1. 多表连接查询 ****
1.1 介绍
内连接 inner join
外连接 left join right join
笛卡尔
1.2 作用
聚合多张表数据,实现查询需求
查询人口数小于100人城市名,国家名,国土面积?
1.3 多表连接的语法
1.3.1 内连接(交集)
FROM A
INNER JOIN B
ON A.x=B.y
1.3.2 外连接
FROM A
LEFT JOIN B
ON A.x=B.y
FROM A
RIGHT JOIN B
ON A.x=B.y
1.3.3 笛卡尔乘积
FROM A
JOIN B
1.4 多表连接例子
1. 查询人口数小于100人城市名,国家名,国土面积?
套路 : 1. 找关联表 2. 找关系列
USE world
SHOW TABLES;
DESC city;
DESC country;
SELECT
city.name,
country.name,
country.SurfaceArea,
city.`Population`
FROM city
JOIN country
ON city.`CountryCode`=country.`Code`
WHERE city.`Population`<100
2. 统计查询每位学员的平均分
SELECT student.xid ,
student.xname,
AVG(score.score)
FROM student
JOIN score
ON student.xid=score.xid
GROUP BY student.xid,student.xname
3. 统计每位学员学习了几门课
SELECT student.xid ,
student.xname,
COUNT(score.score)
FROM student
JOIN score
ON student.xid=score.xid
GROUP BY student.xid,student.xname
4. 查询每位老师教的课程名和价格
SELECT teacher.tname,course.`cname`,course.`cprice`
FROM teacher
JOIN course
ON teacher.`tid`=course.`tid`;
5. 每位老师教的学生的个数和姓名列表
SELECT
teacher.tname,
COUNT(student.xid),
GROUP_CONCAT(student.xname)
FROM student
JOIN score
ON student.xid=score.xid
JOIN course
ON score.cid=course.cid
JOIN teacher
ON course.tid=teacher.tid
GROUP BY teacher.tid,teacher.tname;
综合练习题:
- 查询oldguo老师教的学生名
- 查询oldguo所教课程的平均分数
- 每位老师所教课程的平均分,并按平均分排序
- 查询白龙马,学习的课程名称有哪些?
- 统计每位老师赚了多少钱?
- 统计每门课程学习的人数.
- 查询oldboy老师教的学生不及格的学生名单
- 统计每位老师不及格学生名单
- 每位老师所教课程的平均分,并按平均分排序
- 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
- 查询平均成绩大于60分的同学的学号和平均成绩；
扩展:
- 统计各位老师,所教课程的及格率
- 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
1. 查询oldguo老师教的学生名.
SELECT
CONCAT(teacher.tname,"_",teacher.tid) AS "教师名",
GROUP_CONCAT(student.`xname`) AS "学生列表"
FROM teacher
JOIN course
ON teacher.`tid`=course.`tid`
JOIN score
ON course.`cid`=score.`cid`
JOIN student
ON score.`xid`=student.`xid`
WHERE teacher.tname='oldguo'
GROUP BY teacher.tid,teacher.tname;
2. 查询oldguo所教课程的平均分数.
SELECT
CONCAT(teacher.tname,"_",teacher.tid) AS "教师名",
course.cname AS "课程名",
AVG(score.score) AS "平均分"
FROM teacher
JOIN course
ON teacher.`tid`=course.`tid`
JOIN score
ON course.`cid`=score.`cid`
WHERE teacher.tname='oldguo'
GROUP BY teacher.tid,teacher.tname,course.cname;
3. 每位老师所教课程的平均分,并按平均分排序.
SELECT
CONCAT(teacher.tname,"_",teacher.tid) AS "教师名",
course.cname AS "课程名",
AVG(score.score) AS "平均分"
FROM teacher
JOIN course
ON teacher.`tid`=course.`tid`
JOIN score
ON course.`cid`=score.`cid`
GROUP BY teacher.tid,teacher.tname,course.cname;
4. 查询白龙马,学习的课程名称有哪些.
SELECT
CONCAT(student.xname,"_",student.`xid`) AS "学生姓名",
GROUP_CONCAT(course.`cname`) AS "课程列表"
FROM student
JOIN score
ON student.xid = score.xid
JOIN course
ON score.`cid`=course.`cid`
WHERE student.xname='白龙马'
GROUP BY student.xid,student.xname;
5. 统计每位老师总收入
SELECT teacher.`tname`,COUNT(score.`xid`),SUM(course.`cprice`)
FROM teacher
JOIN course
ON teacher.`tid` = course.`tid`
JOIN score
ON course.`cid` = score.`cid`
GROUP BY teacher.`tid`,teacher.`tname`;
5.1 统计每位老师,每门课程,分别的收入情况
SELECT
CONCAT(teacher.`tname`,"_",teacher.`tid`) AS "讲师名",
course.`cprice`*COUNT(score.xid) AS "课程收入"
FROM teacher
JOIN course
ON teacher.`tid` = course.`tid`
JOIN score
ON course.`cid` = score.`cid`
GROUP BY teacher.`tid`,teacher.`tname` , course.`cid`;
6. 统计每门课程学习的人数.
SELECT course.`cname`,COUNT(score.`xid`)
FROM course
JOIN score
ON course.`cid`=score.`cid`
GROUP BY course.`cname`;
7. 查询oldboy老师教的学生不及格的学生名单.
SELECT
CONCAT(teacher.tname,"_",teacher.tid) AS "教师名"
,GROUP_CONCAT(CONCAT(student.xname,":",score.score))
FROM teacher
JOIN course
ON teacher.`tid`=course.`tid`
JOIN score
ON course.`cid`=score.`cid`
JOIN student
ON score.`xid`=student.`xid`
WHERE teacher.`tname`='oldboy' AND score.`score`<60
GROUP BY teacher.tid,teacher.tname;
8. 统计每位老师不及格学生名单.
SELECT
CONCAT(teacher.tname,"_",teacher.tid) AS "教师名"
,GROUP_CONCAT(CONCAT(student.xname,":",score.score))
FROM teacher
JOIN course
ON teacher.`tid`=course.`tid`
JOIN score
ON course.`cid`=score.`cid`
JOIN student
ON score.`xid`=student.`xid`
WHERE score.`score`<60
GROUP BY teacher.tid,teacher.tname;
9. 每位老师所教课程的平均分,并按平均分排序.
SELECT
CONCAT(teacher.tname,"_",teacher.tid) AS "教师名",
course.`cname`,
AVG(score.`score`)
FROM teacher
JOIN course
ON teacher.`tid`=course.`tid`
JOIN score
ON course.`cid`=score.`cid`
GROUP BY teacher.tid,teacher.tname,course.`cid`;
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.`cid`,MAX(score.`score`),MIN(score.`score`)
FROM course
JOIN score
ON course.`cid`=score.`cid`
GROUP BY course.`cid`;
11. 查询平均成绩大于60分的同学的学号和平均成绩.
SELECT
CONCAT(student.`xname`,"_",student.`xid`),
AVG(score.`score`)
FROM student
JOIN score
ON student.`xid`=score.`xid`
GROUP BY student.`xname`,student.`xid`
HAVING AVG(score.`score`)>60
ORDER BY AVG(score.`score`) DESC ;
扩展:
12. 统计各位老师,所教课程的及格率
SELECT
CONCAT(teacher.tname,"_",teacher.tid) AS "教师名",
course.`cname`,
CONCAT(COUNT(CASE WHEN score.score>60 THEN 1 END)/COUNT(score.xid)*100,"%")
FROM teacher
JOIN course
ON teacher.`tid`=course.`tid`
JOIN score
ON course.`cid`=score.`cid`
GROUP BY teacher.tid,teacher.tname,course.`cid`;
13. 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` AS 课程名称,
GROUP_CONCAT(CASE WHEN score.`score` >= 85 THEN student.xname END ) AS "优秀",
GROUP_CONCAT(CASE WHEN score.`score` >=70 AND score.`score` < 85 THEN student.xname END) AS "良好",
GROUP_CONCAT(CASE WHEN score.`score` >=60 AND score.`score` <70 THEN student.xname END )AS "一般",
GROUP_CONCAT(CASE WHEN score.`score` <60 THEN student.xname END ) AS "不及格"
FROM student
JOIN score
ON student.xid = score.xid
JOIN course
ON score.`cid`=course.`cid`
GROUP BY course.`cid`;
1.5 left/right join 外连接应用
一般应用在强制驱动表时,强制小结果集驱动大表
生产中可以使用left join 强制 驱动表.尽量减少next loop的出现.
为什么要强制? inner join 优化器 自动选择, 按照索引选择的几率较大
select a.name,b.telnum from a left join b on a.id=b.id where a.age>=18
1.6 补充 别名的应用 .
