day 05 多表连接查询
1.select 多表
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;
实例：
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 "学生名"
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.`tname`,teacher.`tid`
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`,
AVG(score.`score`)
FROM teacher JOIN course
ON teacher.`tid`=course.`tid`
JOIN score
ON course.`cid`=score.`cid`
WHERE teacher.`tname`='oldguo'
GROUP BY teacher.`tname`,teacher.`tid`,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`),
course.`cname`,AVG(score.`score`)
FROM teacher JOIN course
ON teacher.`tid`=course.`tid`
JOIN score
ON course.`cid`=score.`cid`
GROUP BY teacher.`tname`,teacher.`tid`,course.`cname`
ORDER BY score.`score` DESC
4.查询白龙马,学习的课程名称有哪些.
SELECT CONCAT(student.`xname`,"_",student.`xid`),
GROUP_CONCAT(course.`cname`)
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.`xname`,student.`xid`
5. 统计每位老师,每门课程,分别的收入情况
SELECT teacher.`tname`,course.`cname`,course.`cprice`*COUNT(score.`xid`)
FROM teacher JOIN course
ON teacher.`tid`=course.`tid`
JOIN score
ON course.`cid`=score.`cid`
GROUP BY teacher.`tname`,teacher.`tid`,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 teacher.`tname`,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.`tname`,teacher.`tid`
8.统计每位老师不及格学生名单.
SELECT teacher.`tname`,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.`tname`,teacher.`tid`
9.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(score.`score`)
FROM teacher JOIN course
ON teacher.`tid`=course.`tid`
JOIN score
ON course.`cid`=score.`cid`
GROUP BY teacher.`tname`,teacher.`tid`,course.`cid`
ORDER BY AVG(score.`score`) DESC
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 score JOIN course
ON score.`cid`=course.`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 score
JOIN student
ON score.`xid`=student.`xid`
GROUP BY student.`xname`,student.`xid`
HAVING AVG(score.`score`)>60
或
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 ;
12.统计各位老师,所教课程的及格率
SELECT teacher.`tname`,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`,
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.`c
1.5 left/right join 外连接应用
inner join 内连接应用
一般应用在强制驱动表时，强制小结果集驱动大表
---生产总可以使用left join 强制驱动表 ，尽量减少next loop 的出现
---为啥要强制？
inner join 优化器 自动选择，按照索引选择的几率较大，导致效率比较低。通过大量的读写磁盘，导致磁盘的损坏。
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`;
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;
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;
--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&效对规则
show charset; #查看字符集
show collation; #查看效对信息
---查看建库&建表信息
mysql> show create database world;
mysql> show create table world.city;
-- 查看用户权限
mysql> show grants for oldguo@'10.0.0.%';
-- 查看支持的存储引擎
mysql> show engines;
-- 查询表中索引信息
mysql> 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 ?
安全 : 只允许查询,不知道操作的是什么对象.
-
方便 : 只需要简单的select语句即可使用
create view a as 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 * from a;
3.2 作用：
1.方便做数据库资产统计
资产就包括： 库 表
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: 索引占用的长度（字节）
TABLE_COMMENT:表注释
---统计信息库information_schema
例：
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')