SQL is an ANSI standard language for accessing databases.
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, DB2, Informix, Microsoft SQL Server, Oracle, Sybase, and many others (but unfortunately most of them also have their own proprietary extensions to the language).
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.
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.
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:
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: