How to increment a counter and return the value in MySQL

Question

I have a table that has a Key column and a Counter column. I need to do something like this:

SELECT counter=counter+1 FROM table WHERE key=\'mykey\'
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Answer 1

update mytable set count=last_insert_id(counter+1) where key='mykey'

Then

select last_insert_id()

last_insert_id() can be passed an argument to 'set' it (and return that value), and calling it without an argument will return the value again. The internal state used by last_insert_id() is per-connection, which means that the same update statement issued on another connection will not affect the first.

Reference: last_insert_id()