Select column value if not null else use another column value

Question

I have 2 columns in mysql table: a and b. a is allways string value and b is sometimes a string value and sometimes it is null.

How to construct a mysql SELECT so that t

Answer 1

IF(expr1,expr2,expr3)

If expr1 is TRUE (expr1 <> 0 and expr1 <> NULL) then IF() returns expr2; otherwise it returns expr3.