如何将此查询编写为完全联接而不是联合左/右联接？

Question

这是代码，显示了输入和所需的输出。

基本上，我正在尝试自我加入，以将我的经纪人声明的结果与我的内部记录相匹配。所以左边的一组列是经纪人的名单，右边是我的名单。如果经纪人有头寸，而我没有，右边的空值。如果我有头寸而经纪人没有头寸，则左侧为 NULL。

left join + right join + union 完全符合我的要求。似乎应该有一些巫术来允许完全加入而无需两次选择即可获得它，但我无法弄清楚。

drop table MatchPositions
go

create table MatchPositions
( 
    mt_source varchar (10),
    mt_symbol varchar (10),
    mt_qty float,
    mt_price float
)

go

insert into MatchPositions values ('BROKER', 'IBM', 100, 50.25)
insert into MatchPositions values ('BROKER', 'MSFT', 75, 30)
insert into MatchPositions values ('BROKER', 'GOOG', 25, 500)
insert into MatchPositions values ('BROKER', 'SPY', 200, 113)

insert into MatchPositions values ('MODEL', 'MSFT', 75, 30)
insert into MatchPositions values ('MODEL', 'GOOG', 25, 500)
insert into MatchPositions values ('MODEL', 'GLD', 300, 150)

go

select * from MatchPositions b
left join MatchPositions m on b.mt_symbol = m.mt_symbol and m.mt_source = 'MODEL'
where b.mt_source = 'BROKER'
union

select * from MatchPositions b
right join MatchPositions m on b.mt_symbol = m.mt_symbol and b.mt_source = 'BROKER'
where m.mt_source = 'MODEL'

这是预期的输出：

mt_source  mt_symbol  mt_qty                 mt_price               mt_source  mt_symbol  mt_qty                 mt_price
---------- ---------- ---------------------- ---------------------- ---------- ---------- ---------------------- ----------------------
NULL       NULL       NULL                   NULL                   MODEL      GLD        300                    150
BROKER     GOOG       25                     500                    MODEL      GOOG       25                     500
BROKER     IBM        100                    50.25                  NULL       NULL       NULL                   NULL
BROKER     MSFT       75                     30                     MODEL      MSFT       75                     30
BROKER     SPY        200                    113                    NULL       NULL       NULL                   NULL

Answer 1

如果您的 RDBMS 支持 FULL JOIN（也称为 FULL OUTER JOIN）：

SELECT *
FROM (SELECT * FROM MatchPositions WHERE mt_source = 'BROKER') b
FULL
JOIN (SELECT * FROM MatchPositions WHERE mt_source = 'MODEL' ) m
  ON b.mt_symbol = m.mt_symbol

此解决方案与 Martin 的解决方案基本相同，只是使用了不同的语法，如果您的 RDBMS 不支持 CTE，这可能会有所帮助。

Answer 2

试试这个：

select *
from      MatchPositions broker
full join MatchPositions model  on model.mt_symbol =  broker.mt_symbol
                               and model.mt_source <> broker.mt_source
where ( broker.mt_source = 'BROKER' or broker.MT_SOURCE is null )
  and ( model.mt_source  = 'MODEL'  or model.MT_SOURCE is null )

从第一个逻辑源表中，您需要代理行或缺失行。

从第二个逻辑源表中，您需要模型行或缺失行。

Answer 3

SELECT *
FROM MatchPositions b
  FULL JOIN MatchPositions m ON b.mt_symbol = m.mt_symbol
                            AND b.mt_source = 'BROKER'
                            AND m.mt_source = 'MODEL'

这会在外部连接之前将表过滤为“BROKER”和“MODEL”部分。

Answer 4

可能使用 isnull 函数：

SELECT *
FROM MatchPositions b
  FULL JOIN MatchPositions m on b.mt_symbol = m.mt_symbol
                            and b.mt_source != m.mt_source
WHERE isnull(b.mt_source, 'BROKER') = 'BROKER'
  and isnull(m.mt_source, 'MODEL') = 'MODEL'

Answer 5

;WITH T1 AS
(
SELECT *
FROM MatchPositions 
WHERE mt_source = 'BROKER'
), T2 AS
(
SELECT *
FROM MatchPositions 
WHERE mt_source = 'MODEL'
)
SELECT *
FROM T1 FULL JOIN T2 ON T1.mt_symbol = T2.mt_symbol