When a row is inserted into a table, the database server does not attempt to put the data in any particular location within the table. Instead, the server simply places the data in the next available location within the file. When a query is sent against the table therefore, the server will need to inspect every row of the table to answer the query. Tables in MySQL can grow very large, and as a table gets bigger, retrievals from it become slower.
An index in MySQL serves to assist in finding row data more quickly and easily, much like an index in the back of a technical manual. Database indexes are used to locate rows in a table. Indexes are special structures that, unlike normal tables, are kept in a specific order. Instead of containing all of the data about an entity, however, an index contains only the column(s) used to locate rows in the data table. It also contains information describing where the rows are physically located. Usually, all the indexes required are created when the tables are created. Indexes can be created on single columns or multiple columns (composite indexes). For example, the phone number for an individual in a phone book would use a single index, whereas the last_name and first_name would be grouped together as a composite index.
MySQL supports several general types of indexes:
PRIMARY KEY
1. Only one PRIMARY KEY is allowed per table
2. Uniquely identifies a single row in the table
3. No NULL values are allowed
4. If a duplicate value is INSERTed or UPDATED MySQL will return an error
UNIQUE (same rules as the PRIMARY KEY with two exceptions)
1. Allows values stored as NULL
2. Can have multiple UNIQUE indexes in a table
3. Each NON-NULL value uniquely identifies a single row in a table
INDEX - Allows multiple identical values
Example:
CREATE TABLE `countrylanguage` (
`CountryCode` char(3) NOT NULL default '',
`Language` char(30) NOT NULL default '',
`IsOfficial` enum('T','F') NOT NULL default 'F',
`Percentage` float(4,1) NOT NULL default '0.0',
PRIMARY KEY (`CountryCode`,`Language`)
) ENGINE=MyISAM COMMENT 'Lists Languages Spoken'
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