Group by month in SQLite

it is always good while you group by MONTH it should check YEAR also

select SUM(transaction) as Price, 
       DATE_FORMAT(transDate, "%m-%Y") as 'month-year' 
       from transaction group by DATE_FORMAT(transDate, "%m-%Y");

FOR SQLITE

select SUM(transaction) as Price, 
       strftime("%m-%Y", transDate) as 'month-year' 
       from transaction group by strftime("%m-%Y", transDate);

You can group on the start of the month:

select  date(DateColumn, 'start of month')
,       sum(TransactionValueColumn)
from    YourTable
group by 
        date(DateColumn, 'start of month')

Try the following:

SELECT SUM(price), strftime('%m', transDate) as month
FROM your_table
GROUP BY strftime('%m', transDate);

Use the corresponding page in SQLite documentation for future references.