multiple EXCEPT support

[mweisgut]

TPC-DS query 87 contains multiple, not nested EXCEPT operations.

TPC-DS query 87, netezza dialect:

select count(*) 
from ((select distinct c_last_name, c_first_name, d_date
       from store_sales, date_dim, customer
       where store_sales.ss_sold_date_sk = date_dim.d_date_sk
         and store_sales.ss_customer_sk = customer.c_customer_sk
         and d_month_seq between 1202 and 1202+11)
       except
      (select distinct c_last_name, c_first_name, d_date
       from catalog_sales, date_dim, customer
       where catalog_sales.cs_sold_date_sk = date_dim.d_date_sk
         and catalog_sales.cs_bill_customer_sk = customer.c_customer_sk
         and d_month_seq between 1202 and 1202+11)
       except
      (select distinct c_last_name, c_first_name, d_date
       from web_sales, date_dim, customer
       where web_sales.ws_sold_date_sk = date_dim.d_date_sk
         and web_sales.ws_bill_customer_sk = customer.c_customer_sk
         and d_month_seq between 1202 and 1202+11)
) cool_cust
;