a database
SQL can retrieve data from a database
SQL can insert new records in a database
SQL can delete records from a database
SQL can update records in a database
SQL is very easy to learn



SQL is a Standard

SQL is an ANSI (American National Standards Institute) standard for accessing database
systems. SQL statements are used to retrieve and update data in a database.

SQL works with database programs like Access, Informix, Microsoft SQL Server,
Oracle, Sybase, and many others (but unfortunately most of them also have their
own proprietary extensions to the language).


Database Tables
Databases contain objects called Tables.
Records of data are stored in these tables. Tables are identified by names (like "Persons", "Orders",
"Suppliers").
Tables contain Columns and Rows with data. Rows contain records (like one
record for each person). Columns contain data (like First Name, Last Name,
Address, and City).
Here is an example of  a Table called "Persons":


LastName
FirstName
Address
City


Hansen
Ola
Timoteivn 10
Sandnes


Svendson
Tove
Borgvn 23
Sandnes


Pettersen
Kari
Storgt 20
Stavanger



LastName, FirstName, Address, and City are table Columns. The Rows contain 3
records about 3 persons.

SQL Queries


With SQL, we can Query a database and have a Result returned in
a tabular form.


A Query like this:




SELECT LastName FROM Persons



Will give a Result like this:




LastName


Hansen


Svendson


Pettersen



Note: Some database systems require a semicolon at the end of the SQL statement.
We don't use the semicolon in our tutorials.

SQL Data Manipulation

As the name suggests, SQL is a syntax for executing queries. But the SQL
language also includes a syntax to update records, insert new records and delete
existing records.

These query and update commands together form the Data Manipulation Language (DML)
part of SQL: 


SELECT - extracts data from a database
UPDATE - updates data in a database
DELETE - deletes data from a database
INSERT - inserts new data into a database



SQL Data Definition

The Data Definition Language (DDL) part of SQL permits database tables to be
created or deleted. We can also define indexes (keys), specify links between tables,
and impose constraints between database tables.

The most important DDL statements in SQL are: 


CREATE TABLE - creates a new database table
ALTER TABLE - alters (changes) a database table
DROP TABLE - deletes a database table
CREATE INDEX - creates an index (search key)
DROP INDEX - deletes an index




SQL and Active Server Pages

SQL is an important part of ASP, because the Active Data Object (ADO) used in
ASP to access databases, rely on SQL for data access.

If you want to know more about ASP and ADO:

Study our ASP School, and our ADO
School.




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