DQL应用
select
作用
获取MySQL中的数据行
单独使用select
select @@xxxx;获取参数信息。
mysql> select @@port;
mysql> show variables like '%innodb%';
select 函数();
mysql> select database();
mysql> select now();
mysql> select version();
SQL92标准的使用语法
select语法执行顺序（单表）
select开始 ---->
from子句 --->
where子句--->
group by子句--->
select后执行条件--->
having子句 ---->
order by ---->
limit
FROM
--- 例子：查询city表中的所有数据
USE world;
SELECT * FROM city; --->适合表数据行较少，生产中使用较少。
SELECT * FROM world.city;
--- 例子： 查询name和population的所有值
SELECT NAME , population FROM city;
SELECT NAME , population FROM world.city;
单表查询练习环境：world数据库下表介绍
SHOW TABLES FROM world;
city(城市):
DESC city;
id: 自增的无关列，数据行的需要
NAME: 城市名字
countrycode：城市所在的国家代号，CHN,USA,JPN。。。。
district : 城市的所在的区域，中国是省的意思，美国是洲的意思
population: 城市的人口数量
说明: 此表是历史数据，仅供学习交流使用。
where
WHERE
--- 例子：
--- WHERE 配合 等值查询(=)
--- 查询中国的城市信息
SELECT *
FROM world.city
WHERE countrycode='CHN';
--- 查询美国的城市信息
SELECT *
FROM world.city
WHERE countrycode='USA';
--- WHERE 配合 不等值(> < >= <= <>)
--- 查询一下世界上人口小于100人的城市
SELECT *
FROM world.city
WHERE population<100;
--- 查询世界上人口大于10000000的城市
略。
--- WHERE 配合 模糊(LIKE)
--- 查询国家代号是C开头的城市
SELECT *
FROM world.city
WHERE countrycode
LIKE 'C%';
--- 注意：like 语句在MySQL中，不要出现%在前面的情况。因为效率很低，不走索引。
--- 错误的
SELECT *
FROM world.city
WHERE countrycode
LIKE '%C%';
--- WHERE 配合 逻辑连接符(AND OR)
--- 查询城市人口在1w到2w之间的城市
SELECT *
FROM city
WHERE population >= 10000
AND Population <= 20000;
SELECT *
FROM city
WHERE population
BETWEEN 10000 AND 20000;
--- 查询一下中国或美国的城市信息
SELECT *
FROM city
WHERE countrycode='CHN' OR countrycode='USA';
SELECT *
FROM city
WHERE countrycode IN ('CHN','USA');
建议改写为，以下语句：
SELECT *
FROM city
WHERE countrycode='CHN'
UNION ALL
SELECT *
FROM city
WHERE countrycode='USA';
GROUP BY 配合聚合函数应用
常用聚合函数：
AVG() ----平均值
COUNT() ----个数
SUM() ----总和
MAX() ----最大值
MIN() ----最小值
GROUP_CONCAT()
--- 统计每个国家的总人口
SELECT countrycode,SUM(population) FROM city GROUP BY countrycode ;
--- 统计每个国家的城市个数
1.拿什么站队
GROUP BY countrycode
2. 拿什么统计
城市id，name
3. 统计的是什么？
COUNT(id)
SELECT countrycode,COUNT(id) FROM city GROUP BY countrycode;
--- 统计并显示 每个国家的省名字列表
SELECT countrycode,GROUP_CONCAT(district) FROM city GROUP BY countrycode;
--- 统计中国每个省的城市名列表
SELECT District,GROUP_CONCAT(NAME)
FROM city
WHERE countrycode='CHN'
GROUP BY district;
--- 统计一下中国,每个省的总人口数
SELECT district ,SUM(population) FROM city
WHERE countrycode='CHN'
GROUP BY district
--- 统计一下中国,每个省的平均人口
SELECT district ,AVG(population) FROM city
WHERE countrycode='CHN'
GROUP BY district
HAVING
--- 统计中国，每个省的总人口大于1000w的省及人口数
SELECT district ,SUM(population) FROM city
WHERE countrycode='CHN'
GROUP BY district
HAVING SUM(population)>10000000
说明: having后的条件是不走索引的，可以进行一些优化手段处理。
ORDER BY
SELECT district ,SUM(population) FROM city
WHERE countrycode='CHN'
GROUP BY district
ORDER BY SUM(population) DESC ;
--- 例子：查询中国所有的城市，并以人口数降序输出
SELECT * FROM city WHERE countrycode='CHN' ORDER BY population DESC;
LIMIT
SELECT *
FROM city
WHERE countrycode='CHN'
ORDER BY population DESC
LIMIT 5;
SELECT *
FROM city
WHERE countrycode='CHN'
ORDER BY population DESC
LIMIT 10;
SELECT *
FROM city
WHERE countrycode='CHN'
ORDER BY population DESC
LIMIT 5,3;
SELECT *
FROM city
WHERE countrycode='CHN'
