Database schema - Configurable fields?

Question

I sell leads and charge my clients like so: (Only one type of payment from the followings can be charged from a client)
Pay Per Lead:
$__ for the first __ leads per month
$__ for the next __ leads per month
$__ for the next __ leads per month
and so on...

Pay per Appointment:
$__ for the first __ leads per month
$__ for the next __ leads per month
$ __ for the next __ leads per month
and so on...

Pay per Percentage of Sale:
__% of the sale price (per sale)

My Question:
What are the best possible database design solutions in such cases?

What i have tried:

+---------+ 
| clients |
+---------+
| id      |
| name    |
+---------+

+---------------+
|     deals     |
+---------------+
| client_id     |
| max_quantity  | 
| cost          |
| unit_type     |
+---------------+

So records for client with the id 1 might look like:

+-----------+--------------+---------------+-------------+
| client_id | max_quantity | cost_per_unit |  unit_type  |
+-----------+--------------+---------------+-------------+
|         1 |           10 |            10 | lead        |
|         1 |           30 |             5 | lead        |
|         1 |          100 |             2 | lead        |
|         1 |           10 |            35 | appointment |
|         1 |           30 |            20 | appointment |
|         1 |          100 |            10 | appointment |
|         1 |         1000 |             5 | appointment |
|         1 |            0 |            50 | sale        |
+-----------+--------------+---------------+-------------+

Now the above table means that:
$10 will be charged per lead upto 10 leads
$5 will be charged per lead upto 30 leads
$2 will be charged per lead upto 100 leads

$35 will be charged per appointment upto 10 leads
$20 will be charged per appointment upto 30 leads
$10 will be charged per appointment upto 100 leads
$5 will be charged per appointment upto 1000 leads

$50 will be charged per sale

Also i want to add x number of such rules (per lead, per appointment, per sale)

I personally don't think that my approach is one of the best solutions. Looking forward to hear for you cleaver folks! Thank you.

P.S. I know that unit_type can be further normalized but this is not the issue :)

Update

Maybe i can store serialized data?

Answer 1

Your proposed schema is a good start and has some merits. IMO the less elegant parts are the denormalized repetition of unit_type values and non-functional max_quantity value for sale.

Would suggest splitting deals into three tables rather than one. Would personally go with singular rather than plural table names** and begin with the same prefix so they are listed close to each other: Something like commission_lead, commission_appointment and commission_sale.

** [Lots of debate on this here]

Would also suggest including both lower and upper bands in each row. This does use more data than is strictly needed but think it is worth doing as it should make the table data more readable and simplify the calculation queries.

So the proposed new schema is:

+---------+ 
| client  |
+---------+
| id      |
| name    |
+---------+

+-----------------+
| commission_lead |
+-----------------+
| client_id       |
| min_quantity    |
| max_quantity    | 
| cost_per_unit   |
+-----------------+

+------------------------+
| commission_appointment |
+------------------------+
| client_id              |
| min_quantity           |
| max_quantity           |
| cost_per_unit          |
+------------------------+

+-----------------+
| commission_sale |
+-----------------+
| client_id       |
| cost_per_unit   |
+-----------------+

And the records for client_id = 1 are:

commission_lead
+-----------+--------------+--------------+---------------+
| client_id | min_quantity | max_quantity | cost_per_unit |
+-----------+--------------+--------------+---------------+
|         1 |            0 |           10 |            10 |
|         1 |           11 |           30 |             5 |
|         1 |           31 |          100 |             2 |
+-----------+--------------+--------------+---------------+

commission_appointment
+-----------+--------------+--------------+---------------+
| client_id | min_quantity | max_quantity | cost_per_unit |
+-----------+--------------+--------------+---------------+
|         1 |            0 |           10 |            35 |
|         1 |           11 |           30 |            20 |
|         1 |           31 |          100 |            10 |
|         1 |          101 |         1000 |             5 |
+-----------+--------------+--------------+---------------+

commission_sale
+-----------+---------------+
| client_id | cost_per_unit |
+-----------+---------------+
|         1 |            50 |
+-----------+---------------+