Question

如果我写这样的查询：

with foo as (
    select
        'foo' as name,
        1 as value
    union all select
        'foo', 2
    union all select
        'foo', 3
    union all select
        'foo', 4
), bar as (
    select
        'bar' as name,
        3 as value
    union all select
        'bar', 4
    union all select
        'bar', 5
    union all select
        'bar', 6
)
select
    f.name,
    b.value,
    b.name
from
    foo as f
full outer join
    bar as b
    on b.value = f.value;

我明白了：

name        value    name
foo         (null)  (null)
foo         (null)  (null)
foo         3       bar
foo         4       bar
(null)      5       bar
(null)      6       bar

这是我的期望。但是，如果我尝试使用像这样的表值函数做类似的事情：

with allSizes as (
        select cast('120X60' as character varying) as size
        union all select '160X600'
        union all select '1X1'
        union all select '300X250'
        union all select '728X90'
        union all select '88X32'
        union all select 'PEEL'
)
select
        msd.region,
        msd.market,
        s.size,
        msd.target,
        msd.actual
from 
        marketdata('2013-01-05') as msd
full outer join
        allSizes as s
        on s.size = msd.size
where
        msd.market = 'Foo, USA';

其中marketdata()是一个表值函数，我没有得到与size列对应的marketdata()列中没有的空行。为什么不呢？

Answer 1

这是因为：

where
        msd.market = 'Foo, USA';

它生成full outer join，然后过滤掉marketdata()中不存在的行（因为它们有msd.market = NULL）

您可以将查询更改为：

with allSizes as (
        select cast('120X60' as character varying) as size
        union all select '160X600'
        union all select '1X1'
        union all select '300X250'
        union all select '728X90'
        union all select '88X32'
        union all select 'PEEL'
)
select
        msd.region,
        msd.market,
        s.size,
        msd.target,
        msd.actual
from 
        (select marketdata('2013-01-05')
         where  market = 'Foo, USA') as msd
full outer join
        allSizes as s
        on s.size = msd.size;

或者只是重写

where
        msd.market = 'Foo, USA' OR msd.market IS NULL;

如果marketdata()没有返回msd.market IS NULL的行。

Postgres Full Outer加入headscratcher

1 个答案: