SQL 中 CASE 表达式的使用方式

　　--- 使用IN谓词

　　SELECT

　　course_name AS '课程名',

　　CASE WHEN courese_id IN (SELECT course_id FROM open_course WHERE mouth = '200706')

　　THEN 'o'

　　ELSE 'x'

　　END AS '6 月'

　　CASE WHEN courese_id IN (SELECT course_id FROM open_course WHERE mouth = '200707')

　　THEN 'o'

　　ELSE 'x'

　　END AS '7 月'

　　CASE WHEN courese_id IN (SELECT course_id FROM open_course WHERE mouth = '200708')

　　THEN 'o'

　　ELSE 'x'

　　END AS '8 月'

　　FROM course_master;

　　--- 或者使用EXIST谓词

　　SELECT CM.course_name,

　　CASE WHEN EXISTS (SELECT course_id FROM OpenCourses OC WHERE month = 200706 AND OC.course_id = CM.course_id)

　　THEN '○' ELSE '×'

　　END AS "6 月",

　　CASE WHEN EXISTS (SELECT course_id FROM OpenCourses OC WHERE month = 200707 AND OC.course_id = CM.course_id)

　　THEN '○' ELSE '×'

　　END AS "7 月",

　　CASE WHEN EXISTS (SELECT course_id FROM OpenCourses OC WHERE month = 200708 AND OC.course_id = CM.course_id)

　　THEN '○' ELSE '×'

　　END AS "8 月"

　　FROM CourseMaster CM;