Mysql select query condition

Question

I have two database columns in a mysql database, A and B. In a select query, I want to implement this logic:

Select the rows where A is 'X'. If A is not set in a row, then check and select the row only if column B='Y'.

So one can say that column B is a fallback for column A.

How could I construct a SELECT query with 'X' and 'Y' as inputs for the WHERE clause?

Answer 1

Use boolean logic:

SELECT *
FROM table
WHERE A = 'X' OR (A IS NULL AND B = 'Y')