postgresql：附加的 WHERE 子句隐藏了行

Question

我的 SQL-请求加入 2 tables，结果是付款金额，按付款方式分组。 payment_history table 中存在付款方式：BlueMedia、Transferuj、BankPayment 和 ChangeSum。 payment_history.payment_method 列也可以包含空字符串或 NULL - 在这种情况下，我想将其检索为 'other' 值。

SQL-查询是：

SELECT 
    CASE
        WHEN COALESCE(payment_history.payment_method, '') = '' THEN 'other'
        ELSE payment_history.payment_method
    END as payment_method, 
    CASE
        WHEN SUM(paid_balance) IS NULL THEN 0.00
        ELSE SUM(paid_balance)
    END as paid_balance, 
    CASE
        WHEN SUM(sum_without_commission) IS NULL THEN 0.00
        ELSE SUM(sum_without_commission)
    END as sum_without_commission                   
FROM 
    payment_history 
    LEFT JOIN 
    participants_list ON payment_history.registration_id = participants_list.registration_id 
WHERE 
    payment_history.client_id = 258 AND
    participants_list.deleted = 0 AND
    participants_list.is_reserved = 0
GROUP BY payment_history.payment_method

结果（没有支付方式Transferuj的记录，没关系，因为id= 258的当前客户不存在）：

other 表示记录存在于 payment_history 中，其中 payment_method 是 NULL 或空字符串。

我不想检索记录，支付方式是ChangeSum。所以，我在查询中添加了适当的条件：

...
WHERE payment_history.client_id = 258 
AND payment_history.payment_method != 'ChangeSum'
...

但在结果集中 other 也是空的：

为什么会出现这种情况，我该如何使用所需的方法获取数据（包括 other，当 payment_method 列包含 NULL|| 空字符串；但没有 ChangeStatus)?

Answer 1

当LEFT JOIN时，把右边的table条件放在ON子句中。（在 WHERE 时，您将得到常规 INNER JOIN 结果。）

SELECT 
    CASE
        WHEN COALESCE(payment_history.payment_method, '') = '' THEN 'other'
        ELSE payment_history.payment_method
    END as payment_method, 
    CASE
        WHEN SUM(paid_balance) IS NULL THEN 0.00
        ELSE SUM(paid_balance)
    END as paid_balance, 
    CASE
        WHEN SUM(sum_without_commission) IS NULL THEN 0.00
        ELSE SUM(sum_without_commission)
    END as sum_without_commission                   
FROM payment_history 
LEFT JOIN participants_list 
ON payment_history.registration_id = participants_list.registration_id 
  AND participants_list.deleted = 0 
  AND participants_list.is_reserved = 0
WHERE payment_history.client_id = 258 
GROUP BY payment_history.payment_method

Answer 2

除了 jarlh 已解释的 left join 和 where 子句之外，您还可以使用 coalesce() 简化查询，并且可以在可为 null 的值周围使用 coalesce()用于比较，因此它们不会 return null.

select 
    coalesce(payment_history.payment_method, 'other') as payment_method
  , coalesce(sum(paid_balance),0.00) as paid_balance
  , coalesce(sum(sum_without_commission),0.00) as sum_without_commission
from payment_history 
  left join participants_list 
    on payment_history.registration_id = participants_list.registration_id 
   and participants_list.deleted = 0 
   and participants_list.is_reserved = 0
where payment_history.client_id = 258 
  and coalesce(payment_history.payment_method,'')!='ChangeSum'
group by payment_history.payment_method

当比较payment_method为null时，null != 'ChangeSum'将returnnull，不成立。 Comparison Operators

扩展 null 比较问题：

Null 被视为 unknown，因此与 null 的比较始终是 unknown，因此如果您过滤 null != 'ChangeSum'，结果不是 true（即 null），而您的 where 仅包含您的条件为 true 的结果。 https://www.postgresql.org/docs/9.1/static/functions-comparison.html

当 payment_method 为 null 时使用 coalesce(payment_history.payment_method,'')!='ChangeSum' returns true，因为 coalesce 正在替换 null 值与 '' 进行比较， '' != 'ChangeSum' 是 true.

Answer 3

感谢@SqlZim 和@jarlh，我更改了我的查询。以下是有效变体：

SELECT 
COALESCE(NULLIF(payment_history.payment_method, ''), 'other') AS payment_method, 
COALESCE(SUM(paid_balance), 0.00) AS paid_balance,
COALESCE(SUM(sum_without_commission), 0.00) AS sum_without_commission                  
FROM payment_history 
INNER JOIN participants_list 
ON payment_history.registration_id = participants_list.registration_id 
WHERE payment_history.client_id = 258 
AND participants_list.deleted = 0 
AND participants_list.is_reserved = 0
AND COALESCE(payment_history.payment_method, '') != 'ChangeSum'
GROUP BY payment_history.payment_method

也可以写成

AND payment_history.payment_method IS DISTINCT FROM 'ChangeSum'

而不是

AND COALESCE(payment_history.payment_method, '') != 'ChangeSum'

，正如@Clodoaldo Neto 所建议的那样。

Answer 4

使用is distinct from:

and payment_history.payment_method is distinct from 'ChangeSum'

postgresql：附加的 WHERE 子句隐藏了行

postgresql: additional WHERE-clause hides rows

sql

postgresql

where