sql - Postgres 全外连接 headscratcher

Question

如果我这样写一个查询:

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

这是我所期望的。但是，如果我尝试对表值函数执行类似的操作，如下所示:

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 列对应的空行，而 marketdata( ) 函数。为什么不呢？

Answer 1

这是因为:

where
        msd.market = 'Foo, USA';

它生成全外连接，然后过滤掉marketdata()中不存在的行(因为它们有msd.market = 空)

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;

或者干脆重写

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

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