取消PostgreSQL中的数据

问题描述

我在Postgresql中有一个具有以下值的表，

empid    hyderabad    bangalore    mumbai    chennai
 1        20           30           40        50
 2        10           20           30        40

我的输出应如下所示

empid    city         nos
1        hyderabad    20
1        bangalore    30
1        mumbai       40
1        chennai      50
2        hyderabad    10
2        bangalore    20
2        mumbai       30
2        chennai      40

我如何在Postgresql中做到这一点？

解决方法

您可以使用横向连接：

select t.empid,x.city,x.nos
from the_table t
  cross join lateral (
     values 
       ('hyderabad',t.hyderabad),('bangalore',t.bangalore),('mumbai',t.mumbai),('chennai',t.chennai)
  ) as x(city,nos)
order by t.empid,x.city;

,

或者这一个：更易于阅读和真实的普通SQL ...

WITH
input(empid,hyderabad,bangalore,mumbai,chennai) AS (
          SELECT 1,20,30,40,50
UNION ALL SELECT 2,10,40
),i(i) AS (
          SELECT 1
UNION ALL SELECT 2
UNION ALL SELECT 3
UNION ALL SELECT 4
)
SELECT
  empid,CASE i 
    WHEN 1 THEN 'hyderabad'
    WHEN 2 THEN 'bangalore'
    WHEN 3 THEN 'mumbai'
    WHEN 4 THEN 'chennai'
    ELSE        'unknown'
  END AS city,CASE i 
    WHEN 1 THEN hyderabad
    WHEN 2 THEN bangalore
    WHEN 3 THEN mumbai
    WHEN 4 THEN chennai
    ELSE        NULL::INT
  END AS city 
FROM input CROSS JOIN i
ORDER BY empid,i;
-- out  empid |   city    | city 
-- out -------+-----------+------
-- out      1 | hyderabad |   20
-- out      1 | bangalore |   30
-- out      1 | mumbai    |   40
-- out      1 | chennai   |   50
-- out      2 | hyderabad |   10
-- out      2 | bangalore |   20
-- out      2 | mumbai    |   30
-- out      2 | chennai   |   40