How do I get distinct items from one table into another?

Question

EDIT : Because I'm using ExactTarget (Salesforce Marketing Cloud), I only have access to the SELECT statement. None of the good SQL that would make this easy.

I have a table of Customers and a table of Coupons. I'd like to assign unique codes to eligible Customers but I'm a little bit stuck on query #1. (Query #2 is an easy join.)

TABLE 1

+-------------+-------------+------------+
| Customer    | CouponType  | CouponCode |
+-------------+-------------+------------+
| Customer1   | 20pctoff    |            |
| Customer2   | 20pctoff    |            |
| Customer3   | 10pctoff    |            |
| Customer4   | NULL        |            |
+-------------+-------------+------------+

TABLE 2

+-------------+-------------+------------+
| CouponType  | CouponCode  | AssignedTo |
+-------------+-------------+------------+
| 10pctoff    | Coupon1     |            |
| 20pctoff    | Coupon2     |            |
| 30pctoff    | Coupon3     |            |
+-------------+-------------+------------+

Query #1 would result in the following:

Customer1 = Coupon2
Customer2 = none (no more 20pctoff available)
Customer3 = Coupon1
Customer4 = none (not eligible)

The second SQL to update table 2 is a simple join (I don't need help with that).

Answer 1

Solved with a third table. Inelegant as heck, but that's the price to pay for using ExactTarget sometimes. Caveat, requires iterating through known CouponCodes 1 at a time.

New table:

+-----+------------------+--------------+
| ID  | Customer         | CouponCode   |
+-----+------------------+--------------+

Target table3, overwrite

SELECT row_number() OVER (order by Customer) as ID, Customer from Table1 where CouponType = '20pctoff'

Target table3, update

SELECT row_number() OVER (order by CouponCode) as ID, CouponCode from Table2 where CouponType = '20pctoff' and AssignedTo is NULL

Target table1, update

SELECT t3.Customer, t3.CouponCode FROM table3 t3 inner join table1 t1 on t1.Customer = t3.Customer where t3.Customer is not NULL and t3.CouponCode is not NULL

Target table2, update

SELECT t3.Customer as AssignedTo, t3.CouponCode FROM table3 t3 inner join table2 t2 on t2.CouponCode = t3.CouponCode where t3.Customer is not null and t3.CouponCode is not null