详解数据库_MySQL: mysql函数

mysql> select left('123456',4);
+------------------+
| left('123456',4) |
+------------------+
| 1234    |
+------------------+
1 row in set (0.00 sec)

mysql> select right('123456',4);
+-------------------+
| right('123456',4) |
+-------------------+
| 3456    |
+-------------------+
1 row in set (0.00 sec)

mysql> select substring('123456',2,4); 
+-------------------------+
| substring('123456',2,4) |
+-------------------------+
| 2345     |
+-------------------------+
1 row in set (0.00 sec)

mysql> select now();
+---------------------+
| now()    |
+---------------------+
| 2019-03-16 14:55:42 |
+---------------------+
1 row in set (0.00 sec)

mysql> select unix_timestamp();
+------------------+
| unix_timestamp() |
+------------------+
|  1552719356 |
+------------------+
1 row in set (0.00 sec)

mysql> select from_unixtime(1552719356);
+---------------------------+
| from_unixtime(1552719356) |
+---------------------------+
| 2019-03-16 14:55:56  |
+---------------------------+
1 row in set (0.00 sec)

mysql> select 
 -> year(now()) as '年',
 -> month(now()) as '月',
 -> day(now()) as '日',
 -> hour(now()) as '时',
 -> minute(now()) as '分',
 -> second(now()) as '秒';
+------+------+------+------+------+------+
| 年 | 月 | 日 | 时 | 分 | 秒 |
+------+------+------+------+------+------+
| 2019 | 3 | 16 | 14 | 59 | 12 |
+------+------+------+------+------+------+

mysql> select if(null,1,2);
+--------------+
| if(null,1,2) |
+--------------+
|   2 |
+--------------+
1 row in set (0.00 sec)

mysql> select if(1,0,2);
+-----------+
| if(1,0,2) |
+-----------+
|   0 |
+-----------+
1 row in set (0.00 sec)

mysql> select ifnull(0,123);
+---------------+
| ifnull(0,123) |
+---------------+
|    0 |
+---------------+
1 row in set (0.00 sec)

mysql> select ifnull('a',123);
+-----------------+
| ifnull('a',123) |
+-----------------+
| a    |
+-----------------+
1 row in set (0.00 sec)

#修改结束符
delimiter //
create function 函数名（参数名 类型,...,参数名n 类型n） returns 返回数据类型
begin
#SQL语句
return 返回值;
end //
delimiter ;

#调用
select 函数名();

#判断函数是否存在，存在就删除
drop function if exists f1;

delimiter //
create function f1() returns varchar(30)
begin
 return 'hello world';
end //
delimiter ;

select f1();

+-------------+
| f1()  |
+-------------+
| hello world |
+-------------+

drop function if exists f2;

delimiter //

create function f2(num1 int, num2 int) returns int
begin
  return num1 + num2;
end //
delimiter ;

select f2(8, 2);

+----------+
| f2(8, 2) |
+----------+
|  10 |
+----------+