数据库连表查询
sql的join功能如上图所示
当 a表 join b表 时 想要a表的内容不损失 就使用left join
建表语句:
1.创建student和score表
CREATE TABLE student (
id INT(10) NOT NULL UNIQUE PRIMARY KEY ,
name VARCHAR(20) NOT NULL ,
sex VARCHAR(4) ,
birth YEAR,
department VARCHAR(20) ,
address VARCHAR(50)
);
创建score表。SQL代码如下:
CREATE TABLE score (
id INT(10) NOT NULL UNIQUE PRIMARY KEY AUTO_INCREMENT ,
stu_id INT(10) NOT NULL ,
c_name VARCHAR(20) ,
grade INT(10)
);
2.为student表和score表增加记录
向student表插入记录的INSERT语句如下:
INSERT INTO student VALUES( 901,'张老大', '男',1985,'计算机系', '北京市海淀区');
INSERT INTO student VALUES( 902,'张老二', '男',1986,'中文系', '北京市昌平区');
INSERT INTO student VALUES( 903,'张三', '女',1990,'中文系', '湖南省永州市');
INSERT INTO student VALUES( 904,'李四', '男',1990,'英语系', '辽宁省阜新市');
INSERT INTO student VALUES( 905,'王五', '女',1991,'英语系', '福建省厦门市');
INSERT INTO student VALUES( 906,'王六', '男',1988,'计算机系', '湖南省衡阳市');
向score表插入记录的INSERT语句如下:
INSERT INTO score VALUES(NULL,901, '计算机',98);
INSERT INTO score VALUES(NULL,901, '英语', 80);
INSERT INTO score VALUES(NULL,902, '计算机',65);
INSERT INTO score VALUES(NULL,902, '中文',88);
INSERT INTO score VALUES(NULL,903, '中文',95);
INSERT INTO score VALUES(NULL,904, '计算机',70);
INSERT INTO score VALUES(NULL,904, '英语',92);
INSERT INTO score VALUES(NULL,905, '英语',94);
INSERT INTO score VALUES(NULL,906, '计算机',90);
INSERT INTO score VALUES(NULL,906, '英语',85);
3.查询student表的所有记录
select * from student;
4.查询student表的第2条到4条记录
select * from student limit 2,2;
5.从student表查询所有学生的学号(id)、姓名(name)和院系(department)的信息
select id,name,department from student;
6.从student表中查询计算机系和英语系的学生的信息
select * from student where department in('计算机系','英语系');
7.从student表中查询年龄34~38岁的学生信息
select * from student where (2024-birth) between 34 and 38;
8.从student表中查询每个院系有多少人
select count(*),department as d from student group by(d);
9.从score表中查询每个科目的最高分
select c_name,max(grade) from score group by c_name;
10.查询李四的考试科目(c_name)和考试成绩(grade)
select name,c_name,grade from student a join score b on (b.stu_id=a.id) where name='李四';
11.用连接的方式查询所有学生的信息和考试信息
select * from student a join score b on (b.stu_id=a.id);
12.计算每个学生的总成绩
select name,sum(grade) 总成绩,a.id from student a left join score b on (b.stu_id=a.id) group by a.id;
13.计算每个考试科目的平均成绩
select avg(grade),c_name from score group by c_name;
14.查询计算机成绩低于95的学生信息
select a.* from student a left join score b on (b.stu_id=a.id) where c_name='计算机' and grade<95;
15.查询同时参加计算机和英语考试的学生的信息
SELECT a.* FROM student a, score b, score c WHERE a.id=b.stu_id AND b.c_name='计算机' AND a.id=c.stu_id ANDc.c_name='英语';
16.将计算机考试成绩按从高到低进行排序
select * from student a left join score b on (b.stu_id=a.id) where c_name='计算机' order by grade ;
17.从student表和score表中查询出学生的学号,然后合并查询结果
select * from student inner join score on student.id=score.stu_id;
18.查询姓张或者姓王的同学的姓名、院系和考试科目及成绩
select * from student a left join score b on (b.stu_id=a.id) WHERE a.name LIKE'张%' or a.name like '王%';
19.查询都是湖南的学生的姓名、年龄、院系和考试科目及成绩
select * from student a left join score b on (b.stu_id=a.id) WHERE a.address like '%湖南%';