时间：2021-07-01 10:21:17 帮助过：10人阅读
2.删除数据库
drop database zhuziqin;
3.创建一个用户表
mysql> use db1; mysql> create table user(id int,name varchar(30),pass varchar(30));
mysql> show tables;
+---------------+
| Tables_in_db1 |
+---------------+
| user |
+---------------+ mysql> select * from user;
Empty set (0.00 sec
4.插入数据
mysql> insert into user(id,name,pass) values("1","zhangsan","123"); Query OK, 1 row affected (0.01 sec) mysql> select * from user; +------+----------+------+ | id | name | pass | +------+----------+------+ | 1 | zhangsan | 123 | +------+----------+------+ 1 row in set (0.00 sec)
5.删除一个表
drop table user;
6.查看表字段
desc table user;
7.查看表数据
mysql> select * from user; +------+----------+------+ | id | name | pass | +------+----------+------+ | 1 | zhangsan | 123 | +------+----------+------+
8.修改表名
mysql> rename table user to hehe;
9.更新数据
mysql> update hehe set pass="newpass" where pass=123; #把密码更新为newpass
mysql> update hehe set id=10,name="lisi" where id =1; #把id记录的name更新为id10和name为lisi
10.重命名字段名
mysql> alter table hehe change pass newname varchar(30);
11.表中添加一个字段
mysql> alter table hehe add age int; Query OK, 0 rows affected (0.18 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> select * from hehe; +------+------+---------+------+ | id | name | newname | age | +------+------+---------+------+ | 10 | lisi | newpass | NULL | +------+------+---------+------+
12.修改字段
alter table user modify age int no null default 20; #当年龄输入为空时，默认为20岁
13.删除表中的一个字段
alter table user drop age;
14.删除一条记录
delete from user where id=1;
15.查找一条记录
select id,name,pass from user where id=1;
16.删除ID记录
delete from user where id>=3 id<=5; #删除大于3小于5的id
delete from user where id in(1,3,5); #删除1,3,5的id
delete from user where id=1 or id=3 or id=5; #删除1,3,5的id
delete from user where id between 1 and 5; #删除1至5的id
高级查询
1.返回结果删除重复项
select distinct id from user;
2.查询字段中为NULL
select * from where pass is null;
select * from where pass is not null; #查询不为NULL的
3.like模糊查询,包含zhang的列出来
select * from user where name like ‘%zhang%‘;
select * from user where name like ‘%zhang%‘ or name like ‘%li%‘;#查找包含zhang或li的列出来
也可以使用正则表达式查询,生产环境一般不用的，因为查询慢，效果一样
select * from user where name regexp ‘li‘;
4.使用order by对查询结果排序(升序/降序)，默认是升序
select id,name,pass from user order by id asc/desc;
5.使用limit取出排名前三个
select * from user order by id desc limit 3;
6.concat函数使用-字符串连接符
mysql> select id,name,pass,concat(id,‘_‘,name) idname from user;
+----+----------+------+------------+
| id | name | pass | idname |
+----+----------+------+------------+
| 1 | zhangsan | 123 | 1_zhangsan |
| 2 | lisi | 123 | 2_lisi |
| 3 | zhaowu | 123 | 3_zhaowu |
+----+----------+------+------------+
7.rand函数随机排序，如随机抽取前三名
select * from user order by rand() limit 3;
8.count统计,如统计多少个id记录
select count(*) count from user;
#count为自定义显示查询结果后字段名，*为mysql优化后的查询方法，要比直接写入id效率高
select count(id) from user where name=‘zhangsan‘; #统计zhangsan多少条记录
9.sum求和，如统计同消费了多少钱
select sum(id) from user where name=‘lisi‘; #所有id数加在一起
10.avg平均数,如求班级平均分
select avg(id) from user;
11.max最大值，如得到一个最高分
select max(id) from user;
12.min最小值
select min(id) from user;
13.group by分组聚合
select name,count(id) from user group by name order by desc;
#使用count来聚合，基于name分组，再order by排序下（一般排名都使用分组聚合）
select name,count(id) count from user group by name having count>=3;
#查询前三名，having是对分组的结果进行筛选，这不能用where，count代表查询结果后显示的字段名
14.普通多表查询（前提：两个表必须有关系）
先创建两个有关系的表：
create table user(id int unsigned auto_increment primary key,name varchar(30),age int);
create table post(id int unsigned auto_increment primary key,uid int,title varchar(200),content text);
结果一：查询用户发的所有记录
select user.name,post.title,post.content from user,post where user.id=post.uid;
结果二：统计每个用户有多少个记录
select user.name,post.title,post.content count(id) from user,post where user.id=post.uid group by user.name;
15.联表查询（查询成绩）
mysql> select * from tb1;
+------+----------+
| id | name |
+------+----------+
| 1 | zhangsan |
| 2 | lisi |
+------+----------+
mysql> select * from tb2;
+------+-------+
| id | score |
+------+-------+
| 1 | 80 |
| 2 | 81 |
+------+-------+
mysql> select tb1.name,tb2.score from tb1,tb2 where tb1.id = tb2.id;
+----------+-------+
| name | score |
+----------+-------+
| zhangsan | 80 |
| lisi | 81 |
+----------+-------+
二、表字段类型
1.数值
int(size)：整型，只能存整数数字，不能为空，默认允许输入null，也可以设置不允许写（not null）
