UPDATE statement to re-assign a column value according to a numeric formula

Question

Given a table that would have values like this:

name, sortval, dept

bob,   2,       1
tom,   4,       1
mary,  6,       1
sam,   8,       1
tim,  10,                


        

Answer 1

A CASE statement may help. In this example:

• the source, sam, is at position 8
• the target, bob, is at position 2

Replacing the variables with the actual values, the following statement shifts everything down 2 from the source, leaves the between members as is, sets the target equal to source, moves the rest down:

postgres=> SELECT * FROM test order by sortval;
 name | sortval
------+---------
 bob  |       2
 tom  |       4
 mary |       6
 sam  |       8
 tim  |      10
(5 rows)


postgres=>      UPDATE test
postgres->        SET sortval = CASE WHEN sortval <= 2 THEN sortval - 2
postgres->                           WHEN sortval = 8  THEN 2
postgres->                           WHEN sortval >= 8 THEN sortval - 2
postgres->                           ELSE sortval
postgres->                           END;
UPDATE 5
postgres=> SELECT * FROM test order by sortval;
 name | sortval
------+---------
 bob  |       0
 sam  |       2
 tom  |       4
 mary |       6
 tim  |       8
(5 rows)

That would move something up the list. Similar logic could be applied for moving down a list. And it assumes negative numbers are fine an that just the relative ordering is of interest.