Introduction to SQL
SQL is a standard computer language for accessing and
manipulating databases.
What is SQL?
- SQL stands for Structured Query Language
- SQL allows you to access a database
- SQL is an ANSI standard computer 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 easy to learn
SQL is a Standard - BUT....
SQL is an ANSI (American National Standards Institute) standard computer
language for accessing and manipulating database
systems. SQL statements are used to retrieve and update data in a database. SQL works with database programs like MS Access, DB2, Informix, MS SQL Server,
Oracle, Sybase, etc.
Unfortunately, there are many different versions of the SQL language, but to be in
compliance with the ANSI standard, they must support the same
major keywords in a similar manner (such as SELECT, UPDATE, DELETE, INSERT,
WHERE, and others).
Note: Most of the SQL database programs also have their own
proprietary extensions in addition to the SQL standard!
SQL Database Tables
A database most often contains one or more tables. Each table is identified
by a name (e.g. "Customers" or "Orders"). Tables contain records (rows) with
data.
Below 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 |
The table above contains three records (one for each person) and four columns (LastName, FirstName, Address, and City).
SQL Queries
With SQL, we can query a database and have a result set returned.
A query like this:
SELECT LastName FROM Persons |
Gives a result set 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 Language (DML)
SQL (Structured Query Language) is a syntax for executing queries. But the SQL
language also includes a syntax to update, insert, and delete
records.
These query and update commands together form the Data Manipulation Language (DML)
part of SQL:
- SELECT - extracts data from a database table
- UPDATE - updates data in a database table
- DELETE - deletes data from a database table
- INSERT INTO - inserts new data into a database table
SQL Data Definition Language (DDL)
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
Whether you're new to XML or already an advanced user, the user-friendly views
and powerful entry helpers, wizards, and debuggers in XMLSpy are designed to
meet your XML and Web services development needs from start to finish.
- XML editor
- Graphical XML Schema / DTD editors
- XSLT 1.0/2.0 editor, debugger, profiler
- XQuery editor, debugger, profiler
- Support for Office Open XML (OOXML)
- Graphical WSDL editor & SOAP debugger
- Java, C#, C++ code generation
- And much more!
Download a free 30-day trial today!
Click to see a demo!
Learn what’s new in v2008
 |
|
Get Your Diploma!
W3Schools' Online Certification Program is the perfect solution for busy
professionals who need to balance work, family, and career building.
The HTML Certificate is for developers who want to document their knowledge of HTML, XHTML, and CSS.
The ASP Certificate is for developers who want to document their knowledge of ASP, SQL, and ADO.
|
| 
HOME
Printer friendly page
