MySQL -行到列

我试着搜索帖子，但我只找到SQL Server/Access的解决方案。我需要一个解决方案在MySQL (5.X)。

我有一个表(称为历史)3列:hostid, itemname, itemvalue。如果我执行select (select * from history)，它会返回

   +--------+----------+-----------+
   | hostid | itemname | itemvalue |
   +--------+----------+-----------+
   |   1    |    A     |    10     |
   +--------+----------+-----------+
   |   1    |    B     |     3     |
   +--------+----------+-----------+
   |   2    |    A     |     9     |
   +--------+----------+-----------+
   |   2    |    C     |    40     |
   +--------+----------+-----------+

如何查询数据库以返回类似的内容

   +--------+------+-----+-----+
   | hostid |   A  |  B  |  C  |
   +--------+------+-----+-----+
   |   1    |  10  |  3  |  0  |
   +--------+------+-----+-----+
   |   2    |   9  |  0  |  40 |
   +--------+------+-----+-----+

当前回答

我的解决方案:

select h.hostid, sum(ifnull(h.A,0)) as A, sum(ifnull(h.B,0)) as B, sum(ifnull(h.C,0)) as  C from (
select
hostid,
case when itemName = 'A' then itemvalue end as A,
case when itemName = 'B' then itemvalue end as B,
case when itemName = 'C' then itemvalue end as C
  from history 
) h group by hostid

它在提交的案例中产生预期的结果。

2014-01-29 21:06:37

其他回答

我找到了一种方法，使用简单的查询使我的报告几乎动态地将行转换为列。你可以在这里在线查看和测试。

查询的列数是固定的，但值是动态的，并且基于行值。所以，我使用一个查询来构建表头，另一个查询来查看值:

SELECT distinct concat('<th>',itemname,'</th>') as column_name_table_header FROM history order by 1;

SELECT
     hostid
    ,(case when itemname = (select distinct itemname from history a order by 1 limit 0,1) then itemvalue else '' end) as col1
    ,(case when itemname = (select distinct itemname from history a order by 1 limit 1,1) then itemvalue else '' end) as col2
    ,(case when itemname = (select distinct itemname from history a order by 1 limit 2,1) then itemvalue else '' end) as col3
    ,(case when itemname = (select distinct itemname from history a order by 1 limit 3,1) then itemvalue else '' end) as col4
FROM history order by 1;

你也可以总结一下:

SELECT
     hostid
    ,sum(case when itemname = (select distinct itemname from history a order by 1 limit 0,1) then itemvalue end) as A
    ,sum(case when itemname = (select distinct itemname from history a order by 1 limit 1,1) then itemvalue end) as B
    ,sum(case when itemname = (select distinct itemname from history a order by 1 limit 2,1) then itemvalue end) as C
FROM history group by hostid order by 1;
+--------+------+------+------+
| hostid | A    | B    | C    |
+--------+------+------+------+
|      1 |   10 |    3 | NULL |
|      2 |    9 | NULL |   40 |
+--------+------+------+------+

RexTester测试结果:

http://rextester.com/ZSWKS28923

作为一个实际使用的例子，下面的报告以列的形式显示了船只/公共汽车出发和到达的时间，并有一个可视化的时间表。你会看到一个额外的列没有使用在最后一个col而不会混淆可视化: **票务系统的在线售票和存在

2017-06-28 13:14:47

我把它变成Group By hostId，然后它只会显示第一行的值，如:

A   B  C
1  10
2      3

2011-04-28 08:54:56

我编辑阿贡Sagita的答案从子查询加入。我不确定这两种方式有多大区别，但只是作为另一个参考。

SELECT  hostid, T2.VALUE AS A, T3.VALUE AS B, T4.VALUE AS C
FROM TableTest AS T1
LEFT JOIN TableTest T2 ON T2.hostid=T1.hostid AND T2.ITEMNAME='A'
LEFT JOIN TableTest T3 ON T3.hostid=T1.hostid AND T3.ITEMNAME='B'
LEFT JOIN TableTest T4 ON T4.hostid=T1.hostid AND T4.ITEMNAME='C'

2012-12-19 07:22:31

我的解决方案:

select h.hostid, sum(ifnull(h.A,0)) as A, sum(ifnull(h.B,0)) as B, sum(ifnull(h.C,0)) as  C from (
select
hostid,
case when itemName = 'A' then itemvalue end as A,
case when itemName = 'B' then itemvalue end as B,
case when itemName = 'C' then itemvalue end as C
  from history 
) h group by hostid

它在提交的案例中产生预期的结果。

2014-01-29 21:06:37

利用Matt Fenwick的想法帮助我解决了这个问题(非常感谢)，让我们把它简化为一个问题:

select
    history.*,
    coalesce(sum(case when itemname = "A" then itemvalue end), 0) as A,
    coalesce(sum(case when itemname = "B" then itemvalue end), 0) as B,
    coalesce(sum(case when itemname = "C" then itemvalue end), 0) as C
from history
group by hostid

2012-10-22 07:07:50

MySQL -行到列

推荐文章

最新文章

标签