按另一列值进行分组和计数

Question

我有一张如下表：

CREATE TABLE public.test_table
(
    "ID" serial PRIMARY KEY NOT NULL,
    "CID" integer NOT NULL,
    "SEG" integer NOT NULL,
    "DDN" character varying(3) NOT NULL
)

和数据看起来像这样：

ID  CID SEG DDN
1   1   1   "711"
2   1   2   "800"
3   1   3   "124"
4   2   1   "711"
5   3   1   "711"
6   3   2   "802"
7   4   1   "799"
8   5   1   "799"
9   5   2   "804"
10  6   1   "799"

我需要按CID列对这些数据进行分组，并且获取列数取决于DDN列的第一个值，但是计数必须给我两个不同的信息，如果它超过1或者不是。

如果不能解释清楚，我真的很抱歉。让我告诉你我需要什么..

DDN END TRA
711 1   2
799 2   1

如您所见，DDN：711有1个单计数记录（ID：4）。这是END列。但是2次有多个SEG计数（ID：1到3和ID：5到6）。这是TRA专栏。

我不能确定group子句中应该有哪个列！

My solution:

刚刚找到了如下的解决方案

WITH x AS (
    SELECT
        (SELECT t1."DDN" FROM public.test_table AS t1
         WHERE t1."CID"=t."CID" AND t1."SEG"=1) AS ddn,
        COUNT("CID") AS seg_count       
    FROM public.test_table AS t
    GROUP BY "CID"
)

SELECT ddn, COUNT(seg_count) AS "TOTAL",    
    SUM(CASE WHEN x.seg_count=1 THEN 1 ELSE 0 END) as "END",
    SUM(CASE WHEN x.seg_count>1 THEN 1 ELSE 0 END) as "TRA"
FROM x
GROUP BY ddn;

Answer 1

等效，更快的查询：

SELECT "DDN"
     , COUNT(*) AS "TOTAL"
     , COUNT(*) FILTER (WHERE seg_count = 1) AS "END"
     , COUNT(*) FILTER (WHERE seg_count > 1) AS "TRA"
FROM  (
   SELECT DISTINCT ON ("CID")
         "DDN"          -- assuming min "SEG" is always 1
       , COUNT(*) OVER (PARTITION BY "CID") AS seg_count
   FROM   test_table
   ORDER  BY "CID", "SEG"
   ) sub
GROUP  BY "DDN";

db <>小提琴here

笔记：

• CTE通常较慢，只应在Postgres需要的地方使用。
• 这相当于问题中的查询，假设每个“CID”的最小“SEG”始终为1 - 因为此查询返回具有最小“SEG”的行，而查询返回带有"SEG" = 1的行。通常情况下，您会希望“第一个”段和我的查询更可靠地实现此要求，但问题并不清楚。
• COUNT(*)比COUNT(column)稍微快一点，而且不涉及NULL值（适用于此处）。有关： PostgreSQL: running count of rows for a query 'by minute'
• 关于DISTINCT ON： Select first row in each GROUP BY group?
• 汇总FILTER语法需要Postgres 9.4+： SQL count if columns

Answer 2

这是我提出的解决方案，我猜这个查询可以简化。

CREATE TABLE test_table
(
    ID serial PRIMARY KEY NOT NULL,
    CID integer NOT NULL,
    SEG integer NOT NULL,
    DDN character varying(3) NOT NULL
);



insert into test_table(CID,SEG,DDN)
values
(   1,   1,   '711'),
(   1,   2,   '800'),
(   1,   3,   '124'),
(   2,   1,   '711'),
(   3,   1,   '711'),
(   3,   2,   '802'),
(   4,   1,   '799'),
(   5,   1,   '799'),
(   5,   2,   '804'),
(   6,   1,   '799');


with summary as (with ddn_t as (select cid,ddn,row_number() OVER( PARTITION BY cid)from test_table)
select a.cid,count(distinct a.ddn),b.ddn
 from ddn_t a 
 join ddn_t b on b.cid=a.cid and b.row_number=1
group by a.cid, b.ddn)
select ddn,
sum (case when count >1 then 1 else 0 end) as TRA,
sum (case when count = 1 then 1 else 0 end) as END
from summary
group by ddn;