尝试考虑最佳的数据库模式来存储需要连接的日期范围

Question

(Table names in quotes)

Let's say there are "users" that try to sells "products". They earn a commission on all "product_sales" (id, product_id, user_id, total, sale_date). I want to somehow store their commission rate based on certain dates. For example, a user will earn 1% from 2015-01-01 to 2015-01-15, 2% from 2015-01-16 to 2015-01-28, and 3% from 2015-01-29 onwards.

1. I want to run a query that calculates the total commissions for a user in January.

2. I want to run a query that calculates daily earnings in January.

How do I store the commission rates? One idea was having a table "user_commissions" that has (id, user_id, commission_rate, from_date, to_date). It would be easy to calculate the rate for (1) if commissions stayed the same, in which case I'd do this:

SELECT (sum(total) * 0.01) as total_commissions FROM product_sales WHERE user_id = 5 and sale_date between '2015-01-01' and '2015-01-31'

But with commission rates variable this is more complex. I need to somehow join the commissions table on each sale to get the right totals.

Another question would be:

• How do I store the users' current commission rate that doesn't have an expiration date and include that in the reports? In my example, "3% from 2015-01-29 onwards". This has no end date.

Answer 1

Your table structure is a very reasonable structure and often used for slowly changing dimensions. Storing the effective and end dates in the structure is important for efficiency.

One way to store to_date for the most recent commission is to use NULL . This allows you to do:

select *
from commissions
where to_date is null

to get the most recent record.

However, a better way is to use some far distant date, such as '9999-12-12'. This allows you get the most recent commission using:

where curdate() between from_date and to_date

This is an expression that can also make use of an index on from_date, to_date .

Answer 2

Honestly, I would store user commission percentages and the effective dates of those commissions in one table.

TABLE: COMMISSION
user_id, date_effective, commission

In the other table I would store sales data. With each sale, I would keep the commission the salesperson got on the sale. (Added bonus, you can change the commission on any sale, like an override of sorts.)

TABLE: SALE
sale_id, sale_date, user_id, sale_amount, commission

When you create the row in your program, you can grab the correct commission rate using the following query:

SELECT commission from commission WHERE user_id=[user's id] AND date_effective<=[sale date, today] ORDER BY date_effective ASC;

MySQL Left Joins, and SQL in general, can get really tricky when trying to join on dates that don't exactly match up. (Looking back, basically.) I am struggling with the same problem right now without the luxury of the solution I just suggested.

(This whole post is based on the assumption that you aren't going to be directly interacting with this database through a DBMS but instead through an application.)