sql查询一个面试题的7步解法及语法区别分析讲解

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。

这张表的名字SC，SNO学号，CNO课程号，SCGRADE成绩。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

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;

所有方法

第一种：

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;

第二种：

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;

第三种

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;

第四种：

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;

第五种：

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;

第六种：

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;

第七种：

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;

方法1和2之间区别是两个连接表的查询字段的多余与否。

方法3演示了inner jion和on的连接使用，并和cross jion的区别。

方法4和3演示了内连接流程顺序，先from内（包含on）走完，再where。内连接和on where的功能相同，但顺序不同。

方法5演示纯的左连接和on多条件使用。

方法6和5演示了左链接流程顺序，先from内（包含on）走完，再where。内连接和on where的功能相同，但顺序不同。

方法7右连接的on多条件使用。

这些区别已经区分非常清楚。具体语法请看我前几篇关于数据库的基本语法文章。

当然，这些不是什么前沿技术，但很细致，对不对？