There's no difference. They are synonyms, though INDEX should be preferred (as INDEX is ISO SQL compliant, while KEY is a MySQL-specific, non-portable, extension).
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.
By "The key attribute PRIMARY KEY can also be specified as just KEY when given in a column definition.", it means that these three CREATE TABLE statements below are equivalent and generate identical TABLE objects in the database:
CREATE TABLE orders1 (
order_id int PRIMARY KEY
);
CREATE TABLE orders2 (
order_id int KEY
);
CREATE TABLE orders3 (
order_id int NOT NULL,
PRIMARY KEY ( order_id )
);
...while these 2 statements below (for orders4, orders5) are equivalent with each other, but not with the 3 statements above, as here KEY and INDEX are synonyms for INDEX, not a PRIMARY KEY:
CREATE TABLE orders4 (
order_id int NOT NULL,
KEY ( order_id )
);
CREATE TABLE orders5 (
order_id int NOT NULL,
INDEX ( order_id )
);
...as the KEY ( order_id ) and INDEX ( order_id ) members do not define a PRIMARY KEY, they only define a generic INDEX object, which is nothing like a KEY at all (as it does not uniquely identify a row).
As can be seen by running SHOW CREATE TABLE orders1...5:
| Table |
SHOW CREATE TABLE... |
orders1 |
CREATE TABLE orders1 (
order_id int NOT NULL,
PRIMARY KEY ( order_id )
) |
orders2 |
CREATE TABLE orders2 (
order_id int NOT NULL,
PRIMARY KEY ( order_id )
) |
orders3 |
CREATE TABLE orders3 (
order_id int NOT NULL,
PRIMARY KEY ( order_id )
) |
orders4 |
CREATE TABLE orders4 (
order_id int NOT NULL,
KEY ( order_id )
) |
orders5 |
CREATE TABLE orders5 (
order_id int NOT NULL,
KEY ( order_id )
) |
