Linux学习--MySQL学习之查询语句

所有实现基于mysql8.0.26实现，事例代码

1：常用函数

• 字符函数
• 数学函数
• 日期函数
• 聚集函数
• 数学计算
• if函数
• case函数

函数：MySQL服务内置命令
语法：函数名(表头名)
select格式
SELECT  函数(表头名)  FROM  库名.表名；
SELECT  函数(表头名)  FROM  库名.表名 WHERE 条件；

1.字符函数(处理字符或字符类型的表头)
mysql> use tarena;
Database changed
mysql> select name from user where name='root';
+------+
| name |
+------+
| root |
+------+
1 row in set (0.00 sec)
#获取字符字节长度length
mysql> select name,length(name) as 字节个数 from user where name='root';
+------+--------------+
| name | 字节个数     |
+------+--------------+
| root |            4 |
+------+--------------+
1 row in set (0.00 sec)
mysql> select name,length(name) from employees where employee_id=3;
+-----------+--------------+
| name      | length(name) |
+-----------+--------------+
| 李玉英    |            9 |
+-----------+--------------+
1 row in set (0.00 sec)
mysql> select name from employees where employee_id=3;
+-----------+
| name      |
+-----------+
| 李玉英    |
+-----------+
1 row in set (0.00 sec)
#获取字符长度char_length
mysql> select name,char_length(name) from employees where employee_id=3;
+-----------+-------------------+
| name      | char_length(name) |
+-----------+-------------------+
| 李玉英    |                 3 |
+-----------+-------------------+
1 row in set (0.00 sec)

mysql> select name from user where uid <=3;
+--------+
| name   |
+--------+
| root   |
| bin    |
| daemon |
| adm    |
+--------+
4 rows in set (0.00 sec)
#upper转大写
mysql> select upper(name) from user where uid <=3;
+-------------+
| upper(name) |
+-------------+
| ROOT        |
| BIN         |
| DAEMON      |
| ADM         |
+-------------+
4 rows in set (0.00 sec)

mysql> select ucase(name) from user where uid <=3;
+-------------+
| ucase(name) |
+-------------+
| ROOT        |
| BIN         |
| DAEMON      |
| ADM         |
+-------------+
4 rows in set (0.00 sec)

mysql> select lower("ABCD");
+---------------+
| lower("ABCD") |
+---------------+
| abcd          |
+---------------+
1 row in set (0.00 sec)
#lcase将信息转换为小写
mysql> select lcase("ABCD");
+---------------+
| lcase("ABCD") |
+---------------+
| abcd          |
+---------------+
1 row in set (0.00 sec)

mysql> select name from employees where employee_id <=3;
+-----------+
| name      |
+-----------+
| 梁伟      |
| 郭岩      |
| 李玉英    |
+-----------+
3 rows in set (0.00 sec)
//substr(s,start,end)从s的start位置开始取出end长度的子串
mysql> select substr(name,2,3) from employees where employee_id<=3;
+------------------+
| substr(name,2,3) |
+------------------+
| 伟               |
| 岩               |
| 玉英             |
+------------------+
3 rows in set (0.00 sec)
mysql> select instr(name,'a') from user where uid <=3;
+-----------------+
| instr(name,'a') |
+-----------------+
|               0 |
|               0 |
|               2 |
|               1 |
+-----------------+
4 rows in set (0.00 sec)
#instr(str,str1)返回str1参数，在str参数内的位置
mysql> select name,instr(name,"英") from employees;
+-----------+-------------------+
| name      | instr(name,"英")  |
+-----------+-------------------+
| 梁伟      |                 0 |
| 郭岩      |                 0 |
| 李玉英    |                 3 |
| 张健      |                 0 |
| 郑静      |                 0 |
| 牛建军    |                 0 |
| 刘斌      |                 0 |
| 汪云      |                 0 |
...
#去除字符串两边空格
mysql> select trim("    ABC  ");
+-------------------+
| trim("    ABC  ") |
+-------------------+
| ABC               |
+-------------------+
1 row in set (0.00 sec)
#2.数学函数(处理数字或数值类型的表头)
//ABS(x)    返回x的绝对值
mysql> select abs(11);
+---------+
| abs(11) |
+---------+
|      11 |
+---------+
1 row in set (0.00 sec)
mysql> select abs(-11);
+----------+
| abs(-11) |
+----------+
|       11 |
+----------+
1 row in set (0.00 sec)
mysql>
//PI()        返回圆周率π，默认显示6位小数
mysql> select pi() ;
+----------+
| pi()     |
+----------+
| 3.141593 |
+----------+
1 row in set (0.00 sec)
//MOD(x,y)    返回x被y除后的余数 
mysql> select mod(10,3);
+-----------+
| mod(10,3) |
+-----------+
|         1 |
+-----------+
1 row in set (0.00 sec)
//输出1-10之间的偶数uid号
mysql> select name , uid from tarena.user where uid between 1 and 10 and  mod(uid,2) = 0 ;
+----------+------+
| name     | uid  |
+----------+------+
| daemon   |    2 |
| lp       |    4 |
| shutdown |    6 |
| mail     |    8 |
+----------+------+
4 rows in set (0.00 sec)
//CEIL(x)、CEILING(x)    返回不小于x的最小整数 (x 是小数)
mysql> select ceil(9.23);
+------------+
| ceil(9.23) |
+------------+
|         10 |
+------------+
1 row in set (0.00 sec)
mysql> select ceiling(9.23);
+---------------+
| ceiling(9.23) |
+---------------+
|            10 |
+---------------+
1 row in set (0.00 sec)
mysql>
//FLOOR(x）            返回不大于x的最大整数 (x 是有小数的数字)
mysql> select floor(9.23);
+-------------+
| floor(9.23) |
+-------------+
|           9 |
+-------------+
1 row in set (0.00 sec)
#3.日期时间函数
![日期时间函数](https://img-blog.csdnimg.cn/cdda5eb7e0f34d9aaf432774b9b5ebbb.png)
mysql> select curtime(); //获取系统时间
+-----------+
| curtime() |
+-----------+
| 17:42:20  |
+-----------+
1 row in set (0.00 sec)
mysql> select curdate();//获取系统日期
+------------+
| curdate()  |
+------------+
| 2023-05-24 |
+------------+
1 row in set (0.00 sec)
mysql> select now() ;//获取系统日期+时间
+---------------------+
| now()               |
+---------------------+
| 2023-05-24 17:42:29 |
+---------------------+
1 row in set (0.00 sec)
mysql> select year(now()) ; //获取系统当前年
+-------------+
| year(now()) |
+-------------+
|        2023 |
+-------------+
1 row in set (0.00 sec)
mysql> select month(now()) ; //获取系统当前月
+--------------+
| month(now()) |
+--------------+
|            5 |
+--------------+
1 row in set (0.00 sec)
mysql> select day(now()) ; //获取系统当前日
+------------+
| day(now()) |
+------------+
|         24 |
+------------+
1 row in set (0.00 sec)
mysql> select hour(now()) ; //获取系统当前小时
+-------------+
| hour(now()) |
+-------------+
|          17 |
+-------------+
1 row in set (0.00 sec)
mysql> select minute(now()) ; //获取系统当分钟
+---------------+
| minute(now()) |
+---------------+
|            46 |
+---------------+
1 row in set (0.00 sec)
mysql> select second(now()) ; //获取系统当前秒
+---------------+
| second(now()) |
+---------------+
|            34 |
+---------------+
1 row in set (0.00 sec)
mysql> select time(now()) ;//获取当前系统时间
+-------------+
| time(now()) |
+-------------+
| 17:47:36    |
+-------------+
1 row in set (0.00 sec)
mysql> select date(now()) ; //获取当前系统日期
+-------------+
| date(now()) |
+-------------+
| 2023-05-24  |
+-------------+
1 row in set (0.00 sec)
mysql> select curdate();//获取当前系统日志
+------------+
| curdate()  |
+------------+
| 2023-05-24 |
+------------+
1 row in set (0.00 sec)
mysql> select dayofmonth(curdate());//获取一个月的第几天
+-----------------------+
| dayofmonth(curdate()) |
+-----------------------+
|                    24 |
+-----------------------+
1 row in set (0.00 sec)
mysql> select dayofyear(curdate());//获取一年中的第几天
+----------------------+
| dayofyear(curdate()) |
+----------------------+
|                  144 |
+----------------------+
1 row in set (0.00 sec)
mysql>
mysql> select monthname(curdate());//获取月份名
+----------------------+
| monthname(curdate()) |
+----------------------+
| May                  |
+----------------------+
1 row in set (0.00 sec)
mysql> select dayname(curdate());//获取星期名
