SQL 使用左外连接或右外连接查询

Question

我将 table 命名为 "Employee" 并带有相应的列（id、employee_id 和状态）。如果状态在 "pending" 中，则采用 "employee_id" 的待定状态并查找 employee_id 列的其他记录。所以 employee_id 存在然后我必须取 id 值并存储到 result_id.

id      employee_id     status  
 1      1000            failed  
 2      1001            failed  
 3      1002            failed  
 4      1005            failed  
 5      1006            failed  
 6      1005            pending
 7      1004            pending
 8      1001            pending
 9      1002            pending
10      1006            pending

示例：id=6,employee_id=1005 和 status='pending' 那么 result_id 应该是 4（即 result_id=4）

输出：

 id     result_id

 1      NULL     
 2      NULL     
 3      NULL     
 4      NULL     
 5      NULL     
 6      4        
 7      NULL     
 8      2        
 9      3        
 10     5

我试过：

select e.id as id ,e2.id as result_id 
from employee as e, employee as e2 
where e.employee_id=e2.employee_id and e.id not in (e2.id)

此查询将 return 只有 result_id 的值，但我也想要 Null 值。

谁能帮我解决这个问题？

Answer 1

检查这个：

架构（MySQL v5.7）

CREATE TABLE t (
  `id` INTEGER,
  `employee_id` INTEGER,
  `status` VARCHAR(12)
);

INSERT INTO t
  (`id`, `employee_id`, `status`)
VALUES
  ('1', '1000', 'failed'),
  ('2', '1001', 'failed'),
  ('3', '1002', 'failed'),
  ('4', '1005', 'failed'),
  ('5', '1006', 'failed'),
  ('6', '1005', 'pending<br/>'),
  ('7', '1004', 'pending<br/>'),
  ('8', '1001', 'pending<br/>'),
  ('9', '1002', 'pending<br/>'),
  ('10', '1006', 'pending<br/>');

查询#1

select id, null x from t where status = 'failed'
union all
select t.id, t2.id from t
  left join t t2 
    on t.employee_id = t2.employee_id and t2.status = 'failed'
where t.status != 'failed';

| id  | x   |
| --- | --- |
| 1   |     |
| 2   |     |
| 3   |     |
| 4   |     |
| 5   |     |
| 8   | 2   |
| 9   | 3   |
| 6   | 4   |
| 10  | 5   |
| 7   |     |

View on DB Fiddle

SQL 使用左外连接或右外连接查询

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