Question

我尝试编写Postgres查询以显示不同数量的人，但计算实际（非独特）人数。

所以，如果我有

1  Ray ray@gmail.com
2  Ray ray@gmail.com
3  Kate kate@gmail.com

我想要展示：

Ray 2
Kate 1

==

SELECT name, email, COUNT(*) 
FROM (SELECT DISTINCT name, email 
FROM people
WHERE degree = 'Gradiate')

我明白了：

ERROR:  subquery in FROM must have an alias
LINE 3: FROM (SELECT DISTINCT name, email

如何解决这个问题？

Answer 1

你根本不需要子查询（这需要一个已经指出的表别名）。对你需要的东西没有意义。

像这样使用GROUP BY：

SELECT name, email, count(*) AS ct
FROM   people
WHERE  degree = 'Graduate'
GROUP  BY 1, 2;

在这种情况下，

GROUP BY 1, 2是GROUP BY name, email的简写。 Example with more details.

当然，name 和 email必须与此匹配，不像您只展示name和计数（与您的查询相矛盾）的示例。

Answer 2

添加

AS some_alias

像：

SELECT name, email, COUNT(*) 
FROM (SELECT DISTINCT name, email 
FROM people
WHERE degree = 'Gradiate') as my_alias

Answer 3

SELECT name, count(*) AS count
FROM   people
WHERE  degree = 'Gradiate'
GROUP  BY name;