+--------------------+
| dayname(curdate()) |
+--------------------+
| Wednesday          |
+--------------------+
1 row in set (0.00 sec)
mysql> select quarter(curdate());//获取一年中的第几季度
+--------------------+
| quarter(curdate()) |
+--------------------+
|                  2 |
+--------------------+
1 row in set (0.00 sec)
mysql> select week(now());//一年中的第几周
+-------------+
| week(now()) |
+-------------+
|          21 |
+-------------+
1 row in set (0.00 sec)
mysql> select weekday(now())；//一周中的周几 
+----------------+
| weekday(now()) |
+----------------+
|              2 |
+----------------+
1 row in set (0.00 sec)
#4.聚合函数
#求和函数sum
mysql> select sum(basic) from salary where employee_id=3 and year(date)=2018;
+------------+
| sum(basic) |
+------------+
|     111595 |
+------------+
1 row in set (0.00 sec)
#avg(表头名) 计算平均值
mysql> select avg(basic) from salary where employee_id=3 and year(date)=2018;
+------------+
| avg(basic) |
+------------+
|  9299.5833 |
+------------+
1 row in set (0.00 sec)
#min(表头名) 获取最小值
mysql> select min(basic) from salary where employee_id=3 and year(date)=2018;
+------------+
| min(basic) |
+------------+
|       9261 |
+------------+
1 row in set (0.00 sec)
#max(表头名) 获取最大值
mysql> select max(basic) from salary where employee_id=3 and year(date)=2018;
+------------+
| max(basic) |
+------------+
|       9724 |
+------------+
1 row in set (0.00 sec)
#count(表头名) 统计表头值个数----统计3号员工2018年奖金小于3000的次数
mysql> select count(bonus) from salary where employee_id=3 and year(date)=2018 and bonus<3000;
+--------------+
| count(bonus) |
+--------------+
|            3 |
+--------------+
1 row in set (0.00 sec)
#5.数学计算
![数学计算](https://img-blog.csdnimg.cn/3f38703a72b14b3fa23de4c7dbdec92c.png)
#输出8号员工2019年1月10工资总和
mysql> select employee_id ,date , basic +  bonus  as 总工资 from salary 
where employee_id = 8 and date=20190110;
+-------------+------------+----------------+
| employee_id | date       |     总工资       |
+-------------+------------+----------------+
|           8 | 2019-01-10 |          24093 |
+-------------+------------+----------------+
#输出8号员工的名字和年龄
mysql> select name,year(now())-year(birth_date) as 年龄 from employees where employee_id=8;
+--------+--------+
| name   | 年龄   |
+--------+--------+
| 汪云   |     30 |
+--------+--------+
1 row in set (0.00 sec)
#查看8号员工2019年1月10 基本工资翻3倍的值
mysql> select employee_id , basic , basic * 3  as 工资翻三倍  from salary 
where  employee_id=8  and date=20190110;
+-------------+-------+-----------------+
| employee_id | basic | 工资翻三倍      |
+-------------+-------+-----------------+
|           8 | 23093 |           69279 |
+-------------+-------+-----------------+
1 row in set (0.00 sec)
#输出员工编号1-10之间偶数员工编号及对应的员工名
mysql> select employee_id , name  from  tarena.employees  
where  employee_id  between 1 and 10  and  employee_id % 2  =  0   ;
+-------------+-----------+
| employee_id | name      |
+-------------+-----------+
|           2 | 郭岩      |
|           4 | 张健      |
|           6 | 牛建军    |
|           8 | 汪云      |
|          10 | 郭娟      |
+-------------+-----------+
5 rows in set (0.00 sec)
#6.if函数
if(条件,v1,v2) 如果条件是TRUE则返回v1，否则返回v2
ifnull(v1,v2) 如果v1不为NULL，则返回v1，否则返回v2
mysql> select  if(1 = 2 , "a","b");
+---------------------+
| if(1 = 2 , "a","b") |
+---------------------+
| b                   |
+---------------------+
1 row in set (0.00 sec)
mysql> select  if( 1 = 1 , "a","b");
+---------------------+
| if(1 = 1 , "a","b") |
+---------------------+
| a                   |
+---------------------+
1 row in set (0.00 sec)
 mysql> select  ifnull("abc","xxx");
+---------------------+
| ifnull("abc","xxx") |
+---------------------+
| abc                 |
+---------------------+
1 row in set (0.00 sec)
mysql> select  ifnull(null,"xxx");
+--------------------+
| ifnull(null,"xxx") |
+--------------------+
| xxx                |
+--------------------+
1 row in set (0.00 sec)
mysql> select name , uid  , 
#cold_boldif(uid < 1000 , "系统用户","创建用户") as 用户类型  from tarena.user;
+-----------------+-------+--------------+
| name            | uid   | 用户类型     |
+-----------------+-------+--------------+
| root            |     0 | 系统用户     |
| bin             |     1 | 系统用户     |
| daemon          |     2 | 系统用户     |
| adm             |     3 | 系统用户     |
| lp              |     4 | 系统用户     |
| sync            |     5 | 系统用户     |
...
+-----------------+-------+--------------+
27 rows in set (0.00 sec)
mysql>  select name , shell  , 
#cold_boldif(shell = "/bin/bash" , "交互用户","非交户用户") as 用户类型 from tarena.user;
+-----------------+----------------+-----------------+
| name            | shell          | 用户类型        |
+-----------------+----------------+-----------------+
| root            | /bin/bash      | 交互用户        |
| bin             | /sbin/nologin  | 非交户用户      |
| daemon          | /sbin/nologin  | 非交户用户      |
| adm             | /sbin/nologin  | 非交户用户      |
| lp              | /sbin/nologin  | 非交户用户      |
| sync            | /bin/sync      | 非交户用户      |
| shutdown        | /sbin/shutdown | 非交户用户      |
...
+-----------------+----------------+-----------------+
27 rows in set (0.00 sec)

