MySQL Merge non empty values from a table to one result row

I have a MYSQL table which has about 350 columns. There are most 'empty' values and I need to merge several rows in one and replace 'empty' values by a non empty value. My table looks like this:

id|col1|col2|col3|cola|col_b|...|col350
1 |12  |4   |3   |-77 |-77  |...|-77 
2 |12  |-77 |-77 |2   |-77  |...|-77 
3 |12  |-77 |-77 |-77 |6    |...|-77 
4 |12  |-77 |-77 |-77 |-77  |...|6
5 |13  |3   |-77 |-77 |-77  |...|-77 
6 |13  |-77 |5   |-77 |-77  |...|-77
...

-77 is a replacement for empty values. In col1 is an id of datasets witch belongs together.

I need a select query to merge non 'empty' values to one result row per col1-id. For col1-id 12 it should looks like this:

1 | 12 | 4 | 3 | 2 | 6 | ... | 6

Does someone have an idea how to do this?