How to combine MySQL records

Question

I have a MySQL table that looks like this:

id |   id_item      | 
1  |   T0001        | 
2  |   T0002        |
2  |   T0003        |
3  |   T0004        |

and I want to change it like this:

id |    id_item           | 
1  |   T0001,T0002        | 
2  |   T0001,T0003        |
2  |   T0001,T0004        |
3  |   T0002,T0003        |
4  |   T0002,T0004        |
5  |   T0003,T0004        |

and I want save in table item_2 like this:

    id |   id_item      | 
    1  |   T0001        | 
    2  |   T0002        |
    3  |   T0001        |
    4  |   T0003        |
    5  |   T0001        |
    6  |   T0004        |
    7  |   T0002        |
    8  |   T0003        |
    9  |   T0002        |
   10  |   T0004        |
   11  |   T0003        |
   12  |   T0004        |

Does anyone know how to do this by using PHP or MySQL?

Answer 1

You can generate this output via the following query:

SELECT
    CONCAT(t1.id_item, ',', t2.id_item) AS id_item
FROM yourTable t1
INNER JOIN yourTable t2
WHERE
    t2.id > t1.id
ORDER BY
    t1.id,
    t2.id

If you wanted to insert this data, you can use the above select to do an INSERT INTO ... SELECT into a new table. I mention new table, because it probably would not make much sense to store CSV and non CSV data in the same column. Actually, I don't at all recommend storing CSV data in the first place. Instead, just generate it if you need it in your presentation layer using a query similar to what I have given above.

Demo here:

Rextester