Question

我正在使用Postgres 9.6。*

我有这个：

street | first_name | last_name
1st    | james      | bond
1st    | mr         | q 
1st    | ms         | m
2nd    | man        | with_golden_gun

我想获得一个不同地址的列表以及每个地址的第一组'first_name'和'last_name'。

我想要的输出：

street | first_name | last_name
1st    | james      | bond
2nd    | man        | with_golden_gun

我按street进行分组，并尝试MIN(first_name)和MIN(last_name) - 然而 - 使用MIN，每组独特的街道都有案例，我可以看似随机混合匹配 first_name和last_name可能不属于同一行。显然，MIN（最小）在这里不是正确的聚合函数。

我的问题：如何强制first_name和last_name 来自同一行？

Answer 1

您可以使用row_number窗口函数查询每个组的一行：

SELECT street, first_name, last_name
FROM   (SELECT street, first_name, last_name,
               ROW_NUMBER() OVER (PARTITION BY street ORDER BY first_name) AS rn
        FROM   mytable) t
WHERE  rn = 1

Answer 2

你需要＆＃34; DISTINCT ON＆＃34;条款，但这需要排序，例如如first_name：

SELECT
 DISTINCT ON (street)
 street, first_name, last_name
FROM table
ORDER BY street, first_name

Answer 3

- 我根据min first_name对此进行分组，并根据该名字

获取姓氏

Select street, first_name, 
(select last_name from person o where o.first_name = x.first_name) 
from (Select street, min(first_name) as first_name  
from person v group by street) as x;

- 输出

street | first_name | last_name
-------------------------------------
1st    | james      | bond
2nd    | man        | with_golden_gun

- 如果可以将名字和姓氏结合起来

Select street, min(concat(first_name , ' ' , last_name)) as name
from person group by street

- 输出

street | name 
----------------------------
1st    | james bond
2nd    | man with_golden_gun

Answer 4

如How to show row numbers in PostgreSQL query?中所述你可以得到一个行号。然后，您可以根据需要对SELECT语句进行ORDER或WHERE。

SQL：如何选择组中同一个SINGLE行的两个字段？

4 个答案: