MySql 左外连接未按预期工作

Question

我有以下表结构。

loan和loan_repayment的关系是一对多的

贷款

---------------------------------------------------------------------------------------
   id   |                 loan_amount         |           loan_duration         
---------------------------------------------------------------------------------------
    1   |                    60000            |                 3               
    2   |                    80000            |                 4
---------------------------------------------------------------------------------------

贷款还款

---------------------------------------------------------------------------------------
   id   |      loan_id     |      amount      |       month      |       status
---------------------------------------------------------------------------------------
    1   |          1       |       20000      |         1        |       Pending
    2   |          1       |       20000      |         2        |       Pending
    3   |          1       |       20000      |         3        |       Pending


    4   |          2       |       20000      |         1        |       Pending
    5   |          2       |       20000      |         2        |       Pending
    6   |          2       |       20000      |         3        |       Pending
    7   |          2       |       20000      |         4        |       Pending

我只想从整个两个表中获取每笔贷款的贷款金额及其各自支付的月数。我正在做这样的查询。

SELECT loan.`loan_amount`, COUNT(loan_repayment.`loan_id`) FROM loan 
LEFT OUTER JOIN loan_repayment ON 
      (loan.`id`=loan_repayment.`loan_id` AND loan_repayment.`status`='Paid' ) 
GROUP BY loan_repayment.`loan_id`

我在这里使用了左外连接，即使根据 where 子句，loan_repayment 表为空，我仍然可以得到结果为 0。但问题是，它只返回以下数据。我不知道为什么它现在返回以下（贷款）行。

---------------------------------------------------------------------------------------
             loan_amount                  |        count(loan_repayment.loan_id)
---------------------------------------------------------------------------------------
                60000                     |                     0
---------------------------------------------------------------------------------------

实际上我已经创建了一个包含许多其他限制的 Hibernate 标准，但是在进一步研究生成的 sql 后，它缩小到这一点。为什么第 2 行数据及其子数据不包含在建议的 sql 的结果中？

如果可以在休眠条件下复制它也会很有帮助（没什么大不了的，我可以弄清楚，但担心那个 sql 的结果）。

更新：

如果我从

更改查询

loan_repayment.`status`='Paid' 

到

loan_repayment.`status`='Pending'

我得到以下结果

---------------------------------------------------------------------------------------
             loan_amount                  |        count(loan_repayment.loan_id)
---------------------------------------------------------------------------------------
                60000                     |                     3
                80000                     |                     4
---------------------------------------------------------------------------------------

对于完全相反的条件，为什么它没有产生与计数为 0 相同的结果，这有点奇怪。

Answer 1

在查看以前没有接受或赞成的答案的问题时遇到了这个问题。

除了group by 子句之外，您的查询是正确的，您已按left join (GROUP BY loan_repayment.loan_id) 的right side 的表字段分组，这就是您没有其他值的原因，你应该将它们按loan.id 分组，让我们测试一下：

架构：

create table loan( id int, loan_amount  int, loan_duration int);
insert into loan values
(1,60000,3),(2,80000,4);
create table loan_repayment (id int,loan_id int,amount int,month int,status varchar(20));
insert into loan_repayment values
(1 ,1 ,20000,1,'Pending'),(2 ,1 ,20000,2,'Pending'),(3 ,1 ,20000,3,'Pending'),
(4 ,2 ,20000,1,'Pending'),(5 ,2 ,20000,2,'Pending'),(6 ,2 ,20000,3,'Pending'),
(7 ,2 ,20000,4,'Pending');

现在有了这个查询：

SELECT l.loan_amount, COUNT(lr.loan_id) as Paied 
FROM loan l
LEFT OUTER JOIN loan_repayment lr ON 
      (l.id=lr.loan_id AND lr.status='paid') 
GROUP BY l.id

你会得到：

loan_amount Paied
60000       0
80000       0

你也可以同时拥有paid和pending一起：

SELECT l.loan_amount, SUM(CASE 
                            WHEN lr.status='paid' THEN 1
                            ELSE 0
                          END) as Paied,
                       SUM(CASE 
                            WHEN lr.status='Pending' THEN 1
                            ELSE 0
                          END) as Pending

FROM loan l
LEFT OUTER JOIN loan_repayment lr ON 
      (l.id=lr.loan_id) 
GROUP BY l.id

结果：

loan_amount Paied   Pending
60000       0       3
80000       0       4

您可以使用 Hibernate HQL 或 Criteria api 实现上述结果，我认为将上述查询（您尝试的第一个查询）映射到 Hibernate HQL 或 Criteria 并不困难.

Answer 2

看起来好像是错误的 GROUP BY，试试下面的GROUP BY loan.id

SELECT loan.`loan_amount`, COUNT(loan_repayment.`loan_id`) FROM loan 
LEFT OUTER JOIN loan_repayment ON 
      (loan.`id`=loan_repayment.`loan_id` AND loan_repayment.`status`='Paid' ) 
GROUP BY loan.id