mysql中order by 可以指定排序

這裡是想讓它以 num 排序 但是 num=-1的那條資料單獨為最大值
法一：field( field1,val1,val2,…)

這裡大小順序是 … >val2 >val1 >field1（其他的資料） 子後向前=》從大到小
order by field 時 Using filesort(一種速度比較慢的外部排序) , 一般不建議使用

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;

資料分為2個部分：
我認為 表中的資料分為兩部分， num=-1的一條拿出來作為最大值(特殊部分)，然後其他的作為一個整體(其他部分) 然後2個部分進行排序，再用 進行一次排序可達到想要的效果
方法中的2個例子結果是一樣的，但是想對來說第二種用出多些，因為它可以自由選擇大、小的作為特殊部分

#表 test 共有2個欄位 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 |
+----+-----+
 

    #兩種方法獲取到相同的排序 num=-1 作為最大值排序
    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 |
    +----+-----+

    #當進行二次排序時，得到了想要的結果
    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 |
    +----+-----+

    #這裡是 num=2 作為最大值進行倒序排序 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 |
    +----+-----+

    # 這裡是 num=2 作為最小值進行正序排序 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 |
    +----+-----+

					<link href="https://csdnimg.cn/release/phoenix/mdeditor/markdown_views-8cccb36679.css" rel="stylesheet">
            </div>
								
				<script>
					(function(){
						function setArticleH(btnReadmore,posi){
							var winH = $(window).height();
							var articleBox = $("div.article_content");
							var artH = articleBox.height();
							if(artH > winH*posi){
								articleBox.css({
									'height':winH*posi+'px',
									'overflow':'hidden'
								})
								btnReadmore.click(function(){
									articleBox.removeAttr("style");
									$(this).parent().remove();
								})
							}else{
								btnReadmore.parent().remove();
							}
						}
						var btnReadmore = $("#btn-readmore");
						if(btnReadmore.length>0){
							if(currentUserName){
								setArticleH(btnReadmore,3);
							}else{
								setArticleH(btnReadmore,1.2);
							}
						}
					})()
				</script>
				</article>