如果selector值为NULL,则不能在WHEN NULL条件下被匹配,相反,可以在WHEN boolean_expression IS NULL条件下使用CASE表达式搜索。 DECLARE grade CHAR(1); -- NULL by default appraisal VARCHAR2(20); BEGIN appraisal := CASE grade WHEN NULL THEN 'No grade assigned' WHEN 'A' THEN 'Excellent' WHEN 'B' THEN 'Very Good' WHEN 'C' THEN 'Good' WHEN 'D' THEN 'Fair' WHEN 'F' THEN 'Poor' ELSE 'No such grade' END; DBMS_OUTPUT.PUT_LINE ('Grade ' || grade || ' is ' || appraisal); END;