Case When expressions in SQL allow users to iterate through conditions and return results depending on which condition evaluates to TRUE. When a condition evaluates to TRUE, the logic breaks for the current observation and a result is returned (i.e. if the first condition evaluates to TRUE for row 1, the second condition will not be evaluated for row 1). This is essentially just like a chain of IF ELSE statements.
Syntax
The syntax for CASE WHEN statements looks like this:
CASE
WHEN col1 = 'foo' THEN 'bar'
WHEN col2 = 'baz' THEN 'buzz'
ELSE 'fizz'
END AS new_col