replace a column with another column when column is null in SELECT statement

Question

Here is my dummy Schema in MYSQL:
A column: which is a nullable string
B column: which is a string but its not null


I only want to select A columns but when facing a null, I want it to be replaced with the B column.
How can I do that?
this is my desired output

Answer 1

Or use ifnull, the sound of which makes it easy to remember. Note that it only works for 2 columns/values

select ifnull(a, b) as a
from t;

Answer 2

coalesce() or

SELECT CASE WHEN A IS NOT NULL THEN A ELSE B END AS value FROM tablename;

Answer 3

Use coalesce():

select coalesce(a, b) as a
from t;