mysql> insert   into   user (name, homedir) values ("jerrya",null);
mysql>  select name  姓名, ifnull(homedir,"NO  home")as 家目录  from  tarena.user;
+-----------------+--------------------+
| 姓名            | 家目录             |
+-----------------+--------------------+
| root            | /root              |
| bin             | /bin               |
| daemon          | /sbin              |
...
| apache          | /usr/share/httpd   |
| mysql           | /var/lib/mysql     |
| bob             | NO  home           |
| jerrya          | NO  home           |
+-----------------+--------------------+
28 rows in set (0.00 sec)
#7.case函数
命令格式
CASE 表头名              
WHEN 值1 THEN 输出结果 
WHEN 值2 THEN 输出结果  
WHEN 值3 THEN 输出结果 
ELSE 输出结果  
END
或
CASE              
WHEN  判断条件1 THEN 输出结果 
WHEN  判断条件2 THEN 输出结果  
WHEN  判断条件3 THEN 输出结果 
ELSE  输出结果  
END
mysql> select  * from tarena.departments;
+---------+-----------+
| dept_id | dept_name |
+---------+-----------+
|       1 | 人事部    |
|       2 | 财务部    |
|       3 | 运维部    |
|       4 | 开发部    |
|       5 | 测试部    |
|       6 | 市场部    |
|       7 | 销售部    |
|       8 | 法务部    |
+---------+-----------+
8 rows in set (0.03 sec)
//输出部门类型
select dept_id, dept_name,
case dept_name
when '运维部' then '技术部门'
when '开发部' then '技术部门'
when '测试部' then '技术部门'
else '非技术部门'
end as  部门类型   from  tarena.departments;
+---------+-----------+-----------------+
| dept_id | dept_name | 部门类型        |
+---------+-----------+-----------------+
|       1 | 人事部    | 非技术部门      |
|       2 | 财务部    | 非技术部门      |
|       3 | 运维部    | 技术部门        |
|       4 | 开发部    | 技术部门        |
|       5 | 测试部    | 技术部门        |
|       6 | 市场部    | 非技术部门      |
|       7 | 销售部    | 非技术部门      |
|       8 | 法务部    | 非技术部门      |
+---------+-----------+-----------------+
8 rows in set (0.00 sec)
或
mysql> select dept_id,dept_name,
    -> case
    -> when dept_name="运维部"  then "技术部"
    -> when dept_name="开发部"  then "技术部"
    -> when dept_name="测试部"  then "技术部"
    -> else "非技术部"
    -> end as 部门类型  from  tarena.departments;
+---------+-----------+--------------+
| dept_id | dept_name | 部门类型     |
+---------+-----------+--------------+
|       1 | 人事部    | 非技术部     |
|       2 | 财务部    | 非技术部     |
|       3 | 运维部    | 技术部       |
|       4 | 开发部    | 技术部       |
|       5 | 测试部    | 技术部       |
|       6 | 市场部    | 非技术部     |
|       7 | 销售部    | 非技术部     |
|       8 | 法务部    | 非技术部     |
+---------+-----------+--------------+
8 rows in set (0.00 sec)
或
mysql> select dept_id,dept_name,
    -> case
    -> when dept_name in ("运维部","开发部","测试部") then "技术部"
    -> else "非技术部"
    -> end as 部门类型  from  tarena.departments;
+---------+-----------+--------------+
| dept_id | dept_name | 部门类型     |
+---------+-----------+--------------+
|       1 | 人事部    | 非技术部     |
|       2 | 财务部    | 非技术部     |
|       3 | 运维部    | 技术部       |
|       4 | 开发部    | 技术部       |
|       5 | 测试部    | 技术部       |
|       6 | 市场部    | 非技术部     |
|       7 | 销售部    | 非技术部     |
|       8 | 法务部    | 非技术部     |
+---------+-----------+--------------+
8 rows in set (0.00 sec)

