DQL-Select 多表连接
1.1连接方式
a.笛卡尔乘积
mysql> select * from a,b;
b.内连接
mysql> select * from a join b on a.id=b.aid;
c.外连接
mysql> select * from a left join b on a.id=b.aid;
mysql> select * from a right join b on a.id=b.aid;
mysql> select * from a left join b on a.id=b.aid and b.aid is null;
1.2 列子
1.2.1 前提条件
student ：学生表
sno： 学号
sname：学生姓名
sage： 学生年龄
ssex： 学生性别
teacher ：教师表
tno： 教师编号
tname：教师名字
course ：课程表
cno： 课程编号
cname：课程名字
tno： 教师编号
score ：成绩表
sno： 学号
cno： 课程编号
score：成绩
项目构建
mysql> create database school charset utf8mb4;
mysql> use school
mysql> source /root/school.sql
Database changed
Query OK, 0 rows affected (0.02 sec)
Query OK, 0 rows affected (0.01 sec)
Query OK, 0 rows affected (0.01 sec)
Query OK, 0 rows affected (0.00 sec)
Query OK, 10 rows affected (0.03 sec)
Records: 10 Duplicates: 0 Warnings: 0
Query OK, 4 rows affected (0.00 sec)
Records: 4 Duplicates: 0 Warnings: 0
Query OK, 4 rows affected (0.00 sec)
Records: 4 Duplicates: 0 Warnings: 0
Query OK, 17 rows affected (0.01 sec)
Records: 17 Duplicates: 0 Warnings: 0
############################/root/school.sql#############################
[root@oldboy ~]# cat school.sql
CREATE DATABASE school CHARSET utf8;
USE school
CREATE TABLE student(
sno INT NOT NULL PRIMARY KEY AUTO_INCREMENT COMMENT '学号',
sname VARCHAR(20) NOT NULL COMMENT '姓名',
sage TINYINT UNSIGNED NOT NULL COMMENT '年龄',
ssex ENUM('f','m') NOT NULL DEFAULT 'm' COMMENT '性别'
)ENGINE=INNODB CHARSET=utf8;
CREATE TABLE course(
cno INT NOT NULL PRIMARY KEY COMMENT '课程编号',
cname VARCHAR(20) NOT NULL COMMENT '课程名字',
tno INT NOT NULL COMMENT '教师编号'
)ENGINE=INNODB CHARSET utf8;
CREATE TABLE sc (
sno INT NOT NULL COMMENT '学号',
cno INT NOT NULL COMMENT '课程编号',
score INT NOT NULL DEFAULT 0 COMMENT '成绩'
)ENGINE=INNODB CHARSET=utf8;
CREATE TABLE teacher(
tno INT NOT NULL PRIMARY KEY COMMENT '教师编号',
tname VARCHAR(20) NOT NULL COMMENT '教师名字'
)ENGINE=INNODB CHARSET utf8;
INSERT INTO student(sno,sname,sage,ssex)
VALUES
(1,'zhang3',18,'m'),
(2,'zhang4',18,'m'),
(3,'li4',18,'m'),
(4,'wang5',19,'f'),
(5,'zh4',18,'m'),
(6,'zhao4',18,'m'),
(7,'ma6',19,'f'),
(8,'oldboy',20,'m'),
(9,'oldgirl',20,'f'),
(10,'oldp',25,'m');
INSERT INTO teacher(tno,tname) VALUES
(101,'oldboy'),
(102,'hesw'),
(103,'oldguo'),
(104,'alex');
INSERT INTO course(cno,cname,tno)
VALUES
(1001,'linux',101),
(1002,'python',102),
(1003,'mysql',103),
(1004,'go',105);
INSERT INTO sc(sno,cno,score)
VALUES
(1,1001,80),
(1,1002,59),
(2,1002,90),
(2,1003,100),
(3,1001,99),
(3,1003,40),
(4,1001,79),
(4,1002,61),
(4,1003,99),
(5,1003,40),
(6,1001,89),
(6,1003,77),
(7,1001,67),
(7,1003,82),
(8,1001,70),
(9,1003,80),
(10,1003,96);
1.2.2 大显身手
#############################################################
核心思路：
1.找到所有相关表
2.找到所有表之间的关联关系
3.罗列其他的查询条件
#############################################################
-- 统计下每个学生平均成绩
-- a.分析题意,找出所有相关表
student
sc
-- b. 找到以上表的直接或间接关联条件
student.sno
sc.sno
-- c. 列举你要查询的列条件
student.sname
sc.score
-- d. 组合
mysql> select student.sname,avg(sc.score)
from student join sc on student.sno = sc.sno
group by student.sno;
-- 练习 :
-- 1. 每位学生学习的课程门数
SELECT student.sname,COUNT(sc.score)
FROM student JOIN sc ON student.sno=sc.sno
GROUP BY student.sno;
-- 2. 每位老师所教的课程门数
SELECT teacher.tname ,COUNT(course.cname)
FROM teacher JOIN course
ON teacher.tno=course.tno
GROUP BY teacher.tno;
-- 3. 每位老师所教的课程门数和名称
SELECT teacher.tname ,COUNT(course.cname),GROUP_CONCAT(course.cname)
FROM teacher JOIN course
ON teacher.tno=course.tno
GROUP BY teacher.tno;
-- 4. 每位学生学习的课程门数和名称
SELECT student.sname,COUNT(sc.score),GROUP_CONCAT(course.cname)
FROM student
JOIN sc
ON student.sno=sc.sno
JOIN course
ON sc.cno=course.cno
GROUP BY student.sno;
-- 多表SQL练习题
-- 1. 统计zhang3,学习了几门课
SELECT student.sname ,COUNT(*)
FROM student
JOIN sc
ON student.sno=sc.sno
WHERE student.sname='zhang3';
-- 2. 查询zhang3,学习的课程名称有哪些?
