Looking at the description, it seems like you want a group based on negative and positive quantity along with date.
You have not tagged your question with any database but you can use the standard sql case .. when as follows:
Select date, price, sum(quantity) as quantity
From your_table t
Group by date, price, case when quantity < 0 the 1 else 2 end;
If you dont want the result to be groupped by price then you can remove the price from the group by clause and use max(price) or any aggregate function as per your requirement in the select clause.
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