Question

I have a table with an auto increment key id, item_no can be either one or two rows in a row (so they always have consecutive ids) that share the same ref but have different right/left (but technically item_no can be repeated multiple times throughout the table but that’s not an issue), and description will sometimes be the same on the consecutive rows but sometimes different:

id | item_no | description | right\left | ref
1  | 1       | a1          | right      | aaa
2  | 1       | a1          | left       | aaa
3  | 2       | b1          | right      | bbb
4  | 3       | c1          | right      | ccc
5  | 3       | c2          | left       | ccc
6  | 4       | d1          | right      | ddd
7  | 4       | d1          | left       | ddd

My issue is that I need item_no to append a -r or -l on to its value if the description of its ‘matching’ row is different.

So the result I am looking for is:

id | item_no | description | right\left | ref
1  | 1       | a1          | right      | aaa
2  | 1       | a1          | left       | aaa
3  | 2       | b1          | right      | bbb
4  | 3-r     | c1          | right      | ccc
5  | 3-l     | c2          | left       | ccc
6  | 4       | d1          | right      | ddd
7  | 4       | d1          | left       | ddd

I am exporting the table to a csv but am not using much php, just a mysql statement and then looping out the results, is this possible within the mysql statement or will I have to rely on a php loop?

Answer 1

I would use this:

update
  items inner join
  (select item_no from items
   group by item_no
   having count(distinct description)>1) dup
  on items.item_no=dup.item_no
set
  items.item_no=concat(items.item_no, '-', substr(rightleft, 1,1))

If rows are always consecutive, you could also use this:

update
  items i1 inner join items i2
  on (i1.id=i2.id+1 or i1.id=i2.id-1) 
     and (i1.item_no=i2.item_no)
     and (i1.description<>i2.description)
set i1.item_no=concat(i1.item_no, '-', substr(i1.rightleft, 1,1))

EDIT: if rows are always consecutive, and you just need a select and not an update, you could use this:

select
  i1.id,
  case when i1.description=i2.description or i2.id is null then i1.item_no else
            concat(i1.item_no, '-', substr(i1.rightleft, 1,1)) end,
  i1.description, i1.rightleft, i1.ref
from
  items i1 left join items i2
  on (i1.id=i2.id+1 or i1.id=i2.id-1) and (i1.item_no=i2.item_no)
order by i1.id