ORDER BY population DESC
LIMIT 3 OFFSET 5;
LIMIT M,N 跳过M行，显示N行
LIMIT X OFFSET Y 跳过Y行，显示X行
多表连接查询
什么时候用？
需要查询的数据是来自于多张表时。
怎么去多表连接查询
1.传统的连接：基于where条件
(1) 找表之间的关系列
(2) 排列查询条件
select name,countrycode from city whrere population<100;
PCN
select name,surfacearea from country where code='PCN'
--- 人口数量小于100人的城市,所在国家的国土面积(城市名，国家名，国土面积)
select city.name,country.name ,country.surfacearea
from city,country
where city.countrycode = country.code
and city.population<100
2. 内连接 *****
A B
A.x B.y
(1) 找表之间的关系列
(2) 将两表放在join左右
(3) 将关联条件了放在on后面
(4) 将所有的查询条件进行罗列
select A.m,B.n
from
A join B
on A.x=B.y
where
group by
order by
limit
--- 例子：
--- 1. 查询人口数量小于100人的国家名，城市名，国土面积
SELECT country.name,city.name,country.surfacearea
FROM
city JOIN country
ON city.countrycode=country.code
WHERE city.population<100;
--- 2. 查询oldguo老师和他教课程名称
SELECT teacher.tname ,course.cname
FROM teacher
JOIN course
ON teacher.tno=course.tno
WHERE teacher.tname='oldguo';
SELECT teacher.`tname` ,course.`cname`
FROM teacher
JOIN course
ON teacher.`tno`=course.`tno`
WHERE teacher.`tname`='oldboy';
--- 3. 统计一下每门课程的总成绩
SELECT course.cname,SUM(sc.score)
FROM course
JOIN sc
ON course.cno = sc.cno
GROUP BY course.cname;
-- 5.7 版本会报错的情况,在sqlyog中以下操作没问题
-- 但是在命令行上是会报错
SELECT course.cno,course.cname,SUM(sc.score)
FROM course
JOIN sc
ON course.cno = sc.cno
GROUP BY course.cname;
mysql> SELECT course.cno,course.cname,SUM(sc.score)
-> FROM course
-> JOIN sc
-> ON course.cno = sc.cno
-> GROUP BY course.cname;
ERROR 1055 (42000): Expression #1 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'school.course.cno' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by
1. 在select后面出现的列，不是分组条件，并且没有在函数中出现。
2. 如果group by 后是主键列或者是唯一条件列，不会报出错误。如下：
SELECT
course.cno,course.cname,SUM(sc.score) FROM course
JOIN sc
ON course.cno = sc.cno
GROUP BY course.cno;
3. 外链接 ****
--- 4. 查询oldguo老师教的学生姓名列表
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.tname;
--- 5. 查询所有老师教的学生姓名列表
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
GROUP BY teacher.tno;
--- 6. 查询oldboy老师教的不及格学生的姓名
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='oldboy' AND sc.score<60
GROUP BY teacher.tno;
--- 7. 统计zhang3,学习了几门课
SELECT student.`sname` ,COUNT(sc.`cno`)
FROM student
JOIN sc
ON student.`sno`=sc.`sno`
WHERE student.sname='zhang3';
--- 8. 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';
--- 9. 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.tname;
--- 10.查询oldguo所教课程的平均分数
SELECT teacher.tname ,course.`cname`,AVG(sc.`score`)
FROM teacher
JOIN course
ON teacher.`tno`=course.`tno`
JOIN sc
ON course.`cno`=sc.`cno`
WHERE teacher.tname='oldguo';
--- 11.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`
ORDER BY AVG(sc.`score`);
--- 12.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;
--- 13.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;
上述案例表结构
use school
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
drop database school;
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');
INSERT INTO student(sno,sname,sage,ssex)
VALUES
(2,'zhang4',18,'m'),
(3,'li4',18,'m'),
(4,'wang5',19,'f');
INSERT INTO student
VALUES
(5,'zh4',18,'m'),
