How to group by / group_concat each set of results? - mysql

Question

Image we have a table like this:
table1
+----------+----------+--------+------------+
| position | epoc | name | value |
+----------+----------+--------+------------+
| 1 | 1 | A | v01 |
| 1 | 1 | B | v02 |
| 1 | 1 | C | v03 |
| 1 | 2 | A | v04 |
| 1 | 2 | B | v05 |
| 1 | 2 | C | v06 |
| 1 | 3 | A | v07 |
| 1 | 3 | B | v08 |
| 1 | 3 | C | v09 |
| 1 | 4 | A | v10 |
| 1 | 4 | B | v11 |
| 1 | 4 | C | v12 |
| 2 | 5 | A | v13 |
| 2 | 5 | B | v14 |
| 2 | 5 | C | v15 |
| 2 | 6 | A | v16 |
| 2 | 6 | B | v17 |
| 2 | 6 | C | v18 |
| 2 | 7 | A | v19 |
| 2 | 7 | B | v20 |
| 2 | 7 | C | v21 |
| 2 | 8 | A | v22 |
| 2 | 8 | B | v23 |
| 2 | 8 | C | v24 |
+----------+----------+--------+------------+
I want to be able to get this table:
table2
+----------+--------------------+
| position | value |
+----------+--------------------+
| 1 | v01,v02,v04,v05 |
| 2 | v13,v14,v16,v17 |
+----------+--------------------+
the conditions are:
JUST the "value" of rows with "name" A OR B;
JUST "epocs" that are the first 2 unique results in "position" (epoc 3,4,7,8 are discarded)
GROUP by table1 position (for each position I want the concat of the values that match previous conditions)

Answer 1

This might be what you are looking for:
select position,
group_concat(value order by overall_row_num) value
from
(
select position,
name,
value,
epoc,
#num := if(#position = `position`, #num + 1, 1) as group_row_number,
#position := `position` as dummy,
overall_row_num
from
(
select position, name,
epoc,
value,
#rn:=#rn+1 overall_row_num
from t1, (SELECT #rn:=0) r
where name in ('A', 'B')
order by position, epoc
) x
order by overall_row_num
) x1
where group_row_number <= 4
group by position
See SQL Fiddle with demo