无法让这个查询工作（左外连接）Postgres

Question

我有这个查询，我想开始工作。我试图在某个非空的日期之后计算外键的所有实例，然后将其与原始表的不同之处连接起来，这样我就可以看到没有条目的变量。这是我的查询：

select fulllist.fk_fc_id 
from 
(
    select distinct fk_fc_id 
    from data_item
) as fulllist 
left outer join
(
    select temp.fk_fc_id, count(*) as number 
    from 
    (
        select * 
        from data_item 
        where di_item_value not like 'Null' 
            and di_timestamp > '2013-11-01 00:00:00'
    )   as temp 
    group by fk_fc_id
)   as lj
    on lj.fc_fk_id=fulllist.fk_fc_id;

错误是

错误：列 lj.fc_fk_id 不存在

第 19 行：在 lj.fc_fk_id=fulllist.fk_fc_id 上；

Answer 1

看起来这只是一个错字。您的子查询正在返回，fk_fc_id但您的连接正在引用fc_fk_id. 似乎您只需要更改查询：

select fulllist.fk_fc_id 
from 
(
    select distinct fk_fc_id 
    from data_item
) as fulllist 
left outer join
(
    select temp.fk_fc_id, count(*) as number 
    from 
    (
        select * 
        from data_item 
        where di_item_value not like 'Null' 
            and di_timestamp > '2013-11-01 00:00:00'
    )   as temp 
    group by fk_fc_id
)   as lj
    on lj.fk_fc_id=fulllist.fk_fc_id;

您似乎还希望将 包含number在最终选择中：

select fulllist.fk_fc_id, coalesce(lj.number, 0) number
from 
(
    select distinct fk_fc_id 
    from data_item
) as fulllist 
left outer join
(
    select temp.fk_fc_id, count(*) as number 
    from 
    (
        select * 
        from data_item 
        where di_item_value not like 'Null' 
            and di_timestamp > '2013-11-01 00:00:00'
    )   as temp 
    group by fk_fc_id
)   as lj
    on lj.fk_fc_id=fulllist.fk_fc_id;

Answer 2

除了拼写错误（已由@blueefet 发现），您的查询还可以简化：

select fk_fc_id,
       COUNT( CASE WHEN di_item_value not like 'Null' 
                        and di_timestamp > '2013-11-01 00:00:00'
                   THEN 1 ELSE NULL END
            ) AS number 
from data_item 
group by fk_fc_id ;