Question

我有两张桌子，一张是预订房间，另一张是“中间”桌子，用来保存预订房间的日期（因为预订可能有多个非连续日期）。 / p>

它看起来像：

Res_table： id，room_id，owner_id

Res_table_mid： id，res_id，date

res_table_mid中的res_id列引用res_table的id。我需要获得预订的开始和结束日期。

所以查询看起来像这样：

SELECT * FROM res_table a
LEFT JOIN (SELECT min(date) as start_date, res_id FROM res_table_mid) AS min ON a.id = min.res_id
LEFT JOIN (SELECT max(date) as end_date, res_id FROM res_table_mid) AS max ON a.id = max.res_id

这是按预期工作的，除非表是空的或没有结果，在这种情况下它会错误

#1048 - Column 'res_id' cannot be null

有没有办法写这个，以便我得到我需要的数据，但如果没有结果也没有错误？

谢谢！

Answer 1

Select id, room_id, owner_id
From Res_table
    Left Join   (
                Select R2.res_id, Min(R2.Date), Max(R2.Date)
                From Res_table_mid As R2
                Group By R2.res_id
                ) As MinMax
            On MinMax.res_Id = Res_table.Id

在原始查询中，派生表均未指示“分组依据”列。相反，你依靠MySQL猜测它应该按res_id分组。如果我不得不下注，我会说这可能是问题的根源。

Answer 2

SELECT   a.id,
         a.room_id,
         a.owner_id,
         MAX(m.date) AS end_date ,
         MIN(m.date) AS start_date
FROM     res_table a
         LEFT JOIN res_table_mid m
         ON a.id = m.res_id
GROUP BY a.id,
         a.room_id,
         a.owner_id;

Answer 3

SELECT min(date) AS start_date FROM (
SELECT * FROM res_table a
LEFT JOIN res_table_mid AS b
ON a.id = b.res_id
WHERE a.id = @reservation)

SELECT max(date) AS end_date FROM (
SELECT * FROM res_table a
LEFT JOIN res_table_mid AS b
ON a.id = b.res_id
WHERE a.id = @reservation)

mysql左连接问题

3 个答案: