Merging rows in MySQL with the same item number but different dates

Question

good day everyone, i would like to ask you guys for help. see i have a table Table1 with columns

Item_id, 
date, 
count

what i need is a MySQL query that will: query conditions *all items with the same id should be merged in one row *sort dates from earliest to latest into one row.

thank you and have a great day.

Table1

Item_id||Date      |Count
1       |01-20-2017|    12
1       |02-02-2017|    5
1       |03-01-2017|    10
2       |03-01-2017|    45
3       |01-26-2017|    15
1       |04-09-2017|    12
2       |04-25-2017|    5
3       |02-15-2017|    45

Table2

Item_id    Date1    Count1     Date2    Count2     Date3    Count3     Date4    Count4
1      01-20-2017      12    02-02-2017     5   03-01-2017    10   04-09-2017   
2      03-01-2017      45    04-25-2017     5   
3      01-26-2017      15    02-15-2017     45  

Answer 1

set @sql = 
(
select  concat
            ('Select item_id,',
            group_Concat(maxs)
            ,' from '
            , '(select t.*,if(item_id <> @p ,@col:=1,@col:=@col+1) col,
                @p:= item_id
             from t cross join (select @col:=0,@p:=0) c
             order by item_id,date) c
             group by item_id;'
            ) 
from
(
select 
         concat
         (
         'max(case when col  = ', col, ' then date else null end) date',col,
         ','
         'max(case when col  = ', col, ' then count else null end) count',col
         ) as maxs

from
(
select @col:=@col + 1 col
from t
cross join (select @col:=0) c
where item_id = 
(
select item_id from
(
select item_id, count(*) col from t group by item_id order by count(*) desc limit 1
) s
)
) x
) y

);

Here the item_id with the most dates is used to build the required number of aggregation statements and wrapped in a select..group by

@sql can then used in a prepared statement

prepare sqlstmt from @sql;
execute sqlstmt;
deallocate prepare sqlstmt;

to give this result

+---------+------------+--------+------------+--------+------------+--------+------------+--------+
| item_id | date1      | count1 | date2      | count2 | date3      | count3 | date4      | count4 |
+---------+------------+--------+------------+--------+------------+--------+------------+--------+
|       1 | 2017-01-20 |     12 | 2017-02-02 |      5 | 2017-03-01 |     10 | 2017-04-09 |     12 |
|       2 | 2017-03-01 |     45 | 2017-04-25 |      5 | NULL       |   NULL | NULL       |   NULL |
|       3 | 2017-01-26 |     15 | 2017-02-15 |     45 | NULL       |   NULL | NULL       |   NULL |
+---------+------------+--------+------------+--------+------------+--------+------------+--------+
3 rows in set (0.00 sec)