SELECT student.sname ,GROUP_CONCAT(course.cname)
FROM student
JOIN sc
ON student.sno=sc.sno
JOIN course
ON sc.cno=course.cno
WHERE student.sname='zhang3'
GROUP BY student.sname
-- 3. 查询oldguo老师教的学生名.
SELECT teacher.tname,GROUP_CONCAT(student.sname)
FROM teacher
JOIN course
ON teacher.tno=course.tno
JOIN sc
ON course.cno=sc.cno
JOIN student
ON sc.sno=student.sno
WHERE teacher.tname='oldguo'
GROUP BY teacher.tno
-- 4. 查询oldguo所教课程的平均分数
SELECT teacher.tname,AVG(sc.score)
FROM teacher
JOIN course
ON teacher.tno=course.tno
JOIN sc
ON course.cno=sc.cno
WHERE teacher.tname='oldguo'
GROUP BY teacher.tno;
-- 5. 每位老师所教课程的平均分,并按平均分排序
SELECT teacher.tname,course.cname,AVG(sc.score)
FROM teacher
JOIN course
ON teacher.tno=course.tno
JOIN sc
ON course.cno=sc.cno
GROUP BY teacher.tno , course.cno
ORDER BY AVG(sc.score) DESC;
-- 6. 查询oldguo所教的不及格的学生姓名
SELECT teacher.tname,GROUP_CONCAT(student.sname)
FROM teacher
JOIN course
ON teacher.tno=course.tno
JOIN sc
ON course.cno=sc.cno
JOIN student
ON sc.sno=student.sno
WHERE teacher.tname='oldguo' AND sc.score<60
GROUP BY teacher.tno
-- 7. 查询所有老师所教学生不及格的信息
SELECT teacher.tname,GROUP_CONCAT(student.sname)
FROM teacher
JOIN course
ON teacher.tno=course.tno
JOIN sc
ON course.cno=sc.cno
JOIN student
ON sc.sno=student.sno
WHERE sc.score<60
GROUP BY teacher.tno
-- 8. 查询平均成绩大于60分的同学的学号和平均成绩；
SELECT student.sno,student.sname,AVG(sc.score)
FROM student
JOIN sc
ON student.sno=sc.sno
GROUP BY student.sno
HAVING AVG(sc.score)>60
-- 9. 查询所有同学的学号、姓名、选课数、总成绩；
SELECT student.sno ,student.sname,COUNT(*),SUM(sc.score)
FROM student
JOIN sc
ON student.sno=sc.sno
JOIN course
ON sc.cno=course.cno
GROUP BY student.sno
-- 10.查询各科成绩最高和最低的分：以如下形式显示：课程ID，最高分，最低分
SELECT course.cno,MAX(sc.score),MIN(sc.score)
FROM course
JOIN sc
ON course.cno=sc.cno
GROUP BY course.cno;
-- 11.查询每门课程被选修的学生数
SELECT course.cname,COUNT(sc.sno)
FROM course
JOIN sc
ON course.cno=sc.cno
GROUP BY course.cno
-- 12.查询出只选修了一门课程的全部学生的学号和姓名
SELECT course.cname,GROUP_CONCAT(CONCAT(student.sno,":",student.sname))
FROM course
JOIN sc
ON course.cno=sc.cno
JOIN student
ON sc.sno=student.sno
GROUP BY course.cno
-- 13.查询选修课程门数超过1门的学生信息
SELECT student.sname,COUNT(*)
FROM student
JOIN sc
ON student.sno=sc.sno
GROUP BY student.sno
HAVING COUNT(*)>1
-- 14.查询平均成绩大于85的所有学生的学号、姓名和平均成绩
SELECT student.sno,student.sname,AVG(sc.score)
FROM student
JOIN sc
ON student.sno=sc.sno
GROUP BY student.sno
HAVING AVG(sc.score)>85
-- 15.统计各位老师,所教课程的及格率
SELECT teacher.tname,COUNT(CASE WHEN sc.score>=60 THEN 1 END)/COUNT(*)
FROM teacher
JOIN course
ON teacher.tno=course.tno
JOIN sc
ON course.cno=sc.cno
GROUP BY teacher.tno
-- 16.统计每门课程:优秀(85分以上),良好(70-85),一般(60-70),不及格(小于60)的学生列表
SELECT
course.cname,
GROUP_CONCAT(CASE WHEN sc.score>=85 THEN student.sname END) AS 优秀,
GROUP_CONCAT(CASE WHEN sc.score>=70 AND sc.score<85 THEN student.sname END) AS 良好,