2：查询结果处理

- 分组
- 排序
- 过滤
- 分页
语法格式
SELECT 表头名 FROM 库名.表名 [WHERE条件] 分组 | 排序 | 过滤 | 分页;
#1.分组练习
#输出符合条件的shell和name
mysql> select shell as 解释器,count(name) as 总人数 from user where shell in ("/bin/bash","/sbin/nologin") group by shell;
+---------------+-----------+
| 解释器        | 总人数    |
+---------------+-----------+
| /bin/bash     |         2 |
| /sbin/nologin |        20 |
+---------------+-----------+
2 rows in set (0.00 sec)
#统计每个部门的总人数
mysql> select dept_name, emp.dept_id,count(name) from employees emp,departments dep where emp.dept_id=dep.dept_id group by emp.dept_id;
+-----------+---------+-------------+
| dept_name | dept_id | count(name) |
+-----------+---------+-------------+
| 人事部    |       1 |           8 |
| 财务部    |       2 |           5 |
| 运维部    |       3 |           6 |
| 开发部    |       4 |          55 |
| 测试部    |       5 |          12 |
| 市场部    |       6 |           9 |
| 销售部    |       7 |          35 |
| 法务部    |       8 |           3 |
+-----------+---------+-------------+
8 rows in set (0.00 sec)
#2.排序
#按uid升序排序
mysql> select name,uid from user where uid is not null and uid between 100 and 1000 order by uid;
+-----------------+------+
| name            | uid  |
+-----------------+------+
| haproxy         |  188 |
| systemd-network |  192 |
| chrony          |  998 |
| polkitd         |  999 |
| plj             | 1000 |
+-----------------+------+
5 rows in set (0.00 sec)
#按uid降序排序
mysql> select name,uid from user where uid is not null and uid between 100 and 1000 order by uid desc;
+-----------------+------+
| name            | uid  |
+-----------------+------+
| plj             | 1000 |
| polkitd         |  999 |
| chrony          |  998 |
| systemd-network |  192 |
| haproxy         |  188 |
+-----------------+------+
5 rows in set (0.00 sec)
#查看2015年1月10号员工编号小于10的工资总额
mysql> select *,basic+bonus as 工资总额 from salary where employee_id<10 and date='20150110';
+----+------------+-------------+-------+-------+--------------+
| id | date       | employee_id | basic | bonus | 工资总额     |
+----+------------+-------------+-------+-------+--------------+
|  2 | 2015-01-10 |           2 | 17000 | 10000 |        27000 |
|  3 | 2015-01-10 |           3 |  8000 |  2000 |        10000 |
|  4 | 2015-01-10 |           4 | 14000 |  9000 |        23000 |
|  6 | 2015-01-10 |           6 | 14000 | 10000 |        24000 |
|  7 | 2015-01-10 |           7 | 19000 | 10000 |        29000 |
+----+------------+-------------+-------+-------+--------------+
5 rows in set (0.00 sec)
#以工资总额升序排，总额相同按员工编号升序排
mysql> select *,basic+bonus as total from salary where date=20150110 and employee_id<10  order by total,employee_id;
+----+------------+-------------+-------+-------+-------+
| id | date       | employee_id | basic | bonus | total |
+----+------------+-------------+-------+-------+-------+
|  3 | 2015-01-10 |           3 |  8000 |  2000 | 10000 |
|  4 | 2015-01-10 |           4 | 14000 |  9000 | 23000 |
|  6 | 2015-01-10 |           6 | 14000 | 10000 | 24000 |
|  2 | 2015-01-10 |           2 | 17000 | 10000 | 27000 |
|  7 | 2015-01-10 |           7 | 19000 | 10000 | 29000 |
+----+------------+-------------+-------+-------+-------+
5 rows in set (0.00 sec)
#3.过滤
#查找到的数据里过滤符合条件的数据
select 表头名  from 库.表  where 筛选条件  having     筛选条件;
#查找部门总人数少于10人的部门名称及人数
mysql> select dept_id,count(name) as total  from employees emp group by dept_id having total<10;
+---------+-------+
| dept_id | total |
+---------+-------+
|       1 |     8 |
|       2 |     5 |
|       3 |     6 |
|       6 |     9 |
|       8 |     3 |
+---------+-------+
5 rows in set (0.00 sec)
mysql> select dep.dept_name,count(name) as total  from employees emp,departments dep where dep.dept_id = emp.dept_id  group by dep.dept_id having total<10;
+-----------+-------+
| dept_name | total |
+-----------+-------+
| 人事部    |     8 |
| 财务部    |     5 |
| 运维部    |     6 |
| 市场部    |     9 |
| 法务部    |     3 |
+-----------+-------+
5 rows in set (0.00 sec)
#4.分页
#限制查询结果显示行数（默认显示全部查询结果）
#使用SELECT查询时，如果结果集数据量很大，比如1万行数据，放在一个页面显示的话数据量太大，可以分100次显示 每次只显示100行。
#语法
SELECT语句  LIMIT  数字;            //显示查询结果前多少条记录
SELECT语句  LIMIT  数字1,数字2;    //显示指定范围内的查询记录
数字1  表示起始行 （0表示第1行） 数字2表示总行数

