Question

所以，我有3张桌子：

guest : 
id_guest | name
1        | John
2        | Nick
3        | James
4        | Paul
5        | Chris
6        | Karen
7        | Peter

room :  
id_room  | status
1        | Clean
2        | Dirty
3        | Dirty
4        | Clean
5        | Clean
6        | Clean

reservation : 
id_guest | id_room  | date
1        | 1        | 2015-04-15
1        | 1        | 2015-04-16
1        | 1        | 2015-04-17
2        | 3        | 2015-04-15
3        | 4        | 2015-04-15
3        | 4        | 2015-04-16
4        | 2        | 2015-04-16
5        | 2        | 2015-04-17
6        | 2        | 2015-04-18

这就是预期的输出应该是：

id_room  | status   | d04-15    | d04-16    | d04-17    | d04-18
1        | Clean    | John      | John      | John      |
2        | Dirty    |           | Paul      | Chris     | Karen
3        | Dirty    | Nick      |           |           |
4        | Clean    | James     | James     |           |
5        | Clean    |           |           |           |
6        | Clean    |           |           |           |

我已经能够显示它直到第三个字段（d04-15），但日期为值，使用：

SELECT room.id_room,
       room.status,
       reservation.date AS d04-15
FROM room
LEFT JOIN reservation
ON room.id_room = reservation.id_room AND reservation.date = '2015-04-15'
GROUP BY room.id_room

但我不确定如何在那里显示名称从另一个JOIN语句中追加新字段（d04-16，d04-17和d04-18）。

任何帮助将不胜感激。感谢。

Answer 1

查询中返回的列无法在运行时更改;它们必须在SQL SELECT语句中静态声明。

这是一个可以达到指定结果的语句示例：

SELECT m.id_room
     , m.status
     , MAX(IF(r.date='2015-04-15',g.name,NULL)) AS `d04-15`
     , MAX(IF(r.date='2015-04-16',g.name,NULL)) AS `d04-16`
     , MAX(IF(r.date='2015-04-17',g.name,NULL)) AS `d04-17`
     , MAX(IF(r.date='2015-04-18',g.name,NULL)) AS `d04-18`
  FROM room m
  LEFT
  JOIN reservation r
    ON r.id_room = m.id_room
   AND r.date IN ('2015-04-15','2015-04-16','2015-04-17','2015-04-18')
  LEFT
  JOIN guest g
    ON g.id_guest = r.id_guest
 GROUP BY m.id_room

在MySQL查询中添加来自另一个JOIN语句的新字段

1 个答案: