The scalar_expression_1 argument may be evaluated twice: once as part of the search condition (see the preceding expanded CASE expression) and again as a return value for the ELSE clause.

Using a nondeterministic function, such as RANDOM, may have unexpected results if the first calculation of scalar_expression_1 is not equal to scalar_expression_2, in which case the result of the CASE expression is the value of the second calculation of scalar_expression_1, which may be equal to scalar_expression_2.

You can use a scalar subquery in a NULLIF expression. However, if you use a non-scalar subquery (a subquery that returns more than one row), a runtime error is returned.