Question

我的SQL请求加入了2个表，结果是付款金额，按付款方式分组。付款方式显示在payment_history表格中： BlueMedia ， Transferuj，BankPayment 和 ChangeSum 。此外，payment_history.payment_method列可以包含空字符串或NULL - 在这种情况下，我想将其检索为“其他”字样。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（没有付款方式 Transferuj 的记录，它没有问题，导致id = 258的当前客户缺席

other表示该记录存在于payment_history中，其中payment_method为NULL或空字符串。

我不希望检索记录，其中付款方式为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也为空：

当other列包含payment_method ||空字符串时，为什么会出现以及如何使用所需方法（包括NULL）获取数据;但不带< / strong> ChangeStatus）？

Answer 1

当LEFT JOIN时，将右侧表条件放在ON子句中。（在WHERE时，您将获得定期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对left join和where条款的解释之外，您还可以使用coalesce()简化查询，并且可以围绕可以为空的值使用coalesce()进行比较所以他们不会返回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

比较payment_method为空时，null != 'ChangeSum'将返回null，而不是真。 Comparison Operators

扩展null比较问题：

Null被视为unknown，因此与null的比较始终为unknown，因此，如果您对null != 'ChangeSum'进行过滤，则结果不是{{1} （true），null仅包含条件为where的结果。 https://www.postgresql.org/docs/9.1/static/functions-comparison.html

当true为coalesce(payment_history.payment_method,'')!='ChangeSum'时，使用true会返回payment_method，因为null将coalesce值替换为null比较和''为'' != 'ChangeSum'。

Answer 3

使用is distinct from：

and payment_history.payment_method is distinct from 'ChangeSum'

Answer 4

感谢@SqlZim和@jarlh我已经改变了我的查询。下面有一个有效的变体：

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

AND payment_history.payment_method IS DISTINCT FROM 'ChangeSum'

而不是

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

，正如@Clodoaldo Neto提出的那样。

postgresql：附加的WHERE子句隐藏行

4 个答案: