A relational database is a collection of data items organized as a set of formally-described tables from which data can be accessed or reassembled in many different ways without having to reorganize the database tables. The relational database was invented by E. F. Codd at IBM in 1970.
The standard user and application program interface to a relational database is the structured query language (SQL).
A relational database is a set of tables containing data fitted into predefined categories. Each table (which is sometimes called a relation) contains one or more data categories in columns. Each row contains a unique instance of data for the categories defined by the columns. For example, a typical business order entry database would include a table that described a customer with columns for name, address, phone number, and so forth. Another table would describe an order: product, customer, date, sales price, and so forth.
A user of the database could obtain a view of the database that fitted the user’s needs. For example, a branch office manager might like a view or report on all customers that had bought products after a certain date. A financial services manager in the same company could, from the same tables, obtain a report on accounts that needed to be paid.
Creating and modifying relations using SQL
SQL Statements
Most of the actions you need to perform on a database are done with SQL statements.
The following SQL statement selects all the records in the “Customers” table:
Example
SELECT * FROM Customers;
Some of The Most Important SQL Commands
- SELECT – extracts data from a database
- UPDATE – updates data in a database
- DELETE – deletes data from a database
- INSERT INTO – inserts new data into a database
- CREATE DATABASE – creates a new database
- ALTER DATABASE – modifies a database
- CREATE TABLE – creates a new table
- ALTER TABLE – modifies a table
- DROP TABLE – deletes a table
- CREATE INDEX – creates an index (search key)
- DROP INDEX – deletes an index
The SELECT statement is used to select data from a database.
The SQL SELECT Statement
The SELECT statement is used to select data from a database.
The result is stored in a result table, called the result-set.
SQL SELECT Syntax
SELECT column_name,column_name
FROM table_name;
and
SELECT * FROM table_name;