SQL的触发器

这里是想让它以 num 排序 但是 num=-1的那条数据单独为最大值 
法一：field( field1,val1,val2,…)

这里大小顺序是 … >val2 >val1 >field1（其他的数据） 子后向前=》从大到小 
order by field 时 Using filesort(一种速度比较慢的外部排序) , 一般不建议使用

select * from test order by field(num,-1) asc,num asc;

法二：case when … then … when … then … else … end 
select * from test order by case when num = -1 then 1 else 0 end desc;

数据分为2个部分： 
我认为 表中的数据分为两部分， num=-1的一条拿出来作为最大值(特殊部分)，然后其他的作为一个整体(其他部分) 然后2个部分进行排序，再用 进行一次排序可达到想要的效果 
方法中的2个例子结果是一样的，但是想对来说第二种用出多些，因为它可以自由选择大、小的作为特殊部分

#表 test 共有2个字段 id,name
mysql> desc test;
+-------+------------------+------+-----+---------+----------------+
| Field | Type             | Null | Key | Default | Extra          |
+-------+------------------+------+-----+---------+----------------+
| id    | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
| num   | int(11)          | NO   |     | 0       |                |
+-------+------------------+------+-----+---------+----------------+
mysql> select * from test;
+----+-----+
| id | num |
+----+-----+
|  1 |  -1 |
|  2 |   0 |
|  3 |   1 |
|  4 |   2 |
|  5 |   3 |
|  6 |   4 |
|  7 |   5 |
+----+-----+

---

    #两种方法获取到相同的排序 num=-1 作为最大值排序
    mysql> select * from test order by field(num,-1) desc;
    mysql> select * from test order by case when num = -1 then 1 else 0 end desc;
    +----+-----+
    | id | num |
    +----+-----+
    |  1 |  -1 |
    |  2 |   0 |
    |  3 |   1 |
    |  4 |   2 |
    |  5 |   3 |
    |  6 |   4 |
    |  7 |   5 |
    +----+-----+

    mysql> select * from test order by field(num,-1) asc;
    mysql> select * from test order by case when num = -1 then 1 else 0 end asc;
    +----+-----+
    | id | num |
    +----+-----+
    |  2 |   0 |
    |  3 |   1 |
    |  4 |   2 |
    |  5 |   3 |
    |  6 |   4 |
    |  7 |   5 |
    |  1 |  -1 |
    +----+-----+

    #当进行二次排序时，得到了想要的结果
    mysql> select * from test order by field(num,-1) desc,num desc;
    mysql> select * from test order by case when num = -1 then 1 else 0 end desc, num desc;
    +----+-----+
    | id | num |
    +----+-----+
    |  1 |  -1 |
    |  7 |   5 |
    |  6 |   4 |
    |  5 |   3 |
    |  4 |   2 |
    |  3 |   1 |
    |  2 |   0 |
    +----+-----+

    mysql> select * from test order by field(num,-1) asc,num asc;
    mysql> select * from test order by case when num = -1 then 1 else 0 end asc, num asc;

    +----+-----+
    | id | num |
    +----+-----+
    |  2 |   0 |
    |  3 |   1 |
    |  4 |   2 |
    |  5 |   3 |
    |  6 |   4 |
    |  7 |   5 |
    |  1 |  -1 |
    +----+-----+

    #这里是 num=2 作为最大值进行倒序排序 所以num=2在最前面
    mysql> select * from test order by case when num = 2 then 1 else 0 end desc;
    +----+-----+
    | id | num |
    +----+-----+
    |  4 |   2 |
    |  1 |  -1 |
    |  2 |   0 |
    |  3 |   1 |
    |  5 |   3 |
    |  6 |   4 |
    |  7 |   5 |
    +----+-----+

    # 这里是 num=2 作为最小值进行正序排序 所以num=2在最前面
    mysql> select * from test order by case when num = 2 then 0 else 1 end;
    +----+-----+
    | id | num |
    +----+-----+
    |  4 |   2 |
    |  1 |  -1 |
    |  2 |   0 |
    |  3 |   1 |
    |  5 |   3 |
    |  6 |   4 |
    |  7 |   5 |
    +----+-----+

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