How to use ON DUPLICATE KEY in insert statement

How to use ON DUPLICATE KEY in insert statement

Problem:
In case of inserting or updating record we use separate logic using if statement or case statement:
e.g.:

if(i==0)
insert here

if(i=1)
update here

we can terminate the above if statement by using On Duplicate Key.
by using this only one statement can be use for insert and update record.

*note: user_nm coulmn must be unique

Example:

INSERT INTO `posts` (`user_nm`, `no_of_update`, `inserted_on`, `update_on`)
VALUES ('a_user', 1, NOW(), NOW())
ON DUPLICATE KEY
UPDATE `no_of_update` = `no_of_update` + 1, `update_on` = NOW()