sql一个题的解法分析讲解

本篇讲述的是对一个sql面试题的细致语法讲解。关于执行流程（on where），内连接，外连接（左右）上实用。关于这些基本的语法知识请参考我前面的sql基本语法。

　　S(SNO,SNAME)学生学号，姓名

　　C（CNO,CNAME,CTEACHER）课程号，课程名，课成老师名。

　　SC(SNO,CNO,SCGRADE)，SNO学号，CNO课程号，SCGRADE成绩。

题1

要求：列出“1”号课程成绩比“2”号课程成绩高的所有学生学号及其“1”号课程和“2”号课程的成绩

sql一个题的解法分析讲解

要求：列出“1”号课程成绩比“2”号课程成绩高的所有学生学号及其“1”号课程和“2”号课程的成绩

1，分别查出1 2号课程的所有列表（包括学号，课程，成绩）。

2，条件1课程表的成绩>2课程表的成绩。两表连接查询。

3，隐士的条件，这两个表中的学号相等。两表连接查询。

4，select 表1或表2的序号，表1.成绩，表2.成绩。

5，分别起别名。

SELECT  A.SNO as '学号',A.SCGRADE AS '1号课程',b.SCGRADE as '2号课程' FROM
(SELECT *FROM SC WHERE CNO=1)AS A CROSS JOIN (SELECT * FROM SC WHERE CNO=2)as b 
WHERE A.SCGRADE>b.SCGRADE AND A.SNO=b.SNO;


SELECT  A.SNO as '学号',A.SCGRADE AS '1号课程',b.SCGRADE as '2号课程' FROM
(SELECT SNO,SCGRADE FROM SC WHERE CNO=1)AS A CROSS JOIN (SELECT SNO,SCGRADE FROM SC WHERE CNO=2)as b 
WHERE A.SCGRADE>b.SCGRADE AND A.SNO=b.SNO;

SELECT  A.SNO as '学号',A.SCGRADE AS '1号课程',b.SCGRADE as '2号课程' FROM
(SELECT SNO,SCGRADE FROM SC WHERE CNO=1)AS A INNER JOIN (SELECT SNO,SCGRADE FROM SC WHERE CNO=2)as b 
ON A.SCGRADE>b.SCGRADE AND A.SNO=b.SNO;

SELECT  A.SNO as '学号',A.SCGRADE AS '1号课程',b.SCGRADE as '2号课程' FROM
(SELECT SNO,SCGRADE FROM SC WHERE CNO=1)AS A INNER JOIN (SELECT SNO,SCGRADE FROM SC WHERE CNO=2)as b 
ON A.SCGRADE>b.SCGRADE WHERE A.SNO=b.SNO;


SELECT  A.SNO as '学号',A.SCGRADE AS '1号课程',b.SCGRADE as '2号课程' FROM
(SELECT SNO,SCGRADE FROM SC WHERE CNO=1)AS A LEFT JOIN (SELECT SNO,SCGRADE FROM SC WHERE CNO=2)as b 
ON A.SCGRADE>b.SCGRADE AND A.SNO=b.SNO;

SELECT  A.SNO as '学号',A.SCGRADE AS '1号课程',b.SCGRADE as '2号课程' FROM
(SELECT SNO,SCGRADE FROM SC WHERE CNO=1)AS A LEFT JOIN (SELECT SNO,SCGRADE FROM SC WHERE CNO=2)as b 
ON A.SCGRADE>b.SCGRADE WHERE A.SNO=b.SNO;


SELECT  A.SNO as '学号',A.SCGRADE AS '1号课程',b.SCGRADE as '2号课程' FROM
(SELECT SNO,SCGRADE FROM SC WHERE CNO=1)AS A RIGHT JOIN (SELECT SNO,SCGRADE FROM SC WHERE CNO=2)as b 
ON A.SCGRADE>b.SCGRADE AND A.SNO=b.SNO;

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