Detailed introduction to mysql index index related commands

show index from springdemo.blog;

ALTER TABLE `table` ADD INDEX `product_id` (`product_id`)

# 创建唯一索引alter table tb_name  add unique index  `unique_idsrc_caffeversion` (`id_src`, `caffe_version`)

#多列索引ALTER TABLE `table` ADD INDEX `index_name` (`col1`,`col2`)

# add primary key 添加主键ALTER TABLE `table_name` ADD PRIMARY KEY ( `column` )

What are differences between Index v.s. Key in MySQL

CREATE TABLE tasks ( 
task_id INT UNSIGNED NOT NULL AUTO_INCREMENT, 
INDEX parent (parent_id), 
....# However I found a code using KEY instead of INDEX as following....KEY order_date (order_date) 
...

There’s no difference. They are synonyms.This was implemented for 
 compatibility with other database systems.