如果 B 不包含特定值，如何从表 A 和 B 中删除记录

Question

我正在尝试编写 SQL 查询以从两个表中删除数据 - A ， B 。 它们与一个键连接，只有我需要检查，如果在B中是一列中的特定值reservation 。

长话短说，我有带rooms和beds的桌子，我需要检查房间里是否有任何room 。 如果没有，我需要删除room本身及其所有beds 。

我尝试了两个查询，但都没有给我预期的结果。 表B相对较大，因此我试图在一个查询中使其尽可能快。

-- gives me always 0 records
DELETE
  a, b
FROM
  rooms a, beds b
WHERE
  a.id = b.id_room AND
NOT EXISTS
  (SELECT 1 FROM beds WHERE id_room = '45' AND state != 'reservation')


-- deletes all records, but rows where state = 'reservation'
DELETE
  a, b
FROM 
  rooms a, beds b
WHERE
  a.id = b.id_room AND id_room = '45' AND state != 'reservation'

示例数据集：

-- rooms
-------------
| id | Name |
-------------
| 1  | 601  |
| 2  | 602  |
| 3  | 603  |

-- beds
-----------------------------|
| id | id_room | state       |
-----------------------------|
| 1  | 1       | free        |
| 2  | 1       | free        |
| 3  | 1       | reservation |
| 4  | 2       | free        |
| 5  | 2       | free        |
| 6  | 3       | reservation |

预期成绩：

房间1和3在桌beds的state列中有值reservation ，因此无法删除。 Room 2没有预订，因此可以删除它，并且应该删除它，包括它的所有床位 - 所以查询应该从 table rooms中删除记录2并从 table beds中删除记录4和5 。

我总是只想删除一个房间（有床），所以我需要包含关于房间ID的条件来计算。

Answer 1

您可以尝试在表之间使用左连接并检查不匹配的值

    DELETE  beds, rooms
    FROM  rooms a
    LEFT  JOIN beds b ON a.id = b.id_room 
        AND b.state = 'reservation'
    WHERE b.id_room is null

并基于您的代码示例

SELECT a.id, b.id 
FROM  rooms a 
LEFT  JOIN beds b ON a.id = b.id_room 
AND b.state = 'reservation' and 
    b.id_room = 45 WHERE b.id_room is null

    DELETE romms, beds
    FROM  rooms a
    LEFT  JOIN beds b ON a.id = b.id_room 
        AND b.state = 'reservation'
    WHERE b.id_room is null
    AND  a.id = 45 

Answer 2

这个查询：

select id_room
from (
  select id_room, state from beds
  union
  select ?, ''
) t 
where id_room = ?
group by id_room
having sum(state = 'reservation') = 0

返回应该被删除的房间的id ，或者因为它的任何床都没有'reservation' ，或者它没有床。
两个都换? 与您搜索的房间的 id。

将其加入到DELETE语句中的表中：

delete r, b
from rooms r 
inner join (
  select id_room
  from (
    select id_room, state from beds
    union
    select ?, ''
  ) t 
  where id_room = ?
  group by id_room
  having sum(state = 'reservation') = 0
) t on t.id_room = r.id
left join beds b
on b.id_room = t.id_room

请参阅演示。

Answer 3

我想你想要：

DELETE r, b
    FROM rooms r JOIN
         beds b
         ON a.id = b.id_room LEFT JOIN
         (SELECT b2.id_room
          FROM beds b2
          WHERE b2.state = 'reservation'
         ) b2
         ON b2.id_room = b.id_room
    WHERE b2.id_room IS NULL;