如果一个值在一个表中但不在另一个表中，如何加入并给出默认值？答案

【问题标题】：How to join and give a default if a value is in one table but not another?如果一个值在一个表中但不在另一个表中，如何加入并给出默认值？
【发布时间】：2017-07-06 09:02:24
【问题描述】：

我正在使用 MySQL，而且我是新手！希望你们能帮我解答一个 SQL 问题。

假设我有 2 个表，我想要一个简单的连接。

表 1：

   id | service_id | user_number
   ----------------------------------------------------------------------
   0  | 1001       | 10
   1  | 1002       | 20
   2  | 1004       | 40

表 2：

   id | service_id | error_number
   ----------------------------------------------------------------------
   0  | 1001       | 1000
   1  | 1003       | 3000
   2  | 1004       | 4000

我想在service_id 上进行连接，如果不存在，则将user_number 和error_number 的默认值设为0。所以：

   id | service_id | user_number | error_number
   ----------------------------------------------------------------------
   0  | 1001       | 10          | 1000
   1  | 1002       | 20          | 0
   3  | 1003       | 0           | 3000
   2  | 1004       | 40          | 4000

我尝试了一些查询，但他们一直给我null 而不是 0。非常感谢。

【问题讨论】：

向我们展示你的尝试:)

标签： mysql sql database

【解决方案1】：

这里你应该先使用union，然后进行聚合：

select t.`service_id`, sum(t.`user_number`) as `user_number`, sum(t.`error_number`) as `error_number`
from (
    select `service_id`, `user_number`, 0 as `error_number` from t1
    union
    select `service_id`, 0 as `user_number`, `error_number` from t2
) t
group by `service_id`

demo 在这里。

【讨论】：

【解决方案2】：

你可以试试这个，伙计：

SELECT
    t1.id,
    t1.service_id, 
    COALESCE(tb1.user_number, 0) `user_number`,
    COALESCE(tb2.error_number, 0) `error_number`
FROM
    (
        SELECT id, service_id
        FROM table1
        UNION
        SELECT id, service_id
        FROM table2
    ) t1
    LEFT JOIN table1 tb1 ON tb1.service_id = t1.service_id
    LEFT JOIN table2 tb2 ON tb2.service_id = t1.service_id;

【讨论】：

【解决方案3】：

试试这个：

select COALESCE(t1_service,t2_service ) as service_id, COALESCE(user_number,0) as user_number , COALESCE(error_number,0) as error_number 
from (
 select t1.service_id as t1_service , t1.user_number , t2.error_number, t2.service_id as t2_service 
 from table_1  t1
 LEFT OUTER JOIN table_2 t2  
 on t1.service = t2.service
union 
 select t1.service_id as t1_service , t1.user_number , t2.error_number, t2.service_id as t2_service 
 from table_1  t1  
 Right OUTER JOIN table_2 t2   
 on t1.service = t2.service
)z1
order by service_id

【讨论】：