CASE Statement instead of inner join - mysql

Question

This is my table structure:
CUST_ID ORDER_MONTH
---------------------
1 1
1 5
2 3
2 4
My objective is to tag these customers as either New or Returning customers.
When I filter the query lets say for month 1 then customer 1 should have the tag 'New' but when I filter it for month 5 then customer 1 should show up as 'Return' as he already made a purchase in month 1.
Same way customer ID 2 should show up as New for month 3 and return for month 4.
I want to do this using a CASE statement and not inner join.
Thanks

Answer 1

If you insist on using a case statement, the logic would be something like "If this is the first month for that user, write new, otherwise write returning." The query would be as follows:
SELECT CASE
WHEN m.month = (SELECT MIN(month) FROM myTable WHERE customer = m.customer) THEN 'New'
ELSE 'Returning' END AS customerType
FROM myTable m;
However, I think this would be nicer and more readable in a JOIN. You can write an aggregation query to get the earliest month for each user, and then use COALESCE() to replace null values with 'Returning'. The aggregation:
SELECT customer, MIN(month) AS minMonth, 'New' AS customerType
FROM myTable
GROUP BY customer ;
To get the rest:
SELECT m.customer, m.month, COALESCE(t.customerType, 'Returning') AS customerType
FROM myTable m
LEFT JOIN(
SELECT customer, MIN(month) AS minMonth, 'New' AS customerType
FROM myTable
GROUP BY customer) t ON t.customer = m.customer AND t.minMonth = m.month;
Here is an SQL Fiddle example that shows both examples.

Answer 2

You don't need a JOIN and a case statement would probably be overkill...
SELECT CUST_ID, IF(COUNT(1)>1, 'Returning', 'New') AS blah
FROM the_table
WHERE ORDER_MONTH <= the_month
GROUP BY CUST_ID
;
Of course, using just month is going to cause problems after a year (or really, after passing December.)
This would be better
SELECT CUST_ID, IF(COUNT(1)>1, 'Returning', 'New') AS blah
FROM the_table
WHERE order_date <= some_date
GROUP BY CUST_ID
;

Answer 3

Well I do not reccomend this way but this is what you want.
select *
,case when order_month = (select MIN(order_month) from #temp t2 where t1.cust_ID =t2.cust_id) THEN 'NEW' ELSE 'Return' end 'Type'
from #temp t1

Answer 4

I think I get what you're trying to do. Your case statement basically just needs to check if the customer's month equals the month you're filtering by. Something like this:
SELECT
<your other fields>,
CASE WHEN Order_Month = <your filter> THEN 'New'
ELSE 'Return'
END AS 'SomeName'
FROM <your table>

Answer 5

Try this query
select a.CUST_ID, a.ORDER_MONTH ,case when b is not null then 'Return' else 'New' end as type
from tablename a
join tablename b on a.CUST_ID=b.CUST_ID and a.ORDER_MONTH>b.ORDER_MONTH

Answer 6

SELECT *,
CASE
WHEN EXISTS (SELECT *
FROM [YourTable] t2
WHERE t1.cust_id = t2.cust_id
AND t2.order_month < t1.order_month) THEN 'Return'
ELSE 'New'
END
FROM [YourTable] t1
This query uses CASE on an EXISTS clause.
The EXISTS is on a subquery which queries the same table for any rows in previous months.
If there are rows for previous months then the EXISTS is true and the CASE returns 'Return'. If there are no rows for previous months then the EXISTS is false and the CASE returns 'New'.