15、排序
15.1、查询所有员工薪资，排序？
select ename,sal from emp order by sal; // 默认是升序！！！ +--------+---------+ | ename | sal | +--------+---------+ | SMITH | 800.00 | | JAMES | 950.00 | | ADAMS | 1100.00 | | WARD | 1250.00 | | MARTIN | 1250.00 | | MILLER | 1300.00 | | TURNER | 1500.00 | | ALLEN | 1600.00 | | CLARK | 2450.00 | | BLAKE | 2850.00 | | JONES | 2975.00 | | FORD | 3000.00 | | SCOTT | 3000.00 | | KING | 5000.00 | +--------+---------+
15.2、怎么降序？
指定降序：
select ename,sal from emp order by sal desc; +--------+---------+ | ename | sal | +--------+---------+ | KING | 5000.00 | | SCOTT | 3000.00 | | FORD | 3000.00 | | JONES | 2975.00 | | BLAKE | 2850.00 | | CLARK | 2450.00 | | ALLEN | 1600.00 | | TURNER | 1500.00 | | MILLER | 1300.00 | | MARTIN | 1250.00 | | WARD | 1250.00 | | ADAMS | 1100.00 | | JAMES | 950.00 | | SMITH | 800.00 | +--------+---------+
指定升序？
select ename,sal from emp order by sal asc; +--------+---------+ | ename | sal | +--------+---------+ | SMITH | 800.00 | | JAMES | 950.00 | | ADAMS | 1100.00 | | WARD | 1250.00 | | MARTIN | 1250.00 | | MILLER | 1300.00 | | TURNER | 1500.00 | | ALLEN | 1600.00 | | CLARK | 2450.00 | | BLAKE | 2850.00 | | JONES | 2975.00 | | FORD | 3000.00 | | SCOTT | 3000.00 | | KING | 5000.00 | +--------+---------+
15.3、可以两个字段排序吗？或者说按照多个字段排序？
查询员工名字和薪资，要求按照薪资升序，如果薪资一样的话，
再按照名字升序排列。
select ename,sal from emp order by sal asc, ename asc; // sal在前，起主导，只有sal相等的时候，才会考虑启用ename排序。 +--------+---------+ | ename | sal | +--------+---------+ | SMITH | 800.00 | | JAMES | 950.00 | | ADAMS | 1100.00 | | MARTIN | 1250.00 | | WARD | 1250.00 | | MILLER | 1300.00 | | TURNER | 1500.00 | | ALLEN | 1600.00 | | CLARK | 2450.00 | | BLAKE | 2850.00 | | JONES | 2975.00 | | FORD | 3000.00 | | SCOTT | 3000.00 | | KING | 5000.00 | +--------+---------+
15.4、了解：根据字段的位置也可以排序
select ename,sal from emp order by 2; // 2表示第二列。第二列是sal
按照查询结果的第2列sal排序。
了解一下，不建议在开发中这样写，因为不健壮。
因为列的顺序很容易发生改变，列顺序修改之后，2就废了。
16、综合一点的案例：
找出工资在1250到3000之间的员工信息，要求按照薪资降序排列。
select ename,sal from emp where sal between 1250 and 3000 order by sal desc; +--------+---------+ | ename | sal | +--------+---------+ | FORD | 3000.00 | | SCOTT | 3000.00 | | JONES | 2975.00 | | BLAKE | 2850.00 | | CLARK | 2450.00 | | ALLEN | 1600.00 | | TURNER | 1500.00 | | MILLER | 1300.00 | | MARTIN | 1250.00 | | WARD | 1250.00 | +--------+---------+
关键字顺序不能变：
select
...
from
...
where
...
order by
...
以上语句的执行顺序必须掌握：
第一步：from
第二步：where
第三步：select
第四步：order by（排序总是在最后执行！）
17、数据处理函数
17.1、数据处理函数又被称为单行处理函数
单行处理函数的特点：一个输入对应一个输出。
和单行处理函数相对的是：多行处理函数。（多行处理函数特点：多个输入，对应1个输出！）
17.2、单行处理函数常见的有哪些？
lower 转换小写
mysql> select lower(ename) as ename from emp; +--------+ | ename | +--------+ | smith | | allen | | ward | | jones | | martin | | blake | | clark | | scott | | king | | turner | | adams | | james | | ford | | miller | +--------+
14个输入，最后还是14个输出。这是单行处理函数的特点。 upper 转换大写
mysql> select * from t_student; +----------+ | name | +----------+ | zhangsan | | lisi | | wangwu | | jack_son | +----------+ mysql> select upper(name) as name from t_student; +----------+ | name | +----------+ | ZHANGSAN | | LISI | | WANGWU | | JACK_SON | +----------+
substr 取子串（substr( 被截取的字符串, 起始下标,截取的长度)）
select substr(ename, 1, 1) as ename from emp;
注意：起始下标从1开始，没有0.
