查询同时存在 ‘A001‘和‘A002’课程情况
成绩表：ex_chengjibiao_95
（1）先查询A001的课程
命令：SELECT * from ex_chengjibiao_95 WHERE CId='A001'
（2）在查询A002的课程
命令：SELECT * from ex_chengjibiao_95 WHERE CId='A002'
（3）两张表合并分别为a和b 最后where筛选
命令：SELECT * FROM
(SELECT * FROM ex_chengjibiao_95 WHERE Cid = 'A001') as a,
(SELECT * FROM ex_chengjibiao_95 WHERE Cid = 'A002') as b
WHERE a.Sid = b.SId
（4）去除冗余（a表分数命名为01_score，b表分数命名为02_score）
命令：SELECT a.Sid,a.score 01_score,b.score 02_score FROM
(SELECT * FROM ex_chengjibiao_95 WHERE Cid = 'A001') AS a,
(SELECT * FROM ex_chengjibiao_95 WHERE Cid = 'A002') AS b
WHERE a.Sid = b.Sid