Question

I need to display a list of records from a database table ordered by some numeric column. The table looks like this:

CREATE TABLE  items (   position int NOT NULL,   name varchar(100) NOT NULL, ); INSERT INTO items (position, name) VALUE (1, 'first'), (5, 'second'), (8, 'third'), (9, 'fourth'), (15, 'fifth'), (20, 'sixth'); 

Now, the order of the list should change according to a parameter provided by the user. This parameter specifies which record comes first like this:

position = 0 order should be = 1, 5, 8, 9, 15, 20  position = 1 order should be = 20, 1, 5, 8, 9, 15  position = 2 order should be = 15, 20, 1, 5, 8, 9 

In other words the last record becomes the first and so on. Can you think of a way to do this in SQL?

I’m using MySQL but an example in any SQL database will do.

Thanks

Answer 1

See how this works for you. Uses generic SQL so it should be valid for MySql (untested) as well.

DECLARE @user_sort INTEGER SET @user_sort = 0  SELECT position, name FROM  (   SELECT I1.position, I1.name, COUNT(*) AS rownumber, (SELECT COUNT(*) FROM items) AS maxrows   FROM items I1, items I2   WHERE I2.position <= I1.position   GROUP BY I1.position, I1.name ) Q1 ORDER BY    CASE WHEN maxrows - rownumber < (@user_sort % maxrows) THEN 1 ELSE 2 END, position 

Note: * If the user provided sort index is greater than the row count, the value will wrap to within the valid range. To remove this functionality, remove the ‘% maxrows’ from the ORDER BY.

Results:

SET @user_sort = 0

position    name 1   first 5   second 8   third 9   fourth 15  fifth 20  sixth 

SET @user_sort = 1

position    name 20  sixth 1   first 5   second 8   third 9   fourth 15  fifth 

SET @user_sort = 2

position    name 15  fifth 20  sixth 1   first 5   second 8   third 9   fourth 

SET @user_sort = 9

9   fourth 15  fifth 20  sixth 1   first 5   second 8   third