如何从初始行创建多行

Question

我用的是mysql db engine，我想知道有没有可能将表中的一行数据转移到另一个表中，这个表由两列组成，id和value 每个传输的值都将进入一行，并且行看起来像 ID，值，并且只要它具有传输到新行的值，只要它具有属于的 id 的值，就维护 id从中转移的行

初始表的样子

id  |country_name   |city_1      |city_2      |city_3      |city_4
------------------------------------------------------------------------
1   |Some_country   |some_city1  |some_city2  |some_city3  |some_city4

想要的桌子看起来像

 id | city_name
 1  |  some_city1
 1  |  some_city2
 1  |  some_city3
 1  |  some_city4

Answer 1

将此用于特定的ID

select id, city_name from(
    select id, city_1 as city_name from yourTable    
    union all
    select id, city_2 from yourTable    
    union all
    select id, city_3 from yourTable    
    union all
    select id, city_4 from yourTable
) as t where id= yourID

http://sqlfiddle.com/#!9/7ee1f/1

整个表都用这个

 select id, city_name from(
    select id, city_1 as city_name from yourTable    
    union all
    select id, city_2 from yourTable    
    union all
    select id, city_3 from yourTable    
    union all
    select id, city_4 from yourTable
) as t
order by id

Answer 2

您要查找的内容通常称为垂直旋转：您希望将诸如四个城市名称的数组（硬连线到表定义中）旋转成四个垂直行。

解决方案是与一个临时表进行交叉连接，该表具有从 1 开始的多个连续整数，因为您有要旋转的列，并​​结合使用该系列整数的 CASE-WHEN 表达式。

看这里：

WITH foo(id,country_name,city_1,city_2,city_3,city_4) AS (
SELECT 1,'Some_country','some_city1','some_city2','some_city3','some_city4'
)
,    four_indexes(idx) AS (
          SELECT 1
UNION ALL SELECT 2
UNION ALL SELECT 3
UNION ALL SELECT 4
)
SELECT
  id  AS country_id
, idx AS city_id
, CASE idx 
    WHEN 1 THEN city_1
    WHEN 2 THEN city_2
    WHEN 3 THEN city_3
    WHEN 4 THEN city_4
    ELSE ''
  END AS city_name
FROM foo CROSS JOIN four_indexes
;
country_id|city_id|city_name
         1|      1|some_city1
         1|      3|some_city3
         1|      2|some_city2
         1|      4|some_city4

就在前几天，我回答了一个问题，该问题正在寻找反转我们在这里执行的操作：水平旋转。 如果你好奇，请看这里... How to go about a column with different values in a same row in sql?

玩得开心-

理智的马可