找出员工名字第一个字母是A的员工信息？
第一种方式：模糊查询
select ename from emp where ename like 'A%';
第二种方式：substr函数
select
ename
from
emp
where
substr(ename,1,1) = 'A';
首字母大写？
select name from t_student;
select upper(substr(name,1,1)) from t_student;
select substr(name,2,length(name) - 1) from t_student;
select concat(upper(substr(name,1,1)),substr(name,2,length(name) - 1)) as result from t_student;
+----------+
| result |
+----------+
| Zhangsan |
| Lisi |
| Wangwu |
| Jack_son |
+----------+
concat函数进行字符串的拼接
select concat(empno,ename) from emp; +---------------------+ | concat(empno,ename) | +---------------------+ | 7369SMITH | | 7499ALLEN | | 7521WARD | | 7566JONES | | 7654MARTIN | | 7698BLAKE | | 7782CLARK | | 7788SCOTT | | 7839KING | | 7844TURNER | | 7876ADAMS | | 7900JAMES | | 7902FORD | | 7934MILLER | +---------------------+
length 取长度
select length(ename) enamelength from emp; +-------------+ | enamelength | +-------------+ | 5 | | 5 | | 4 | | 5 | | 6 | | 5 | | 5 | | 5 | | 4 | | 6 | | 5 | | 5 | | 4 | | 6 | +-------------+
trim 去空格
mysql> select * from emp where ename = ' KING'; Empty set (0.00 sec) mysql> select * from emp where ename = trim(' KING'); +-------+-------+-----------+------+------------+---------+------+--------+ | EMPNO | ENAME | JOB | MGR | HIREDATE | SAL | COMM | DEPTNO | +-------+-------+-----------+------+------------+---------+------+--------+ | 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL | 10 | +-------+-------+-----------+------+------------+---------+------+--------+
str_to_date 将字符串转换成日期
date_format 格式化日期
format 设置千分位
case..when..then..when..then..else..end
当员工的工作岗位是MANAGER的时候，工资上调10%，当工作岗位是SALESMAN的时候，工资上调50%,其它正常。
（注意：不修改数据库，只是将查询结果显示为工资上调）
select ename, job, sal as oldsal, (case job when 'MANAGER' then sal*1.1 when 'SALESMAN' then sal*1.5 else sal end) as newsal from emp; +--------+-----------+---------+---------+ | ename | job | oldsal | newsal | +--------+-----------+---------+---------+ | SMITH | CLERK | 800.00 | 800.00 | | ALLEN | SALESMAN | 1600.00 | 2400.00 | | WARD | SALESMAN | 1250.00 | 1875.00 | | JONES | MANAGER | 2975.00 | 3272.50 | | MARTIN | SALESMAN | 1250.00 | 1875.00 | | BLAKE | MANAGER | 2850.00 | 3135.00 | | CLARK | MANAGER | 2450.00 | 2695.00 | | SCOTT | ANALYST | 3000.00 | 3000.00 | | KING | PRESIDENT | 5000.00 | 5000.00 | | TURNER | SALESMAN | 1500.00 | 2250.00 | | ADAMS | CLERK | 1100.00 | 1100.00 | | JAMES | CLERK | 950.00 | 950.00 | | FORD | ANALYST | 3000.00 | 3000.00 | | MILLER | CLERK | 1300.00 | 1300.00 | +--------+-----------+---------+---------+
round 四舍五入
select 字段 from 表名;
select ename from emp;
select 'abc' from emp; // select后面直接跟“字面量/字面值”
mysql> select 'abc' as bieming from emp; +---------+ | bieming | +---------+ | abc | | abc | | abc | | abc | | abc | | abc | | abc | | abc | | abc | | abc | | abc | | abc | | abc | | abc | +---------+ mysql> select abc from emp; ERROR 1054 (42S22): Unknown column 'abc' in 'field list'
这样肯定报错，因为会把abc当做一个字段的名字，去emp表中找abc字段去了。
select 1000 as num from emp; // 1000 也是被当做一个字面量/字面值。 +------+ | num | +------+ | 1000 | | 1000 | | 1000 | | 1000 | | 1000 | | 1000 | | 1000 | | 1000 | | 1000 | | 1000 | | 1000 | | 1000 | | 1000 | | 1000 | +------+
结论：select后面可以跟某个表的字段名（可以等同看做变量名），也可以跟字面量/字面值（数据）。
