mysql - 如何在mysql中选择每列的前3个值

Question

您好，我有一张这样的表格，其中包含当天人员的类次。我需要按类次对它进行分类

+------------+----------+-----------------+
| shift_date | name     | shift           |
+------------+----------+-----------------+
| 2016-07-01 | Swamy    | Morning Shift   |
| 2016-07-01 | Hitesh   | Morning Shift   |
| 2016-07-01 | Naresh   | Afternoon Shift |
| 2016-07-01 | Rambabu  | Afternoon Shift |
| 2016-07-01 | Srinivas | Night Shift     |
| 2016-07-01 | Ahmed    | Night Shift     |
| 2016-07-01 | Naveen   | Week Off        |
| 2016-07-01 | Karthik  | Week Off        |
+------------+----------+-----------------+ 

我需要一个查询来获得结果

+------------+---------+-----------+----------+---------+
| shift_date | Morning | Afternoon | Night    | WeekOff |
+------------+---------+-----------+----------+---------+
| 2016-07-01 | Swamy   | Naresh    | Srinivas | Naveen  |
| 2016-07-01 | Hitesh  | Rambabu   | Ahmed    | Karthik |
+------------+---------+-----------+----------+---------+

我写了这个查询

 select
     shift_date,
     CASE shift
       WHEN shift <>'Morning Shift'
       then name
     end as Morning,
     CASE shift
       WHEN shift<>'Afternoon Shift'
       then name
     end as Afternoon,
     CASE shift
       WHEN shift<>'Night Shift'
       then name
     end as Night,
     CASE shift
       WHEN shift<>'Week Off'
       then name
     end as WeekOff
 from roster
 where shift_date ='2016-7-1'
 order by morning desc ,afternoon desc, night desc ,weekoff desc; 

但它得到的结果是

+------------+---------+-----------+----------+---------+
| shift_date | Morning | Afternoon | Night    | WeekOff |
+------------+---------+-----------+----------+---------+
| 2016-07-01 | Swamy   | NULL      | NULL     | NULL    |
| 2016-07-01 | Hitesh  | NULL      | NULL     | NULL    |
| 2016-07-01 | NULL    | Rambabu   | NULL     | NULL    |
| 2016-07-01 | NULL    | Naresh    | NULL     | NULL    |
| 2016-07-01 | NULL    | NULL      | Srinivas | NULL    |
| 2016-07-01 | NULL    | NULL      | Ahmed    | NULL    |
| 2016-07-01 | NULL    | NULL      | NULL     | Naveen  |
| 2016-07-01 | NULL    | NULL      | NULL     | Karthik |
+------------+---------+-----------+----------+---------+

我需要结果作为

+------------+---------+-----------+----------+---------+
| shift_date | Morning | Afternoon | Night    | WeekOff |
+------------+---------+-----------+----------+---------+
| 2016-07-01 | Swamy   | Naresh    | Srinivas | Naveen  |
| 2016-07-01 | Hitesh  | Rambabu   | Ahmed    | Karthik |
+------------+---------+-----------+----------+---------+

Answer 1

select m.shift_date  ,Morning , Afternoon 
from (select shift_date,name as Morning  from roster where shift = 'Morning Shift') as m, 
LEFT JOIN (select shift_date,name as Afternoon  from roster where shift = 'Afternoon Shift') as n on m.shift_date = n.shift_date

在您的问题中，所有轮类日期都相同。 因此，通过这个加入日期也将获得 shift_date 的名称。

这不是一个合适的解决方案。 希望能有点帮助