postgresql：额外的WHERE子句隐藏行

Question

我的SQL请求加入2个表，结果是支付金额，按付款方式分组。付款方式存在于payment_history表中：BlueMedia,Transferuj，BankPayment和ChangeSum。另外payment_history.payment_method列可以包含空字符串或NULL - 在这种情况下，我想将其作为“其他”值进行检索。

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（以付款方式Transferuj没有记录，这是好的，因为它是缺席了当前客户端id = 258）：

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

我不想检索记录，其中付款方式为ChangeSum。在查询所以，我已经添加了相应的条件：

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

但在结果集也other是空的：

为什么会出现这种问题，我能做些什么来获得所需要的数据方法（包括other，当payment_method列包含NULL ||空字符串;但没有ChangeStatus）？

Answer 1

除了什么jarlh已经expalined约left join和where条款，你可以用coalesce()简化您的查询，您可以使用coalesce()围绕一个空值比较，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 

当比较payment_method为null，null != 'ChangeSum'将返回null，事实并非如此。 Comparison Operators

扩大对null比较问题：

Null被视为unknown，所以比较null总是unknown，所以如果你在null != 'ChangeSum'筛选结果不是true（这是null），和您的where只包括您的条件为true的结果。https://www.postgresql.org/docs/9.1/static/functions-comparison.html

使用coalesce(payment_history.payment_method,'')!='ChangeSum'返回true当payment_method是null，因为​​3210与''用于比较和'' != 'ChangeSum'取代null值是true。

Answer 2

当LEFT JOIN，在ON条款中放置右侧表条件。 （当WHERE，你会得到定期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 3

感谢@SqlZim和@jarlh我已经改变了我的查询。下面有有效的变体：

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 

也有可能写，而不是

AND payment_history.payment_method IS DISTINCT FROM 'ChangeSum'

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

，作为@Clodoaldo内托提议。

Answer 4

使用is distinct from：

and payment_history.payment_method is distinct from 'ChangeSum'