SQL Syntax IF/THEN/ELSE

So I've been dreading asking this question - mostly because I'm terrible at logic in excel, and transferring logic statements to SQL is such a struggle for me, but I'm going to try and make this as clear as possible.

I have two tables. One table is historic_events and the other is future_events. Based on future_events, I have another table confidence_interval that calculates a z-score telling me based on how many future events will occur, how many past event data points I will need to calculate a reliable average. Each record in historic_events has a unique key called event_id. Each record in future_events has a field called service_id that is non-unique, however, this is how I map the future event type to the past event types.

So, with all that being said, based on the count of future events by service_id, my confidence_interval table calculates the z-score. I then need to select records from the historic_events table for each service_id that satisfy the following parameters

Select * OFS_ID 
From historic_events
where END_DATE is within two calendar years from todays date
and count of `OFS_ID` is >= `confidence_interval.Z_SCORE`

if those parameters are not met, then I want to widen the date value to being within three years.

if those parameters are still not met, I want to widen the date value to being within four years, and then again to five years. If there still aren't enough datapoints after five years, ohwell, we'll settle for what we have. We do not want to look at datapoints that are older than five years.

I want my end result to be a table that has a list of the OFS_ID and I would re-run the SQL query for each service_id.

I hope this makes sense - I can figure out the SELECT and FROM, but totally getting stuck on the WHERE.