This is an expansion (and slight variation) of Joe’s answer in StackOverflow regarding the thread Find the maximum consecutive years for each ID’s in a table (Oracle SQL) using SQL Server. I’ve tested this in SQL Server 2014, but should work from SQL Server 2005 onwards where the ranking functions are supported.
The problem Joe solved is not uncommon – i.e. how to get the consecutive years of something (consecutive year sales, consecutive years in school, consecutive years volunteering etc), and his solution is pretty clever.
Let’s assume you are working on a sales database. You have multiple clients who could be purchasing from your store several times a year, or a couple of times every five years. You may want to know what’s the maximum consecutive years they’ve purchased from you. Why? Perhaps in a marketing campaign, you may want to give your loyal customers (purchased in 5 or more consecutive years) a special discount.
Your data may look like the following screenshot. Notice that in this example, Client 00001, 00002 and 00003 purchased only once. Client 00004 purchased several times, and it looks like there were purchases in consecutive years.
Let’s take it step by step to understand both the problem and solution better. It will be easy to visually identify which clients have purchased consecutive years if we first display all the unique years that client has purchased:
SELECT DISTINCT CLIENTID, CLIENTNAME, YEAR(REVENUEDATE) REVENUEYEAR FROM SAMPLESALES
Here’s the result:
Here we can see Client 00004 purchased in 2004 and 2005 (2 consecutive years), but did not purchase in 2006. Starting 2007, Client 0004 started purchasing again every year until 2012 (6 consecutive years).