Question

我有这样的数据：

table 1
|id|fieldname1|fieldname2|price|

table 2
|id|fieldname3|fieldname4|price|

desired result:
|table1_id|table2_id|fieldname1|fieldname2|fieldname3|fieldname4|table1_min_price|table2_min_price|

我基本上可以从表1到表2的左边连接（fieldname1 = fieldname3）从每个表中获得最低价格。

但是，如果表2返回0个查询，则根本不会加入任何结果。我的意图是如果表2或表1没有结果，它仍然会连接表并创建具有空值的字段。

任何想法如何做到这一点？

Answer 1

您可能需要一个FULL OUTER JOIN，它将为您提供Table1中的值或Table2中的值或两者。但是，我不认为你可以在它的“无论如何”时获得一排

Answer 2

将'UNION ALL'更改为'UNION'：

SELECT * from 
    (SELECT origin,destination,min(price) as oneway_cheapest FROM farestable_test where flight_type=0  group by origin,destination )as t1 
     LEFT JOIN
    (SELECT origin,destination,min(price) as return_cheapest FROM farestable_test where flight_type=1 group by origin,destination )as t2

ON (t1.origin = t2.origin AND t1.destination=t2.destination)

UNION 

SELECT * from 
    (SELECT origin,destination,min(price) as oneway_cheapest FROM farestable_test where flight_type=0  group by origin,destination )as t1 
     RIGHT JOIN
    (SELECT origin,destination,min(price) as return_cheapest FROM farestable_test where flight_type=1 group by origin,destination )as t2

ON (t1.origin = t2.origin AND t1.destination=t2.destination)

Answer 3

您可以完成以下全部外部联接：

select *
from table1 left outer join table2 on(fieldname1=fieldname3)
union 
select *
from table1 right outer join table2 on(fieldname1=fieldname3)

Answer 4

以下是我提出的表格。


CREATE TABLE IF NOT EXISTS `farestable_test` (
  `id` tinyint(4) NOT NULL,
  `origin` varchar(3) NOT NULL,
  `destination` varchar(3) NOT NULL,
  `flight_type` tinyint(4) NOT NULL,
  `price` float NOT NULL
) ENGINE=MyISAM DEFAULT CHARSET=latin1;



SELECT * from 
    (SELECT origin,destination,min(price) as oneway_cheapest FROM farestable_test where flight_type=0  group by origin,destination )as t1 
     LEFT JOIN
    (SELECT origin,destination,min(price) as return_cheapest FROM farestable_test where flight_type=1 group by origin,destination )as t2

ON (t1.origin = t2.origin AND t1.destination=t2.destination)

UNION ALL

SELECT * from 
    (SELECT origin,destination,min(price) as oneway_cheapest FROM farestable_test where flight_type=0  group by origin,destination )as t1 
     RIGHT JOIN
    (SELECT origin,destination,min(price) as return_cheapest FROM farestable_test where flight_type=1 group by origin,destination )as t2

ON (t1.origin = t2.origin AND t1.destination=t2.destination)

我想要达到的目标是：

如果数据


INSERT INTO `farestable_test` (`id`, `origin`, `destination`, `flight_type`, `price`) VALUES
(1, 'syd', 'mky', 0, 100),
(2, 'syd', 'mky', 0, 200),

它显示了正确的结果：

origin  destination     oneway_cheapest     origin  destination     return_cheapest
syd     mky     100     null    null    null

然后......

如果数据

INSERT INTO `farestable_test` (`id`, `origin`, `destination`, `flight_type`, `price`) VALUES
(3, 'syd', 'mky', 1, 300),
(4, 'syd', 'mky', 1, 400);

它将显示正确的结果：

origin  destination     oneway_cheapest     origin  destination     return_cheapest
null    null    null    syd     mky     300

但是，如果数据低于

INSERT INTO `farestable_test` (`id`, `origin`, `destination`, `flight_type`, `price`) VALUES
(1, 'syd', 'mky', 0, 100),
(2, 'syd', 'mky', 0, 200),
(3, 'syd', 'mky', 1, 300),
(4, 'syd', 'mky', 1, 400);

这里的结果是错误的：

origin  destination     oneway_cheapest     origin  destination     return_cheapest
syd     mky     100     syd     mky     300
syd     mky     100     syd     mky     300

它有重复的数据，这是错误的，我不能完全连接没有与mysql的联合。还有更好的方法吗？

mysql连接表即使第二个表返回0查询？

4 个答案: