成为（SQL大师）的秘诀——极少人知道的秘诀

一.前期准备

建表语句

学⽣表 Student

create table Student(
    SId varchar(10),
    Sname varchar(10),
    Sage datetime,
    Ssex varchar(10)
); 

#-- 学生表 Student
#-- 学生表 Student
insert into Student values('01' , '赵雷' , '1990-01-01' , '男'); 
insert into Student values('02' , '钱电' , '1990-12-21' , '男'); 
insert into Student values('03' , '孙风' , '1990-12-20' , '男'); 
insert into Student values('04' , '李云' , '1990-12-06' , '男'); 
insert into Student values('05' , '周梅' , '1991-12-01' , '女'); 
insert into Student values('06' , '吴兰' , '1992-01-01' , '女'); 
insert into Student values('07' , '郑竹' , '1989-01-01' , '女'); 
insert into Student values('09' , '张三' , '2017-12-20' , '女'); 
insert into Student values('10' , '李四' , '2017-12-25' , '女'); 
insert into Student values('11' , '李四' , '2012-06-06' , '女'); 
insert into Student values('12' , '赵六' , '2013-06-13' , '女'); 
insert into Student values('13' , '孙七' , '2014-06-01' , '女'); 

课程表 Course

create table Course(
    CId varchar(10),
    Cname nvarchar(10),
    TId varchar(10)
);
# -- 科⽬表 Course 
insert into Course values('01' , '语文' , '02'); 
insert into Course values('02' , '数学' , '01'); 
insert into Course values('03' , '英语' , '03');

教师表 Teacher

create table Teacher(
    TId varchar(10),
    Tname varchar(10)
);
# -- 教师表 Teacher 
insert into Teacher values('01' , '张三'); 
insert into Teacher values('02' , '李四'); 
insert into Teacher values('03' , '王五'); 

成绩表 SC

create table SC(
    SId varchar(10),
    CId varchar(10),
    score decimal(18,1)
);
# -- 成绩表 SC 
insert into SC values('01' , '01' , 80); 
insert into SC values('01' , '02' , 90); 
insert into SC values('01' , '03' , 99); 
insert into SC values('02' , '01' , 70); 
insert into SC values('02' , '02' , 60); 
insert into SC values('02' , '03' , 80); 
insert into SC values('03' , '01' , 80); 
insert into SC values('03' , '02' , 80); 
insert into SC values('03' , '03' , 80); 
insert into SC values('04' , '01' , 50); 
insert into SC values('04' , '02' , 30); 
insert into SC values('04' , '03' , 20); 
insert into SC values('05' , '01' , 76); 
insert into SC values('05' , '02' , 87); 
insert into SC values('06' , '01' , 31); 
insert into SC values('06' , '03' , 34); 
insert into SC values('07' , '02' , 89); 
insert into SC values('07' , '03' , 98);

二.50道封神题

1.查询" 01 “课程比” 02 "课程成绩高的学生的信息及课程分数

分析：
1、找出有01成绩的学生成绩信息
2、找出有02成绩的学生成绩信息
3、通过SId将两表取别名t1、t2进行左连接
4、加上满足01‘语文’ > 02’数学’的条件

2.查询不存在" 01 “课程但存在” 02 "课程的情况

查询平均成绩⼤于等于 60 分的同学的学生编号和学生姓名和平均成绩