Mysql, reshape data from long / tall to wide

Solution 1:

Cross-tabs or pivot tables is the answer. From there you can SELECT FROM ... INSERT INTO ... or create a VIEW from the single SELECT.

Something like:

SELECT country, 
       MAX( IF( key='President', value, NULL ) ) AS President,
       MAX( IF( key='Currency', value, NULL ) ) AS Currency,
       ...

FROM table 
GROUP BY country;

For more info: http://dev.mysql.com/tech-resources/articles/wizard/index.html

Solution 2:

I think I found the solution, which uses VIEWS and INSERT INTO (as suggested by e4c5).

You have to get your list of AttrNames/Keys yourself, but MYSQL does the other heavy lifting.

For the simple test case above, create the new_table with the appropriate columns (don't forget to have an auto-increment primary key as well). Then

CREATE VIEW a
AS SELECT country, attrValue
WHERE attrName="President";

CREATE VIEW b
AS SELECT country, attrValue
WHERE attrName="Currency";


INSERT INTO newtable(country, President, Currency)
SELECT a.country, a.attrValue, b.attrValue
FROM  a
INNER JOIN b  ON a.country=b.country;

If you have more attrNames, then create one view for each one and then adjust the last statement accordingly.

INSERT INTO newtable(country, President, Currency, Capital, Population)
SELECT a.country, a.attrValue, b.attrValue, c.attrValue, d.attrValue
FROM  a
INNER JOIN b  ON a.country=b.country
INNER JOIN c  ON a.country=c.country
INNER JOIN d  ON a.country=d.country;

Some more tips

• use NATURAL LEFT JOIN and you don't have to specify the ON clause

Solution 3:

If you were using SQL Server, this would be easy using UNPIVOT. As far as I am aware, this is not implemented in MySQL, so if you want to do this (and I'd advise against it) you'll probably have to generate the SQL dynamically, and that's messy.