GROUP_CONCAT(CASE WHEN sc.score>=60 AND sc.score<70 THEN student.sname END) AS 一般,
GROUP_CONCAT(CASE WHEN sc.score<60 THEN student.sname END) AS 差
FROM course
JOIN sc
ON course.cno=sc.cno
JOIN student
ON sc.sno=student.sno
GROUP BY course.cno
2.元数据获取
2.1 show
mysql> show databases;
mysql> show tables;
mysql> show privileges;
mysql> show grants for root@'localhost';
mysql> show create database school;
mysql> show charset ;
mysql> show collation;
mysql> show variables like '%char%';
mysql> show status like '%binglog%'
mysql> show processlist;
mysql> show index from school.student;
mysql> show engines;
mysql> show binlog events ;
mysql> show master status ;
mysql> show slave status ;
mysql> show slave hosts;
mysql> show plugins;
mysql> show engine innodb status ;
mysql> show relaylog events;
mysql> show open tables;
3.1.1介绍
视图. 查询元数据的方法
3.2.1 tables视图介绍
TABLE_SCHEMA 表所在库
TABLE_NAME 表名
ENGINE 存储引擎
TABLE_ROWS 表的行数(粗略统计)
AVG_ROW_LENGTH 平均行长度(粗略统计)
INDEX_LENGTH 索引长度(粗略统计)
DATA_FREE 碎片数
TABLE_COMMENT 表注释
3.2.2 tables视图应用例子:
1. 统计所有库,对应的表个数和名字列表
mysql> select table_schema,count(*),group_concat(table_name)
-> from information_schema.tables
-> group by table_schema;
| world | 3 | countrylanguage,country,city
2. 统计每个库的数据量大小
mysql> select table_schema,sum(table_rows*avg_row_length+index_length)/1024
-> from information_schema.tables
-> group by table_schema;
+--------------------+--------------------------------------------------+
| table_schema | sum(table_rows*avg_row_length+index_length)/1024 |
+--------------------+--------------------------------------------------+
| information_schema | NULL |
| mysql | 2338.7188 |
| performance_schema | 0.0000 |
| school | 63.9834 |
| sys | 15.9961 |
| test | 16.0000 |
| world | 781.9990 |
+--------------------+--------------------------------------------------+
3. 拼接语句
a. 查询一下业务数据库中,非InnoDB的表
mysql> SELECT table_schema , table_name ,engine FROM information_schema.tables
-> WHERE
-> table_schema NOT IN ('mysql','sys','information_schema','performance_schema')
-> AND ENGINE <>'innodb';
b.将非InnoDB表批量替换为InnoDB
SELECT concat("alter table ",table_schema,".",table_name," engine=innodb;") FROM information_schema.tables
WHERE
table_schema NOT IN ('mysql','sys','information_schema','performance_schema','world')
AND ENGINE <>'innodb' into outfile '/tmp/alter.sql' ;
mysqldump -uroot -p123 world city >/databak/world_city.sql
SELECT concat("mysqldump -uroot -p123 ",table_schema," ",table_name," >/databak/",table_schema,"_",table_name,".sql") FROM information_schema.tables
WHERE
table_schema NOT IN ('mysql','sys','information_schema','performance_schema') into outfile '/tmp/bak.sh' ;
3.2.3 columns
TABLE_SCHEMA
TABLE_NAME
COLUMN_NAME
DATA_TYPE
COLUMN_KEY
COLUMN_COMMENT
select table_schema,table_name ,column_name from information_schema.columns
where table_schema NOT IN ('mysql','sys','information_schema','performance_schema');