16.20 - Usage Notes - Teradata Database

Teradata Vantageā„¢ - SQL Functions, Expressions, and Predicates

Advanced SQL Engine
Teradata Database
March 2019
Programming Reference

WHEN clauses are processed sequentially.

The first WHEN clause search_condition_n that is TRUE returns the value of its associated scalar_expression_n as its result. The evaluation process then ends.

If no search_condition_n is TRUE, then scalar_expression_m, the argument of the ELSE clause, is the result.

If no ELSE clause is defined, then the default value for the result is NULL.

You can use a scalar subquery in the WHEN clause, THEN clause, and ELSE clause of a CASE expression. If you use a non-scalar subquery (a subquery that returns more than one row), a runtime error is returned.

Recommendation: Do not use the built-in functions CURRENT_DATE or CURRENT_TIMESTAMP in a CASE expression that is specified in a partitioning expression for a partitioned primary index (PPI). In this case, all rows are scanned during reconciliation.