if-then-else construction in complex stored procedure - sql

Question

I am relatively new to sql queries and I was wondering how to create a complex stored procedure. My database runs on SQL server.
I have a table customer (id, name) and a table customer_events (id, customer_id, timestamp, action_type). I want add a calculated field customer_status to table customer which is
0: (if there is no event for this customer in customer_events) or (the most recent event is > 5 minutes ago)
1: if the most recent event is < 5 minutes ago and action_type=0
2: if the most recent event is < 5 minutes ago and action_type=1
Can I use if-then-else constructions or should I solve this challenge differently?

Answer 1

As you mentioned in comments, you actually want to add a field to a select query, and in a general sense what you want is a CASE statement. They work like this:
SELECT field1,
field2,
CASE
WHEN some_condition THEN some_result
WHEN another_condition THEN another_result
END AS field_alias
FROM table
Applied to your specific scenario, well it's not totally straightforward. You're certainly going to need to left join your status table, you also want to aggregate to find the most recent event, along with that event's action type. Once you have that information, the case statement is straightforward.
Always hard to write sql without access to your data, but something like:
SELECT c.id,
c.name,
CASE
WHEN e.id IS NULL OR DATEDIFF(minute,e.timestamp,getDate())>=5 THEN 0
WHEN DATEDIFF(minute,e.timestamp,getDate())<5 AND s.action_type=1 THEN 1
WHEN DATEDIFF(minute,e.timestamp,getDate())<5 AND s.action_type=0 THEN 2
END as customer_status
FROM clients c
LEFT JOIN (
SELECT id, client_id, action_type,
rank() OVER(partition by client_id order by timestamp desc) AS r
FROM customer_events
) e
ON c.id=e.client_id AND e.r=1
The core of this is the subquery in the middle, it's using a rank funtion to give a number to each status by client_id ordered by the timestamp descending. Therefore every record with a rank of 1 will be the most recent (for that client). Thereafter, you simply join it on to the client table, and use it to determine the right value for customer_status

Answer 2

Presuming you get the event info into "Most_Recent_Event_Mins_Ago". If none it will be NULL.
SELECT Id, Name,
CASE
WHEN Most_Recent_Event_Mins_Ago IS NULL THEN 0
WHEN Most_Recent_Event_Mins_Ago <5 AND Action_type = 0 THEN 1
WHEN Most_Recent_Event_Mins_Ago <5 AND Action_type = 1 THEN 0
..other scenarions
ELSE yourDefaultValueForStatus
END as Status
FROM customer
WHERE
...
...