float：浮点型，可以写入整数或浮点数
1.1字段属性
unsigned：无符号，全是整数
zerofill：与长度无关，不够3位时前面补0，默认看不见
null与not null：允许输入null和不允许输入
default：不允许null情况下，当输入空时，则使用默认值
auto_increment：一般自增ID
1.2示例
例如，设置id为自增：
create table user(id int unsigned auto_increment primary key,name varchar(30),pass varchar(30));
primary key：有auto_increment必须设置为主键索引，提供查询速度，再插入就不用写id列了，如果你写了，就以你写的为准，继续自增。
例如，创建一个表，当输入性别为空时，默认则为男：
create table user2(id int unsigned auto_increment primary key,name varchar(30),sex varchar(5) not null default "nan");
查看字段信息：
mysql> desc user2;
+-------+------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------------+------+-----+---------+----------------+
| id | int(10) unsigned | NO | PRI | NULL | auto_increment |
| name | varchar(30) | YES | | NULL | |
| sex | varchar(5) | NO | | nan | |
+-------+------------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)
2.字符串
char(size）：占用size的字节，但查询速度快，最大支持255个字符
varchar(size)：存多少占多少，剩点空间，最大支持65535个字符
text：支持65535字节
longtext：支持42亿字节
3.日期类型（有的会使用int类型来存时间戳）
以下是官方说法占用的字节，但实际会有不符。
date：年月日，占用3个字节
time：时分秒，占用3个字节
datetime：年月日时分秒，占用8个字节
year：年，占用1个字节
三、用户管理
1.设置Mysql密码
方法一：set password=password（‘newpass‘）;
方法二：update user set password=password（‘newpass‘） where user=‘root‘;
方法三：grant all on *.* to ‘root‘@‘localhost‘ identified by ‘newpass‘;
再刷新：flush privileges;
2.创建用户并授权
grant all privileges on *.* to ‘user‘@‘localhost‘ identified by ‘pass‘;
*.*：对应的是数据库/表的权限
localhost：只允许本地访问，也可以设置%为所有访问
3.撤销用户权限
revoke all privileges on *.* from ‘root‘@‘localhost‘ identified by ‘pass‘;
四、数据库字符集
1.查看服务器基本信息\s
|
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 |
mysql> \s -------------- mysql Ver 14.14 Distrib 5.5.34, for Linux (x86_64) using EditLine wrapper Connection id: 141700 Current database: lab Current user: root@localhost SSL: Not in use Current pager: stdout Using outfile: ‘‘ Using delimiter: ; Server version: 5.5.34-log Source distribution Protocol version: 10 Connection: Localhost via UNIX socket Server characterset: latin1 Db characterset: latin1 Client characterset: latin1 Conn. characterset: latin1 UNIX socket: /tmp/mysql.sock Uptime: 47 days 4 hours 54 min 1 sec Threads: 4 Questions: 189434783 Slow queries: 1 Opens: 283512 Flush tables: 23 Open tables: 46 Queries per second avg: 46.447 -------------- |
可以看到四种默认字符集是latin1，修改四种默认字符集，client和connection两个要是一样的。
set character_set_server=utf8;
set character_set_database=utf8;
set character_set_client=utf8;
set character_set_connection=utf8;
也可以修改my.cnf默认字符集
[mysqld]
character_set_server=utf8
[mysql]
default-character-set=utf8
重启mysql！
2.查看创建数据库时使用的字符集
show create database mydb;
五、创建表字段索引
1.主键索引
1.1创建一个主键索引，一般创建表时已经设置
alter table user add primary key(id);
1.2删除一个主键索引
alter table user drop primary key;
2.普通索引
2.1创建一个普通索引
alter table user add index index_name(name);
2.2删除一个普通索引
alter table user drop index index_name;
2.3查看主键索引和普通索引
show index from user;
show keys from user;
注：修改字段名不影响索引。
3.查看表中创建的索引
mysql> desc user2;
+-------+------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------------+------+-----+---------+----------------+
| id | int(10) unsigned | NO | PRI | NULL | auto_increment |
| name | varchar(30) | YES | MUL | NULL | |
| pass | varchar(30) | YES | | NULL | |
+-------+------------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)
上面key中的PRI代表是主键索引，MUL代表是普通索引。
4.查看索引信息
mysql> show index from user2;
+-------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| user2 | 0 | PRIMARY | 1 | id | A | 1 | NULL | NULL | | BTREE | | |
| user2 | 1 | index_name | 1 | name | A | 1 | NULL | NULL | YES | BTREE | | |
+-------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.00 sec)
可以看到user表，做了PRIMARY主键索引和index_name名字的普通索引。
5.测试是否使用索引
mysql> desc select id,name from user2 where name=‘zhangsan‘;
+----+-------------+-------+------+---------------+------------+---------+-------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------------+---------+-------+------+--------------------------+
| 1 | SIMPLE | user2 | ref | index_name | index_name | 33 | const | 1 | Using where; Using index |
+----+-------------+-------+------+---------------+------------+---------+-------+------+--------------------------+
1 row in set (0.00 sec)
用desc检测语句，可以看到查询一条张三的记录，使用了普通索引index_name这个名字，检索了一行(rows)就找到了这条记录。
SQL的使用
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