MySQL入门篇-MySQL常用流程控制函数小结
备注:测试数据库版本为MySQL 8.0
这个blog我们来聊聊常见的流程控制函数
如需要scott用户下建表及录入数据语句，可参考:
scott建表及录入数据sql脚本
流程控制函数
| 函数名 | 函数用途 |
|---|---|
| CASE | case语句用于条件判断 |
| if() | if/else条件判断 |
| ifnull() | null数据处理 |
| nullif() | return null if expr1 = expr2 |
CASE语句
case语句语法:
CASE value WHEN [compare_value] THEN result [WHEN [compare_value] THEN result …] [ELSE result] END
CASE WHEN [condition] THEN result [WHEN [condition] THEN result …] [ELSE result] END
-- 值判断，根据职位英文名判断职位中文名
select e.empno,e.job,case when e.job = 'SALESMAN' then '销售员'when e.job = 'MANAGER' then '管理人员'else '未知' end as job_china
from emp e;-- 条件判断,小于某个时间点入职的是老员工，大于等于的是新员工
select e.empno,e.hiredate,case when e.hiredate < '1982-01-01' then '老员工' else '新员工' end is_old
from emp e;-- 没有else语句且判断不成功的时候，会返回为空
SELECT CASE 'c'WHEN 'a' THEN 1 WHEN 'b' THEN 2 END is_c;
mysql> -- 值判断，根据职位英文名判断职位中文名
mysql> select e.empno,-> e.job,-> case when e.job = 'SALESMAN' then '销售员'-> when e.job = 'MANAGER' then '管理人员'-> else '未知' end as job_china-> from emp e;
+-------+-----------+--------------+
| empno | job | job_china |
+-------+-----------+--------------+
| 7369 | CLERK | 未知 |
| 7499 | SALESMAN | 销售员 |
| 7521 | SALESMAN | 销售员 |
| 7566 | MANAGER | 管理人员 |
| 7654 | SALESMAN | 销售员 |
| 7698 | MANAGER | 管理人员 |
| 7782 | MANAGER | 管理人员 |
| 7788 | ANALYST | 未知 |
| 7839 | PRESIDENT | 未知 |
| 7844 | SALESMAN | 销售员 |
| 7876 | CLERK | 未知 |
| 7900 | CLERK | 未知 |
| 7902 | ANALYST | 未知 |
| 7934 | CLERK | 未知 |
+-------+-----------+--------------+
14 rows in set (0.00 sec)mysql>
mysql> -- 条件判断,小于某个时间点入职的是老员工，大于等于的是新员工
mysql> select e.empno,-> e.hiredate,-> case when e.hiredate < '1982-01-01' then '老员工' else '新员工' end is_old-> from emp e;
+-------+------------+-----------+
| empno | hiredate | is_old |
+-------+------------+-----------+
| 7369 | 1980-12-17 | 老员工 |
| 7499 | 1981-02-20 | 老员工 |
| 7521 | 1981-02-22 | 老员工 |
| 7566 | 1981-04-02 | 老员工 |
| 7654 | 1981-09-28 | 老员工 |
| 7698 | 1981-05-01 | 老员工 |
| 7782 | 1981-06-09 | 老员工 |
| 7788 | 1987-06-13 | 新员工 |
| 7839 | 1981-11-17 | 老员工 |
| 7844 | 1981-09-08 | 老员工 |
| 7876 | 1987-06-13 | 新员工 |
| 7900 | 1981-12-03 | 老员工 |
| 7902 | 1981-12-03 | 老员工 |
| 7934 | 1982-01-23 | 新员工 |
+-------+------------+-----------+
14 rows in set (0.00 sec)mysql>
mysql> -- 没有else语句且判断不成功的时候，会返回为空
mysql> SELECT CASE 'c'-> WHEN 'a' THEN 1 WHEN 'b' THEN 2 END is_c;
+------+
| is_c |
+------+
| NULL |
+------+
1 row in set (0.00 sec)mysql>
IF函数
if语法:
IF(expr1,expr2,expr3)
If expr1 is TRUE (expr1 <> 0 and expr1 <> NULL), IF() returns expr2. Otherwise, it returns expr3.
