The general CASE function (searched_case_function) is a special function that analyzes a quantity of search conditions to determine a result expression.
WHEN <search_condition> THEN <result_expression>
<result_expression> ::= <expression>
<default_expression> ::= <expression>
SQL Tutorial, Functions
CASE checks the search conditions (search_condition) in succession. As soon as a search condition that is true is found, the result of the general CASE function is the value of the expression result_expression that belongs to the search condition.
If no true search expression is found, then CASE gets the result of the expression default_expression. If default_expression is not specified, the result of CASE is the NULL value.
All search conditions without ROWNO Predicate are permissible. The data types of the expressions result_expression and default_expression must be comparable.