多个升序在Mysql中不起作用

Question

我有一张名为request 的表，三列名为Id, want, teach。我必须按升序显示want 和teach 列，我不想显示NULL 和空白行。

我尝试仅查询want 列以升序显示，但teach 列未以升序显示。

  SELECT want, teach FROM request WHERE want IS NOT NULL OR want !='' AND teach IS NOT NULL OR teach !='' GROUP BY want, teach ORDER BY want ASC, teach ASC

如果我写 ORDER BY 教导 ASC，想要 ASC，然后教导列以升序显示，而想要显示 DESC 顺序。 你会在这方面帮助我吗？

Chech 左侧栏是要，右侧栏是教。我得到这样的输出。左侧列是 ASC 顺序，但右侧显示 DESC

Answer 1

您在这里没有得到多订单的意义。

首先，默认情况下，大多数 dbms 排序为ASC。

多个排序取决于排序列的顺序，这意味着-可以这么说-:-

“嘿 sql！从我的表中选择一些结果，按 col1 排序并按此顺序分组，然后按排序或分组的数据集按 col2 排序。”

可以说是一种多维排序。

以此为例：

+------+---------+-------+                   +------+---------+-------+
|  id  |   want  | teach |                   |  id  |   want  | teach |
+------+---------+-------+                   +------+---------+-------+
|   1  |    1    |   2   |    ORDER BY       |   2  |    1    |   1   |
|   2  |    1    |   1   |       want asc,   |   1  |    1    |   2   |
|   3  |    2    |   1   |       teach asc   |   4  |    1    |   2   |
|   4  |    1    |   2   |                   |   3  |    2    |   1   |
+------+---------+-------+                   +------+---------+-------+

当您像这样恢复排序列的顺序时：

+------+---------+-------+                   +------+---------+-------+
|  id  |   want  | teach |                   |  id  |   want  | teach |
+------+---------+-------+                   +------+---------+-------+
|   1  |    1    |   2   |    ORDER BY       |   2  |    1    |   1   |
|   2  |    1    |   1   |       teach asc,  |   3  |    2    |   1   |
|   3  |    2    |   1   |       want asc    |   1  |    1    |   2   |
|   4  |    1    |   2   |                   |   4  |    1    |   2   |
+------+---------+-------+                   +------+---------+-------+

Answer 2

首先你需要了解order by

如果您给出 1st,2nd asc 顺序，它将首先 asc 第一列，并依赖于此它会 asc 2nd 列。 例如

1st  2nd               
1    b
2    d
3    a
2    a
1    a

按转换排序

 1st   2nd 
  1     a
  1     b
  2     a
  2     d
  3     a

所以你需要不同的查询

     SELECT want FROM request WHERE want IS NOT NULL OR want !='' GROUP BY 
 want ORDER BY want ASC

     SELECT teach FROM request WHERE teach IS NOT NULL OR teach !='' GROUP
 BY  teach ORDER BY teach ASC