爲什麼這個左外連接查詢不能正常工作？

我有一個表，看起來像這樣：爲什麼這個左外連接查詢不能正常工作？

peter=> \d aggregated_accounts_by_month 
Table "public.aggregated_accounts_by_month" 
    Column | Type | Modifiers 
-----------+---------+----------- 
xtn_month | date | 
account | text | 
commodity | text | 
amount | numeric | 
Indexes: 
    "idx_aggregated_accounts_by_month_account" btree (account) 
    "idx_aggregated_accounts_by_month_month" btree (xtn_month)

而且看起來像這樣的另一個表：

peter=> \d months 
    Table "pg_temp_2.months" 
    Column | Type | Modifiers 
-----------+------+----------- 
xtn_month | date |

months包含此：

xtn_month 
------------ 
2011-01-01 
2011-02-01 
2011-03-01 
2011-04-01 
2011-05-01 
2011-06-01 
2011-07-01

aggregated_accounts_by_month包含此相關的數據：

xtn_month | account | amount 
------------+---------------+-------- 
2011-01-01 | Expenses:Fuel | 111.31 
2011-02-01 | Expenses:Fuel | 89.29 
2011-03-01 | Expenses:Fuel | 97.41 
2011-04-01 | Expenses:Fuel | 101.70 
2011-05-01 | Expenses:Fuel | 52.9 
2011-07-01 | Expenses:Fuel | 49.55

我試圖運行的查詢是：

select 
    months.xtn_month, 
    account, 
    amount 
from 
    aggregated_accounts_by_month a 
    left outer join months on months.xtn_month = a.xtn_month 
where 
    account = 'Expenses:Fuel' 
order by 
    xtn_month;

我想要的這個查詢做的就是給我的結果：

xtn_month | account | amount 
------------+---------------+-------- 
2011-01-01 | Expenses:Fuel | 111.31 
2011-02-01 | Expenses:Fuel | 89.29 
2011-03-01 | Expenses:Fuel | 97.41 
2011-04-01 | Expenses:Fuel | 101.70 
2011-05-01 | Expenses:Fuel | 52.9 
2011-06-01 | Expenses:Fuel | 
2011-07-01 | Expenses:Fuel | 49.55

但它實際上給我這個：

xtn_month | account | amount 
------------+---------------+-------- 
2011-01-01 | Expenses:Fuel | 111.31 
2011-02-01 | Expenses:Fuel | 89.29 
2011-03-01 | Expenses:Fuel | 97.41 
2011-04-01 | Expenses:Fuel | 101.70 
2011-05-01 | Expenses:Fuel | 52.9 
2011-07-01 | Expenses:Fuel | 49.55

我明顯在做錯事。有任何想法嗎？我在Mac OS X 10.6.7上運行PostgreSQL 9.0.4。

編輯：考慮到這一點之後，我需要對抗外部連接不僅僅是個月，還要對賬戶。該查詢不正是我想要的東西：

select 
    xtn_month, 
    account, 
    coalesce(amount, 0) 
from 
    (
     select 
      xtn_month, 
      account 
     from 
     (
      select 
       distinct xtn_month 
      from 
       aggregated_accounts_by_month 
     ) x 
     cross join 
     (
      select 
       distinct account 
      from 
       aggregated_accounts_by_month 
     ) y 
    ) z 
    left outer join aggregated_accounts_by_month 
     using (xtn_month, account) 
where 
    account = 'Expenses:Fuel' 
order by 
    xtn_month;

ypercube的回答是幾乎正確的，但它並沒有在account欄中填入。這個查詢當然相當昂貴，那裏有那個交叉產品。沒關係，但是，因爲aggregated_accounts_by_month有超過四年的數據少於2000行。

來源

2011-07-04 Pete Keen

兩件事情：

從WHERE條件反轉兩個表的順序在LEFT JOIN和
移動到ON子句。

。

select 
    months.xtn_month, 
    a.account, 
    a.amount 
from 
    months 
    left outer join aggregated_accounts_by_month a 
     on months.xtn_month = a.xtn_month 
     and a.account = 'Expenses:Fuel' 
order by 
    xtn_month;

來源

2011-07-04 00:27:03

這完全有效，並且實際解釋了爲什麼我的查詢不起作用。 'account'列結尾爲null，因爲它不在月份表中，因此'where'子句過濾出該行。謝謝！ –

您的aggregated_accounts_by_month不包含2011-06-01。你實際上可能會尋找一個完整的加盟：

select 
    months.xtn_month, 
    account, 
    amount 
from 
    aggregated_accounts_by_month a 
    full join months on months.xtn_month = a.xtn_month 
where 
    account = 'Expenses:Fuel' 
order by 
    xtn_month;

另外，左連接其他兩個答案建議，即months，然後aggregated_accounts_by_month。

來源

2011-07-04 00:32:34

爲什麼這個左外連接查詢不能正常工作？

回答

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