1.6.1 列别名
SELECT
course.`cname` AS 课程名称,
GROUP_CONCAT(CASE WHEN score.`score` >= 85 THEN student.xname END ) AS "优秀",
GROUP_CONCAT(CASE WHEN score.`score` >=70 AND score.`score` < 85 THEN student.xname END) AS "良好",
GROUP_CONCAT(CASE WHEN score.`score` >=60 AND score.`score` <70 THEN student.xname END )AS "一般",
GROUP_CONCAT(CASE WHEN score.`score` <60 THEN student.xname END ) AS "不及格"
FROM student
JOIN score
ON student.xid = score.xid
JOIN course
ON score.`cid`=course.`cid`
GROUP BY course.`cid`;
说明: 1. 为了显示的好看. 2. 可以在 having 或 order by 子句中调用
1.6.2 表别名
SELECT
CONCAT(te.tname,"_",te.tid) AS "教师名"
,GROUP_CONCAT(CONCAT(st.xname,":",sc.score))
FROM teacher as te
JOIN course as co
ON te.`tid`=co.`tid`
JOIN score as sc
ON co.`cid`=sc.`cid`
JOIN student as st
ON sc.`xid`=st.`xid`
WHERE sc.`score`<60
GROUP BY te.tid,te.tname;
SELECT
CONCAT(te.tname,"_",te.tid) AS "教师名"
,GROUP_CONCAT(CONCAT(st.xname,":",sc.score))
FROM teacher AS te
JOIN course AS co
ON te.`tid`=co.`tid`
JOIN score AS sc
ON co.`cid`=sc.`cid`
JOIN student AS st
ON sc.`xid`=st.`xid`
WHERE sc.`score`<60
GROUP BY te.tid,te.tname;
2. show 语句的列表介绍 ***
-- 查看所有的库
show databases;
-- 查看当前库下的所有表
show tables;
show tables from world;
-- 查看当前并发会话信息
show processlist;
show full processlist;
-- 查看数据库支持的权限
show privileges;
-- 查看数据库参数信息
show variables
show variables like '%trx%';
-- 查看字符集&校对规则
show charset;
show collation;
-- 查看建库&建表语句
show create database world;
show create table world.city;
-- 查看用户权限
show grants for root@'localhost';
-- 查看支持的存储引擎
show engines;
-- 查询表中索引信息
show index from world.city;
-- 查看数据库当前状态信息
show status;
show status like '%lock%';
-- 查看InnoDB引擎相关的状态信息(内存,事务,锁,线程...)
show engine innodb status\G
-- 查看二进制日志相关信息
show binary logs ;
show master status;
show binlog events in 'xxxx';
-- 查看主从复制相关信息
show relaylog events in 'xxxx';
show slave status \G
mysql> help show ;
3. Information_schema 统计信息库
3.1 介绍:
视图 ?
1. 安全 : 只允许查询,不知道操作的是什么对象.
2. 方便 : 只需要简单的select语句即可使用.
3.2 作用:
1. 方便做数据库资产统计
库\表 :
个数
数据量(大小,行数)
每张表的数据字典信息
2. 获取到Server层状态信息
3. 获取到InnoDB引擎层的状态信息
3.3 应用举例:
TABLES :
TABLE_SCHEMA : 表所在的库
TABLE_NAME : 表名
ENGINE : 表的引擎
TABLE_ROWS : 表的行数
AVG_ROW_LENGTH: 平均行长度(字节)
INDEX_LENGTH : 索引占用长度(字节)
TABLE_COMMENT : 表注释
-- 例子:
-- 1. 简单查询体验TABLES信息
SELECT * FROM TABLES;
-- 2. 所有业务库和表的名字.
SELECT table_schema , table_name
FROM information_schema.tables
WHERE table_schema NOT IN ('sys','information_schema','performance_schema','mysql');
-- 3. 统计每个业务库,表的个数和列表
SELECT table_schema , COUNT(table_name),GROUP_CONCAT(table_name)
FROM information_schema.tables
WHERE table_schema NOT IN ('sys','information_schema','performance_schema','mysql')
GROUP BY table_schema;
-- 4. 统计业务数据库的总数据量
SELECT SUM(table_rows * AVG_ROW_LENGTH+index_length)/1024 AS total_KB
FROM information_schema.tables
WHERE table_schema NOT IN ('sys','information_schema','performance_schema','mysql');
-- 5. 每个业务库分别统计数据量
SELECT table_schema,SUM(table_rows * AVG_ROW_LENGTH+index_length)/1024 AS total_KB
FROM information_schema.tables
WHERE table_schema NOT IN ('sys','information_schema','performance_schema','mysql')
GROUP BY table_schema
ORDER BY total_KB DESC ;
-- 6. top 3 数据量大的表
SELECT table_schema,table_name,(table_rows * AVG_ROW_LENGTH+index_length)/1024 AS table_kb
FROM information_schema.tables
WHERE table_schema NOT IN ('sys','information_schema','performance_schema','mysql')
ORDER BY table_kb DESC
LIMIT 3;
-- 7. 查询所有非INNODB的表
SELECT table_schema,table_name ,ENGINE FROM information_schema.tables
WHERE table_schema NOT IN ('sys','information_schema','performance_schema','mysql')
AND ENGINE <> 'innodb';
-- 8. 查询所有非INNODB的表 , 并且提出修改建议
SELECT
table_schema,
table_name ,
ENGINE ,
CONCAT("alter table ",table_schema,".",table_name," engine=innodb;") AS "修改建议"
FROM information_schema.tables
WHERE table_schema NOT IN ('sys','information_schema','performance_schema','mysql')
AND ENGINE <> 'innodb';
-- 9. 所有业务库和表的名字,并且生成备份语句
SELECT
table_schema ,
table_name ,
CONCAT("mysqldump ",table_schema," ",table_name," > /bak/",table_schema,"_",table_name,".sql") AS "备份"
FROM information_schema.tables
WHERE table_schema NOT IN ('sys','information_schema','performance_schema','mysql');
