indexing – What’s the difference between using INDEX vs KEY in MySQL?

The Question :

200 people think this question is useful

I know how to use INDEX as in the following code. And I know how to use foreign key and primary key.

CREATE TABLE tasks ( 
  task_id INT UNSIGNED NOT NULL AUTO_INCREMENT, 
  parent_id INT UNSIGNED NOT NULL DEFAULT 0, 
  task VARCHAR(100) NOT NULL, 
  date_added TIMESTAMP NOT NULL, 
  date_completed TIMESTAMP, 
  PRIMARY KEY (task_id), 
  INDEX parent (parent_id), 
  ....


However I found a code using KEY instead of INDEX as following.

...
KEY order_date (order_date) 
...


I could not find any explanation on the official MySQL page. Could anyone tell me what is the differences between KEY and INDEX?

The only difference I see is that when I use KEY ..., I need to repeat the word, e.g.
KEY order_date (order_date).

The Question Comments :

The Answer 1

278 people think this answer is useful

There’s no difference. They are synonyms.

From the CREATE TABLE manual entry:

KEY is normally a synonym for INDEX. The key attribute PRIMARY KEY can also be specified as just KEY when given in a column definition. This was implemented for compatibility with other database systems.

The Answer 2

13 people think this answer is useful

Here is a nice description about the “difference”:

“MySQL requires every Key also be indexed, that’s an implementation detail specific to MySQL to improve performance.”

The Answer 3

5 people think this answer is useful

It is mentioned as a synonym for INDEX in the ‘create table’ docs: MySQL 5.5 Reference Manual :: 13 SQL Statement Syntax :: 13.1 Data Definition Statements :: 13.1.17 CREATE TABLE Syntax

Nos already cited the section and linked the help for 5.1.

Like PRIMARY KEY creates a primary key and an index for you, KEY creates an index only.

The Answer 4

4 people think this answer is useful

Keys are special fields that play very specific roles within a table, and the type of key determines its purpose within the table.

An index is a structure that RDBMS(database management system) provides to improve data processing. An index has nothing to do with a logical database structure.

SO…

Keys are logical structures you use to identify records within a table and indexes are physical structures you use to optimize data processing.

Source: Database Design for Mere Mortals

Author: Michael Hernandez

The Answer 5

-1 people think this answer is useful

A key is a set of columns or expressions on which we build an index.

  1. While an index is a structure that is stored in database, keys are strictly a logical concept.

  2. Index help us in fast accessing a record, whereas keys just identify the records uniquely.

  3. Every table will necessarily have a key, but having an index is not mandatory.

Check on https://docs.oracle.com/cd/E11882_01/server.112/e40540/indexiot.htm#CNCPT721

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