如何将多列合并为一个？

Question

我有以下示例表：

| name   | email1          | email2            | email3         | 
.................................................................
| John   | john@gmail.com  | john@hotmail.com  | john@yahoo.com |
| Jane   | jane@gmail.com  | NULL              | NULL           |

我需要一个查询来合并电子邮件列，使其结果看起来像这样，忽略空值：

| name   | email            |
.............................
| John   | john@gmail.com   |
| John   | john@hotmail.com | 
| John   | john@yahoo.com   |
| Jane   | jane@gmail.com   |

我该怎么做？

Answer 1

您可以使用横向连接：

select t.name, v.email
from t cross join lateral
     (values (email1), (email2), (email3)) v(email)
where v.email is not null;

Answer 2

您可以在每个电子邮件列上union all进行一系列查询：

SELECT name, email1
WHERE  email1 IS NOT NULL
UNION ALL
SELECT name, email2
WHERE  email2 IS NOT NULL
UNION ALL
SELECT name, email3
WHERE  email3 IS NOT NULL