Check the database for an order number

Question

This code will check the database for an order number. If none exist it creates one. If one does exist then it will increment it by one and update the database. Is this the most efficient way to handle this?

// Order Number check
let updateOrderNumber bizLocId = 

    // order number record exist
    let numExist =
        query{
            for row in tool.db.Order_Order_OrderNumber do
            where(row.BusinessLocationId = bizLocId)
            count
        } 

    // retrieve number
    if numExist < 1 then
        let firstOrder =
            dbSchema.ServiceTypes.Order_Order_OrderNumber(
                BusinessLocationId = bizLocId,
                OrderNumber = 1
            )
        // save to db
        tool.insertRowIn tool.db.Order_Order_OrderNumber firstOrder
        tool.saveToDb ()
        1
    else
        let numRecord =
            query{
                for row in tool.db.Order_Order_OrderNumber do
                where(row.BusinessLocationId = bizLocId)
                select row
                exactlyOneOrDefault
            }
        // update order number in db
        let newOrderNumber = 
            numRecord.OrderNumber + 1
        numRecord.OrderNumber <- newOrderNumber
        tool.saveToDb ()

        newOrderNumber