-- 如果1>2条件成立,则返回2，否则返回3
select if(1>2,2,3);
-- 如果第一个值不为0或null，则返回2，否则返回3
select if(0,2,3);
select if(null,2,3);
select if(1,2,3);
mysql> -- 如果1>2条件成立,则返回2，否则返回3
mysql> select if(1>2,2,3);
+-------------+
| if(1>2,2,3) |
+-------------+
| 3 |
+-------------+
1 row in set (0.00 sec)mysql> -- 如果第一个值不为0或null，则返回2，否则返回3
mysql> select if(0,2,3);
+-----------+
| if(0,2,3) |
+-----------+
| 3 |
+-----------+
1 row in set (0.00 sec)mysql> select if(null,2,3);
+--------------+
| if(null,2,3) |
+--------------+
| 3 |
+--------------+
1 row in set (0.00 sec)mysql> select if(1,2,3);
+-----------+
| if(1,2,3) |
+-----------+
| 2 |
+-----------+
1 row in set (0.00 sec)
IFNULL函数
ifnull语法:
IFNULL(expr1,expr2)
If expr1 is not NULL, IFNULL() returns expr1; otherwise it returns expr2.
如果expr1非空,则返回expr1，否则返回expr2
-- 将奖金为0的员工显示为0
select e.ename,e.job,e.comm comm from emp e;
select e.ename,e.job,ifnull(e.comm,0) comm from emp e;
mysql> -- 将奖金为0的员工显示为0
mysql> select e.ename,e.job,e.comm comm from emp e;
+--------+-----------+---------+
| ename | job | comm |
+--------+-----------+---------+
| SMITH | CLERK | NULL |
| ALLEN | SALESMAN | 300.00 |
| WARD | SALESMAN | 500.00 |
| JONES | MANAGER | NULL |
| MARTIN | SALESMAN | 1400.00 |
| BLAKE | MANAGER | NULL |
| CLARK | MANAGER | NULL |
| SCOTT | ANALYST | NULL |
| KING | PRESIDENT | NULL |
| TURNER | SALESMAN | 0.00 |
| ADAMS | CLERK | NULL |
| JAMES | CLERK | NULL |
| FORD | ANALYST | NULL |
| MILLER | CLERK | NULL |
+--------+-----------+---------+
14 rows in set (0.00 sec)mysql> select e.ename,e.job,ifnull(e.comm,0) comm from emp e;
+--------+-----------+---------+
| ename | job | comm |
+--------+-----------+---------+
| SMITH | CLERK | 0.00 |
| ALLEN | SALESMAN | 300.00 |
| WARD | SALESMAN | 500.00 |
| JONES | MANAGER | 0.00 |
| MARTIN | SALESMAN | 1400.00 |
| BLAKE | MANAGER | 0.00 |
| CLARK | MANAGER | 0.00 |
| SCOTT | ANALYST | 0.00 |
| KING | PRESIDENT | 0.00 |
| TURNER | SALESMAN | 0.00 |
| ADAMS | CLERK | 0.00 |
| JAMES | CLERK | 0.00 |
| FORD | ANALYST | 0.00 |
| MILLER | CLERK | 0.00 |
+--------+-----------+---------+
14 rows in set (0.00 sec)
nullif 函数
nullif语法:
NULLIF(expr1,expr2)
Returns NULL if expr1 = expr2 is true, otherwise returns expr1
This is the same as CASE WHEN expr1 = expr2 THEN NULL ELSE expr1 END.
如果expr1=expr2则返回null，否则返回expr1
mysql> select nullif(1,1);
+-------------+
| nullif(1,1) |
+-------------+
| NULL |
+-------------+
1 row in set (0.00 sec)mysql> select nullif(1,2);
+-------------+
| nullif(1,2) |
+-------------+
| 1 |
+-------------+
1 row in set (0.00 sec)