一天一点MySQL复习——获取数据库系统时间、变量赋值、变量比较

一、SQL获取系统时间

---

 mysql> select now() from dual;
 +---------------------+
 | now()               |
 +---------------------+
 | 2016-05-24 08:34:12 |
 +---------------------+
 1 row in set (0.00 sec)
 mysql> select curdate() from dual;
 +------------+
 | curdate()  |
 +------------+
 | 2016-05-24 |
 +------------+
 1 row in set (0.01 sec)
 mysql> select curtime() from dual;
 +-----------+
 | curtime() |
 +-----------+
 | 08:36:23  |
 +-----------+
 1 row in set (0.00 sec)

二、变量赋值

---

　　赋值方式分为'select'与'set'

　　select 方式：

　　　　select @name:=value;

 mysql> select @b:=4;
 +-------+
 | @b:=4 |
 +-------+
 |     4 |
 +-------+
 1 row in set (0.00 sec)

 mysql> select @b;
 +------+
 | @b   |
 +------+
 |    4 |
 +------+
 1 row in set (0.00 sec)

　　　　select row_name from table INTO @name;

 mysql> select now() from dual INTO @test1;
 Query OK, 1 row affected (0.00 sec)

 mysql> select @test1;
 +---------------------+
 | @test1              |
 +---------------------+
 | 2016-05-24 08:48:21 |
 +---------------------+
 1 row in set (0.00 sec)

　set方式：

　　　　赋值：set @name:=value;

　　　　　　   set @name=value;

 mysql> set @c=1;
 Query OK, 0 rows affected (0.00 sec)

 mysql> select @c;
 +------+
 | @c   |
 +------+
 |    1 |
 +------+
 1 row in set (0.00 sec)

 mysql> set @c:=2;
 Query OK, 0 rows affected (0.00 sec)

 mysql> select @c;
 +------+
 | @c   |
 +------+
 |    2 |
 +------+
 1 row in set (0.00 sec)

三、变量比较

---

　　

 mysql> select @time1,@time2;
 +---------------------+---------------------+
 | @time1              | @time2              |
 +---------------------+---------------------+
 | 2016-05-24 08:54:44 | 2016-05-24 08:27:57 |
 +---------------------+---------------------+
 1 row in set (0.00 sec)

 mysql> select now() from dual INTO @time1;
 Query OK, 1 row affected (0.00 sec)

 mysql> select now() from dual INTO @time2;
 Query OK, 1 row affected (0.00 sec)

 mysql> select @time1,@time2;
 +---------------------+---------------------+
 | @time1              | @time2              |
 +---------------------+---------------------+
 | 2016-05-24 08:58:23 | 2016-05-24 08:58:38 |
 +---------------------+---------------------+
 1 row in set (0.00 sec)

 mysql> select @time1 < @time2;
 +-----------------+
 | @time1 < @time2 |
 +-----------------+
 |               1 |
 +-----------------+
 1 row in set (0.00 sec)

 mysql> select @time1 > @time2;
 +-----------------+
 | @time1 > @time2 |
 +-----------------+
 |               0 |
 +-----------------+
 1 row in set (0.00 sec)