Short note about MySQL INDEX, KEY

MySQL KEY = INDEX - helps to find data faster via using indexing in needed columns

PRIMARY KEY (PRI) - repetitions in this column (or group of columns) are not allowed, NULL values are not allowed

UNIQUE KEY (UNI - one column) - repetitions in this column are not allowed, NULL values are allowed

UNIQUE KEY (MUL - multiple columns) - repetitions allowed in one column bit not in both at he same time, NULL values are allowed

INDEX=KEY - repetitions allowed, NULL values allowed

FOREIGN KEY = only value existing in other table can be added to this column