Return an error when no rows are returned by a query that is expected to return multiple rows-CodePudding

I have the code snippet below, the idea is to get the loyalty points of a customer from all our warehouses. There is a possibility that a customer will have an id but is not registered for loyalty.

Using that customer's id will cause the query below to return zero rows, which is not an error.

I would like to return an error when that happens so that I can handle it later. I am looking for the best possible way to do this.

I am new to programming and golang, I don't really know my way around just yet.

query := `select coalesce(lc.total_points, 0), 
        w.machine_name from 
        lcustomerpoints lc
        join warehouses w
        on lc.machine_ip = w.machine_ip
        where lc.loyaltycard_number = $1;`

    rows, err := l.DB.Query(query, user_id)
    if err != nil {
        return nil, err
    }

    defer rows.Close()

    loyaltyPoints := []*LoyaltyPoint{}

    for rows.Next() {
        l := LoyaltyPoint{}

        err := rows.Scan(&l.Points, &l.Outlet)
        if err != nil {
            return nil, err
        }

        loyaltyPoints = append(loyaltyPoints, &l)

    }

    if err = rows.Err(); err != nil {
        return nil, err
    }

CodePudding user response：

Count the number of times the for loop iterates:

    count := 0
    for rows.Next() {
        count  
        ...
    }
    if count == 0 {
       return fmt.Errorf("Got 0 rows from query")
    }