Question

UPDATE main_cats set cat_order=(
    CASE         
         when cat_order='4' THEN (4-1) 
         WHEN cat_order=(4-1) THEN '4'
    END)

When I run the above query on the bottom data I get.

cat_id  cat_name     cat_order cat_visible
 0      NO MAIN CAT  0           N
 5      CLOTHES      1           N
 6      GIFTS        2           N
 7      ARTWORK      3           Y
 8      TEST CATEGO  4           N

I end up with

cat_id  cat_name     cat_order cat_visible
 0      NO MAIN CAT  0           N
 5      CLOTHES      0           N
 6      GIFTS        0           N 
 7      ARTWORK      3           Y
 8      TEST CATEGO  3           N

Not sure why the others go to ZERO in the CASE statement.

Answer 1

It is because you are updating the column without a where and your conditional is only setting a value in two scenarios. For the rest I would guess it assumes zero with no input. I think this could work:

UPDATE main_cats set cat_order=(
CASE         
     when cat_order='4' THEN (4-1) 
     WHEN cat_order=(4-1) THEN '4'
     ELSE cat_order
END)