PostgreSQL：使用同一表中的某些条件根据另一个列值更改列值答案

【问题标题】：PostgreSQL : Change column values based on another column value using some condition in same tablePostgreSQL：使用同一表中的某些条件根据另一个列值更改列值
【发布时间】：2022-11-04 02:04:37
【问题描述】：

我有一个表，想根据某些条件用其他列值的值替换列值。

 
+---------------------+
| Cntry | Code | Value |    
+---------------------+
| US  | C11  | A     | 
| US  | C12  | B     |
| US  | C13  | C     |
| US  | C14  | D     |
| US  | C15  | E     |
| UK  | C11  | A     | 
| UK  | C12  | B     |
| UK  | C13  | C     |
| UK  | C14  | D     |
| UK  | C15  | E     |

+---------------------+

我想根据Cntry的C11的值替换C14的值

所以我的输出应该是这样的。

+---------------------+
| Cntry | Code | Value |
+---------------------+
| US  | C11  | A     | 
| US  | C12  | B     |
| US  | C13  | C     |
| US  | C14  | A     |<====Repalce with C11 for US
| US  | C15  | E     |
| UK  | C11  | G     | 
| UK  | C12  | B     |
| UK  | C13  | C     |
| UK  | C14  | G     |<====Repalce with C11 for UK
| UK  | C15  | E     |

+---------------------+

无论如何在postgresql中可以做到这一点？

谢谢

【问题讨论】：

标签： postgresql

【解决方案1】：

创建样本数据：

CREATE TABLE table1 (
    cntry varchar NULL,
    code varchar NULL,
    value varchar NULL
);

INSERT INTO table1 (cntry, code, value) VALUES('US', 'C11', 'A');
INSERT INTO table1 (cntry, code, value) VALUES('US', 'C12', 'B');
INSERT INTO table1 (cntry, code, value) VALUES('US', 'C13', 'C');
INSERT INTO table1 (cntry, code, value) VALUES('US', 'C14', 'D');
INSERT INTO table1 (cntry, code, value) VALUES('US', 'C15', 'E');
INSERT INTO table1 (cntry, code, value) VALUES('UK', 'C11', 'G');
INSERT INTO table1 (cntry, code, value) VALUES('UK', 'C12', 'B');
INSERT INTO table1 (cntry, code, value) VALUES('UK', 'C13', 'C');
INSERT INTO table1 (cntry, code, value) VALUES('UK', 'C14', 'D');
INSERT INTO table1 (cntry, code, value) VALUES('UK', 'C15', 'E');

示例查询：

select 
    t1.cntry, 
    t1.code, 
    case when t2.value is not null then t2.value else t1.value end as "value"
from table1 t1  
left join (
    select 
        cntry, 
        'C14' as code, 
        value 
    from table1
    where code = 'C11'
) t2 on t1.cntry = t2.cntry and t1.code = t2.code 

-- Result: 
cntry   code    value
US      C11     A
US      C12     B
US      C13     C
US      C14     A
US      C15     E
UK      C11     G
UK      C12     B
UK      C13     C
UK      C14     G
UK      C15     E

【讨论】：