Ranking within multiple groups & Efficient query for multiple table updates - sql-server

Question

I'm trying to add rank by sales by month and also change the date column to a 'month end' field that would show only last day of month.
Can i do two sets in a row like that without adding an update?
I'm looking for top 2 within each month - does limit and group by work?
I feel like this is right and most efficient query, but its not working - any help appreciated!!
UPDATE table1
SET DATE=EOMONTH(DATE) AS MONTH_END;
ALTER TABLE table1
ADD COLUMN RANK INT AFTER sales;
UPDATE table1
SET RANK=
RANK() OVER(PARTITION BY cust ORDER BY sales DESC);
LIMIT 2
orig table
+------+----------+-------+--+
| CUST | DATE | SALES | |
+------+----------+-------+--+
| 36 | 3-5-2018 | 50 | |
| 37 | 3-15-18 | 100 | |
| 38 | 3-25-18 | 65 | |
| 37 | 4-5-18 | 95 | |
| 39 | 4-21-18 | 500 | |
| 40 | 4-45-18 | 199 | |
+------+----------+-------+--+
desired output
+------+-----------+-------+------+
| CUST | Month End | SALES | Rank |
+------+-----------+-------+------+
| | | | |
| 37 | 3-31-18 | 100 | 1 |
| 38 | 3-31-18 | 65 | 2 |
| 39 | 4-30-18 | 500 | 1 |
| 40 | 4-30-18 | 199 | 2 |
+------+-----------+-------+------+

Answer 1

I do not know why you want EOMONTH as a stored value, but what you have for that will work.
I would not use [rank] as a column name as I avoid any words that are used in SQL, maybe [sales_rank] or similar.
ALTER TABLE table1
ADD COLUMN [sales_rank] INT AFTER sales;
with cte as (
select
cust
, DENSE_RANK() OVER(PARTITION BY cust ORDER BY sales DESC) as ranking
from table1
)
update cte
set sales_rank = ranking
where ranking < 3
;
LIMIT 2 is not something that can be used in SQL Server by the way, and it sure can't be used "per grouping". When you use a "window function" such as rank() or dense_rank() you can use the output of those in the where clause of the next "layer". i.e. use those functions in a subquery (or cte) and then use a where clause to filter rows by the calculated values.
Also note I used dense_rank() to guarantee that no rank numbers are skipped, so that the subsequent where clause will be effective.