CASE WHEN

CASE
    WHEN condition_1 THEN result_1
    WHEN condition_2 THEN result_2
    [WHEN...]
    [ELSE result_n]
END

• condition - условие, возвращающее bool ;

• result_1 - результат или действие в случае с PL\pgSQL.

Пример

SELECT product_name, unit_price, units_in_stock,
	CASE WHEN units_in_stock >= 100 THEN 'lots_of'
		 WHEN units_in_stock >= 50 THEN 'average'
	  	 WHEN units_in_stock < 50 THEN 'low number'
		 ELSE 'unknown'
	END AS amount
FROM products
ORDER BY units_in_stock DESC;