如何编写SQL来转换表，使行变为列，反之亦然

Question

This is my table:

+--------+-----------+
|country |city       |
+--------+-----------+
| india  | hydrabad  |
| india  | bangalore |
| india  | mumbai    |
| UK     | London    |
| UK     | france    |
| UK     | Conberry  |
| USA    | new jersy |
| USA    | texas     |
| USA    | New YORK  |
+--------+-----------+

I want to show it like this:

+--------+----------+--------+------------+
|country |city1     |city2   |city3       |
+--------+----------+--------+------------+
| india  | hydrabad | mumbai | Bangalore  | 
| UK     | London   | France | Conberry   |
| USA    | newjersy | texas  | New YORK   |
+--------+----------+--------+------------+

How can I write a MySQL query to do this?

Answer 1

you can use a query like this:

SELECT country,
  GROUP_CONCAT(IF(nr=0,city,NULL)) AS city1,
  GROUP_CONCAT(IF(nr=1,city,NULL)) AS city2,
  GROUP_CONCAT(IF(nr=2,city,NULL)) AS city3
FROM (
  SELECT 
  @nr := IF(@country_last = c.country, @nr:=(@nr+1),@nr:=0) AS nr,
  @country_last := c.country AS country,
  c.city
  FROM country c
  CROSS JOIN (SELECT @nr := 0, @country_last := '') param
  ORDER BY c.country
) AS d
GROUP BY country;

sample

MariaDB [yourschema]> select * from country;
+---------+-----------+
| country | city      |
+---------+-----------+
| india   | hydrabad  |
| india   | bangalore |
| india   | mumbai    |
| uk      | London    |
| uk      | france    |
| uk      | Conberry  |
+---------+-----------+
6 rows in set (0.00 sec)

MariaDB [yourschema]>

MariaDB [yourschema]> SELECT country,
    ->   GROUP_CONCAT(IF(nr=0,city,NULL)) AS city1,
    ->   GROUP_CONCAT(IF(nr=1,city,NULL)) AS city2,
    ->   GROUP_CONCAT(IF(nr=2,city,NULL)) AS city3
    -> FROM (
    ->   SELECT
    ->   @nr := IF(@country_last = c.country, @nr:=(@nr+1),@nr:=0) AS nr,
    ->   @country_last := c.country AS country,
    ->   c.city
    ->   FROM country c
    ->   CROSS JOIN (SELECT @nr := 0, @country_last := '') param
    ->   ORDER BY c.country
    -> ) AS d
    -> GROUP BY country;
+---------+----------+-----------+----------+
| country | city1    | city2     | city3    |
+---------+----------+-----------+----------+
| india   | hydrabad | bangalore | mumbai   |
| uk      | London   | france    | Conberry |
+---------+----------+-----------+----------+
2 rows in set (0.00 sec)

MariaDB [yourschema]>

Answer 2

Use join query for it( If cities are fixed)

SELECT A.country,A.city city1,B.city city2,C.city city3 FROM tblname A 
    LEFT JOIN tblname B ON A.country=B.country
    LEFT JOIN tblname C ON A.country=C.country
WHERE A.city!=B.city and A.city!=C.city and B.city!=C.city
GROUP BY A.country

If you want to use all cities separated by comma in single column use like this,

SELECT country, GROUP_CONCAT(city) as Cities FROM tblname GROUP BY country

Answer 3

Try this query,

Select Country, City1, City2, City3
From
(
  Select Country, City,
    'City'+
      cast(row_number() over(partition by Country order by Country)
             as varchar(10)) ColumnSequence
  From Country
) Temp
Pivot
(
  Max(City)
  For ColumnSequence in (City1, City2, City3)
) Piv