分组并添加列

Question

我有一个名为 'ward_councillors' 的 table 和两列 'ward' 和 'councillor'。此 table 包含所有议员及其负责的选区的列表。每个选区有三名议员，我需要做的是分组，这样我每个选区都有一个不同的记录和三个新列，每个选区的三名议员负责。

例如目前有：

WARD   | COUNCILLOR
ward a | cllr 1
ward a | cllr 2
ward a | cllr 23
ward b | cllr 4
ward b | cllr 5
ward b | cllr 8

努力实现：

WARD    | COUNCILLOR 1| COUNCILLOR 2| COUNCILLOR 3
ward a  | cllr 1      | cllr 2      | cllr 23
ward b  | cllr 4      | cllr 5      | cllr 8

提前致谢，克里斯

Answer 1

我倾向于使用条件聚合来处理此类查询：

select ward,
       max(case when seqnum = 1 then councillor end) as councillor1,
       max(case when seqnum = 2 then councillor end) as councillor2,
       max(case when seqnum = 3 then councillor end) as councillor3
from (select wc.*,
             row_number() over (partition by ward order by councillor) as seqnum
      from ward_councillors wc
     ) wc
group by ward;

Answer 2

我对 sqlFiddle 有疑问，但我在本地 postgres 上写了这个：

CREATE TABLE ward_councillors (
  ward VARCHAR
  , councillor VARCHAR
);

INSERT INTO ward_councillors
VALUES 
('ward a', 'cllr 1')
, ('ward a', 'cllr 2')
, ('ward a', 'cllr 23')
, ('ward b', 'cllr 4')
, ('ward b', 'cllr 5')
, ('ward b', 'cllr 8')
;


SELECT * 
FROM crosstab('SELECT ward, null /*doesnt really matter*/, councillor  FROM  ward_councillors') 
    AS (ward VARCHAR, councillor_1 VARCHAR, councillor_2 VARCHAR, councillor_3 VARCHAR, councillor_4 VARCHAR)
;

似乎输出了你想要的：

  ward  | councillor_1 | councillor_2 | councillor_3 | councillor_4
--------+--------------+--------------+--------------+--------------
 ward a | cllr 1       | cllr 2       | cllr 23      |
 ward b | cllr 4       | cllr 5       | cllr 8       |
(2 rows)

都在这里描述了：http://www.postgresql.org/docs/9.1/static/tablefunc.html#AEN142967

分组并添加列

Group By and add columns

sql

postgresql

pivot

crosstab