Selecting and joining data from 2 mysql tables

Question

MySQL

hello, I'm trying to solve the following problem:
there is a table with id , m1 , m2
fields m1 and m2 of the same type, which cannot be null at the same time.
you need to make a selection, from n records sorted by id
records are selected in this way
if m1 is null , then the record is not taken.
if m1 is not null , then 2 cases are possible
1) if m2 is not null , then m2 AS m1 is taken
2) if m2 is null , then m1 is taken
I can do it separately, i.e. select n records of the form 1) and n records of the form 2 ) and create the resulting array using some language, but I would like everything in sql, if possible.

Answer 1

Use an IF statement:
www.mysql.ru/docs/man/Control_flow_functions.html
It can also be nested:
komlenic.com/254/mysql-nested-if-in-select-queries

Answer 2

IF's will only confuse everything. There is a special coalesce function for your task :

select coalesce(m2, m1)
from my_table
where m1 is not null