六、数据处理函数
6.1 什么是数据处理函数
数据处理函数又被称为单行处理函数。
特点：一个输入对应一个输出。------> 多行处理函数：多个输入，对应一个输出。
6.2 常见的单行处理函数
6.2.1 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 rows in set (0.00 sec)
# 14个输入，最后还是14个输出。
6.2.2 upper转换大写
mysql> select upper(ename) as name from emp;
6.2.3 substr取子串
substr(被截取的字符串，起始下标，截取的长度)
# 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：
# 法1
mysql> select ename from emp where ename like 'A%';
+-------+
| ename |
+-------+
| ALLEN |
| ADAMS |
+-------+
2 rows in set (0.00 sec)
mysql> select ename from emp where substr(ename, 1, 1) = 'A';
+-------+
| ename |
+-------+
| ALLEN |
| ADAMS |
+-------+
2 rows in set (0.00 sec)
6.2.4 concat进行字符串的拼接
mysql> select concat(empno, ename) from emp;
+----------------------+
| concat(empno, ename) |
+----------------------+
| 7369SMITH |
| 7499ALLEN |
| 7521WARD |
| 7566JONES |
| 7654MARTIN |
| 7698BLAKE |
| 7782CLARK |
| 7788SCOTT |
| 7839KING |
| 7844TURNER |
| 7876ADAMS |
| 7900JAMES |
| 7902FORD |
| 7934MILLER |
+----------------------+
14 rows in set (0.00 sec)
6.2.5 length取长度
mysql> select length(ename) 'ename length' from emp;
+--------------+
| ename length |
+--------------+
| 5 |
| 5 |
| 4 |
| 5 |
| 6 |
| 5 |
| 5 |
| 5 |
| 4 |
| 6 |
| 5 |
| 5 |
| 4 |
| 6 |
+--------------+
14 rows in set (0.00 sec)
6.2.6 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 |
+-------+-------+-----------+------+------------+---------+------+--------+
1 row in set (0.00 sec)
6.2.7 str_to_data将字符串转换成日期
6.2.8 data_format格式化日期
6.2.9 format设置千分位
6.2.10 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：
mysql> 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 |
+--------+-----------+---------+---------+
14 rows in set (0.00 sec)
6.2.11 round四舍五入
select 后面可以跟某个表的字段名（可以等同看作变量名），也可以跟字面量/字面值（数据）。
mysql> select ename from emp;
+--------+
| ename |
+--------+
| SMITH |
| ALLEN |
| WARD |
| JONES |
| MARTIN |
| BLAKE |
| CLARK |
| SCOTT |
| KING |
| TURNER |
| ADAMS |
| JAMES |
| FORD |
| MILLER |
+--------+
14 rows in set (0.00 sec)
mysql> select 'abc' as bieming from emp; # 如果写成abc会报错，因为数据库会把他看作一个字段名，从emp表中找abc字段
+---------+
| bieming |
+---------+
| abc |
| abc |
| abc |
| abc |
| abc |
| abc |
| abc |
| abc |
| abc |
| abc |
| abc |
| abc |
| abc |
| abc |
+---------+
14 rows in set (0.00 sec)
mysql> select 1000 as num from emp;
+------+
| num |
+------+
| 1000 |
| 1000 |
| 1000 |
| 1000 |
| 1000 |
| 1000 |
| 1000 |
| 1000 |
| 1000 |
| 1000 |
| 1000 |
| 1000 |
| 1000 |
| 1000 |
+------+
14 rows in set (0.00 sec)
# round四舍五入
mysql> select round(1236.567, 0) as result from emp; # 保留整位数
+--------+
| result |
+--------+
| 1237 |
| 1237 |
| 1237 |
| 1237 |
| 1237 |
| 1237 |
| 1237 |
| 1237 |
| 1237 |
| 1237 |
| 1237 |
| 1237 |
| 1237 |
| 1237 |
+--------+
14 rows in set (0.00 sec)
mysql> select round(1236.567, 2) as result from emp; # 保留两位小数
+---------+
| result |
+---------+
| 1236.57 |
| 1236.57 |
| 1236.57 |
| 1236.57 |
| 1236.57 |
| 1236.57 |
| 1236.57 |
| 1236.57 |
| 1236.57 |
| 1236.57 |
| 1236.57 |
| 1236.57 |
| 1236.57 |
| 1236.57 |
+---------+
14 rows in set (0.00 sec)
mysql> select round(1236.567, -1) as result from emp; # 保留到十位
+--------+
| result |
+--------+
| 1240 |
| 1240 |
| 1240 |
| 1240 |
| 1240 |
| 1240 |
| 1240 |
| 1240 |
| 1240 |
| 1240 |
| 1240 |
| 1240 |
| 1240 |
| 1240 |
+--------+
14 rows in set (0.00 sec)
6.2.12 rand()生成随机数
mysql> select round(rand()*100, 0) from emp; # 生成100以内的随机数
+----------------------+
| round(rand()*100, 0) |
+----------------------+
| 100 |
| 18 |
| 92 |
| 4 |
| 45 |
| 12 |
| 24 |
| 87 |
| 60 |
| 39 |
| 15 |
| 57 |
| 43 |
| 42 |
+----------------------+
14 rows in set (0.00 sec)
6.2.13 ifnull 将null转换成一个具体值
ifnull是空处理函数，专门处理空的。
在所有数据库中，只要有NULL参与的数学运算，最终结果就是NULL。为了避免这个现象，需要使用ifnull函数。
ifnull函数用法：ifnull(数据, 被当作哪个值)
计算每个员工的年薪：年薪 = (月薪 + 月补助) * 12
mysql> select ename, sal + comm as salcomm from emp;
+--------+---------+
| ename | salcomm |
+--------+---------+
| SMITH | NULL |
| ALLEN | 1900.00 |
| WARD | 1750.00 |
| JONES | NULL |
| MARTIN | 2650.00 |
| BLAKE | NULL |
| CLARK | NULL |
| SCOTT | NULL |
| KING | NULL |
| TURNER | 1500.00 |
| ADAMS | NULL |
| JAMES | NULL |
| FORD | NULL |
| MILLER | NULL |
+--------+---------+
14 rows in set (0.01 sec)
mysql> select ename, (sal + comm)*12 as yearsal from emp;
+--------+----------+
| ename | yearsal |
+--------+----------+
| SMITH | NULL |
| ALLEN | 22800.00 |
| WARD | 21000.00 |
| JONES | NULL |
| MARTIN | 31800.00 |
| BLAKE | NULL |
| CLARK | NULL |
| SCOTT | NULL |
| KING | NULL |
| TURNER | 18000.00 |
| ADAMS | NULL |
| JAMES | NULL |
| FORD | NULL |
| MILLER | NULL |
+--------+----------+
14 rows in set (0.00 sec)
补助为NULL的时候，将补助当做0：
mysql> select ename, (sal + ifnull(comm, 0))*12 as yearsal from emp;
+--------+----------+
| ename | yearsal |
+--------+----------+
| SMITH | 9600.00 |
| ALLEN | 22800.00 |
| WARD | 21000.00 |
| JONES | 35700.00 |
| MARTIN | 31800.00 |
| BLAKE | 34200.00 |
| CLARK | 29400.00 |
| SCOTT | 36000.00 |
| KING | 60000.00 |
| TURNER | 18000.00 |
| ADAMS | 13200.00 |
| JAMES | 11400.00 |
| FORD | 36000.00 |
| MILLER | 15600.00 |
+--------+----------+
14 rows in set (0.00 sec)