#只显示查询结果的第1行
mysql> select * from user where shell is not null limit 1;
+----+------+----------+------+------+---------+---------+-----------+
| id | name | password | uid  | gid  | comment | homedir | shell     |
+----+------+----------+------+------+---------+---------+-----------+
|  1 | root | x        |    0 |    0 | root    | /root   | /bin/bash |
+----+------+----------+------+------+---------+---------+-----------+
1 row in set (0.00 sec)
#只显示查询结果的前3行
mysql> select * from user where shell is not null limit 3;
+----+--------+----------+------+------+---------+---------+---------------+
| id | name   | password | uid  | gid  | comment | homedir | shell         |
+----+--------+----------+------+------+---------+---------+---------------+
|  1 | root   | x        |    0 |    0 | root    | /root   | /bin/bash     |
|  2 | bin    | x        |    1 |    1 | bin     | /bin    | /sbin/nologin |
|  3 | daemon | x        |    2 |    2 | daemon  | /sbin   | /sbin/nologin |
+----+--------+----------+------+------+---------+---------+---------------+
3 rows in set (0.00 sec)
#从查询结果的第3行
mysql> mysql> select * from user where shell is not null limit 3,4;
+----+----------+----------+------+------+----------+----------------+----------------+
| id | name     | password | uid  | gid  | comment  | homedir        | shell          |
+----+----------+----------+------+------+----------+----------------+----------------+
|  4 | adm      | x        |    3 |    4 | adm      | /var/adm       | /sbin/nologin  |
|  5 | lp       | x        |    4 |    7 | lp       | /var/spool/lpd | /sbin/nologin  |
|  6 | sync     | x        |    5 |    0 | sync     | /sbin          | /bin/sync      |
|  7 | shutdown | x        |    6 |    0 | shutdown | /sbin          | /sbin/shutdown |
+----+----------+----------+------+------+----------+----------------+----------------+
4 rows in set (0.00 sec)
#查看uid 号最大的用户名和UID
mysql> select name,uid from user order by uid desc limit 1;
+-----------+-------+
| name      | uid   |
+-----------+-------+
| nfsnobody | 65534 |
+-----------+-------+
1 row in set (0.00 sec)

