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)