(6,'zhao4',18,'m'),
(7,'ma6',19,'f');
INSERT INTO student(sname,sage,ssex)
VALUES
('oldboy',20,'m'),
('oldgirl',20,'f'),
('oldp',25,'m');
INSERT INTO teacher(tno,tname) VALUES
(101,'oldboy'),
(102,'hesw'),
(103,'oldguo');
DESC course;
INSERT INTO course(cno,cname,tno)
VALUES
(1001,'linux',101),
(1002,'python',102),
(1003,'mysql',103);
DESC sc;
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);
SELECT * FROM student;
SELECT * FROM teacher;
SELECT * FROM course;
SELECT * FROM sc;
show
show databases; #查看所有数据库
show tables; #查看当前库的所有表
SHOW TABLES FROM #查看某个指定库下的表
show create database world #查看建库语句
show create table world.city #查看建表语句
show grants for root@'localhost' #查看用户的权限信息
show charset； #查看字符集
show collation #查看校对规则
show processlist; #查看数据库连接情况
show index from #表的索引情况
show status #数据库状态查看
SHOW STATUS LIKE '%lock%'; #模糊查询数据库某些状态
SHOW VARIABLES #查看所有配置信息
SHOW variables LIKE '%lock%'; #查看部分配置信息
show engines #查看支持的所有的存储引擎
show engine innodb status\G #查看InnoDB引擎相关的状态信息
show binary logs #列举所有的二进制日志
show master status #查看数据库的日志位置信息
show binlog evnets in #查看二进制日志事件
show slave status \G #查看从库状态
SHOW RELAYLOG EVENTS #查看从库relaylog事件信息
desc (show colums from city) #查看表的列定义信息
Information_schema
DESC information_schema.TABLES
TABLE_SCHEMA ---->库名
TABLE_NAME ---->表名
ENGINE ---->引擎
TABLE_ROWS ---->表的行数
AVG_ROW_LENGTH ---->表中行的平均行（字节）
INDEX_LENGTH ---->索引的占用空间大小（字节）
查询整个数据库中所有库和所对应的表信息
SELECT table_schema,GROUP_CONCAT(table_name)
FROM information_schema.tables
GROUP BY table_schema;
统计所有库下的表个数
SELECT table_schema,COUNT(table_name)
FROM information_schema.TABLES
GROUP BY table_schema
查询所有innodb引擎的表及所在的库
SELECT table_schema,table_name,ENGINE FROM information_schema.`TABLES`
WHERE ENGINE='innodb';
统计world数据库下每张表的磁盘空间占用
SELECT table_name,CONCAT((TABLE_ROWS*AVG_ROW_LENGTH+INDEX_LENGTH)/1024," KB") AS size_KB
FROM information_schema.tables WHERE TABLE_SCHEMA='world';
统计所有数据库的总的磁盘空间占用
SELECT
TABLE_SCHEMA,
CONCAT(SUM(TABLE_ROWS*AVG_ROW_LENGTH+INDEX_LENGTH)/1024," KB") AS Total_KB
FROM information_schema.tables
GROUP BY table_schema;
mysql -uroot -p123 -e "SELECT TABLE_SCHEMA,CONCAT(SUM(TABLE_ROWS*AVG_ROW_LENGTH+INDEX_LENGTH)/1024,' KB') AS Total_KB FROM information_schema.tables GROUP BY table_schema;"
生成整个数据库下的所有表的单独备份语句
模板语句：
mysqldump -uroot -p123 world city >/tmp/world_city.sql
SELECT CONCAT("mysqldump -uroot -p123 ",table_schema," ",table_name," >/tmp/",table_schema,"_",table_name,".sql" )
FROM information_schema.tables
WHERE table_schema NOT IN('information_schema','performance_schema','sys')
INTO OUTFILE '/tmp/bak.sh' ;
CONCAT("mysqldump -uroot -p123 ",table_schema," ",table_name," >/tmp/",table_schema,"_",table_name,".sql" )
107张表，都需要执行以下2条语句
ALTER TABLE world.city DISCARD TABLESPACE;
ALTER TABLE world.city IMPORT TABLESPACE;
SELECT CONCAT("alter table ",table_schema,".",table_name," discard tablespace")
FROM information_schema.tables
WHERE table_schema='world'
INTO OUTFILE '/tmp/dis.sql';
熟悉业务：
刚入职时，DBA的任务
1. 搞清楚架构
通过公司架构图，搞清楚数据库的物理架构
1-2天
逻辑结构：
（1）生产库的信息（容易达到）
（2）库下表的信息（非常复杂）
1. 开发和业务人员，搞好关系
2. 搞到ER图（PD）
3. 啥都没有怎么怎么办？
（1） 找到建表语句，如果有注释，读懂注释。如果没有注释，只能根据列名翻译
（2） 找到表中部分数据 ，分析数据特点，达到了解列功能的目录