3：管理表记录

- 插入表记录
- 修改表记录
- 删除表记录

#插入1条记录
mysql> insert into user values(40,"jingyaya","x",1001,1001,"teacher","/home/jingyaya","/bin/bash");
Query OK, 1 row affected (0.05 sec)
#查看表记录
mysql> select  * from  tarena.user where name="jingyaya";
+----+----------+----------+------+------+---------+----------------+-----------+
| id | name     | password | uid  | gid  | comment | homedir        | shell     |
+----+----------+----------+------+------+---------+----------------+-----------+
| 40 | jingyaya | x        | 1001 | 1001 | teacher | /home/jingyaya | /bin/bash |
+----+----------+----------+------+------+---------+----------------+-----------+
1 row in set (0.00 sec)
#插入多行记录给所有列赋值
insert into tarena.user values
(41,"jingyaya2","x",1002,1002,"teacher","/home/jingyaya2","/bin/bash"),
(42,"jingyaya3","x",1003,1003,"teacher","/home/jingyaya3","/bin/bash");

#插入1行给指定列赋值，必须写列名，没赋值的列 没有数据 后通过设置的默认值赋值
mysql> insert into tarena.user(name,uid,shell)values("benben",1002,"/sbin/nologin");
#插入多行给指定列赋值，必须写列名，没赋值的列 没有数据 后通过设置的默认值赋值
mysql> insert into tarena.user(name,uid,shell)values("benben2",1002,"/sbin/nologin"),("benben3",1003,"/sbin/nologin");
#查看记录
mysql> select  * from tarena.user where name like  "benben%";
+----+---------+----------+------+------+---------+---------+---------------+
| id | name    | password | uid  | gid  | comment | homedir | shell         |
+----+---------+----------+------+------+---------+---------+---------------+
| 41 | benben  | NULL     | 1002 | NULL | NULL    | NULL    | /sbin/nologin |
| 42 | benben2 | NULL     | 1002 | NULL | NULL    | NULL    | /sbin/nologin |
| 43 | benben3 | NULL     | 1003 | NULL | NULL    | NULL    | /sbin/nologin |
+----+---------+----------+------+------+---------+---------+---------------+
3 rows in set (0.00 sec)

#2.修改表记录
mysql> update tarena.user set comment=NULL where id <= 10 ;
Query OK, 10 rows affected (0.09 sec)
Rows matched: 10  Changed: 10  Warnings: 0
//修改后查看
mysql> select  name , comment from tarena.user where id <= 10 ;
+----------+---------+
| name     | comment |
+----------+---------+
| root     | NULL    |
| bin      | NULL    |
| daemon   | NULL    |
...
| shutdown | NULL    |
| halt     | NULL    |
| mail     | NULL    |
| operator | NULL    |
+----------+---------+
10 rows in set (0.00 sec) [root@localhost ~]#
//修改前查看
mysql> select name , homedir  from tarena.user;
+------------------+--------------------+
| name             | homedir            |
+------------------+--------------------+
| root             | /root              |
| bin              | /bin               |
| daemon           | /sbin              |
| adm              | /var/adm           |
...
| plj              | /home/plj          |
| apache           | /usr/share/httpd   |
| mysql            | /var/lib/mysql     |
| bob              | NULL               |
| jerrya           | NULL               |
| jingyaya         | /home/jingyaya     |
| benben           | NULL               |
| benben2          | NULL               |
| benben3          | NULL               |
| mysql.infoschema | NULL               |
| mysql.session    | NULL               |
| mysql.sys        | NULL               |
| root             | NULL               |
+------------------+--------------------+
36 rows in set (0.00 sec)
//不加条件批量修改
mysql> update  tarena.user set homedir="/student" ;
Query OK, 36 rows affected (0.09 sec)
Rows matched: 36  Changed: 36  Warnings: 0
//修改后查看
mysql> select name , homedir  from tarena.user;
+------------------+----------+
| name             | homedir  |
+------------------+----------+
| root             | /student |
| bin              | /student |
| daemon           | /student |
| adm              | /student |
| lp               | /student |
| sync             | /student |
| shutdown         | /student |
| halt             | /student |
...
| mysql.sys        | /student |
| root             | /student |
+------------------+----------+
36 rows in set (0.00 sec)
#3.删除表记录
//仅删除与条件匹配的行
mysql> delete from tarena.user where id <= 10 ;
Query OK, 10 rows affected (0.06 sec)
//查不到符合条件的记录了
mysql> select  * from tarena.user where id <= 10 ;
Empty set (0.00 sec)