When does mainstream support end for SQL 2005

No read that as 'if there is another service pack, then the support for this one expires 12 months after that one starts'.

Support for SP4 therefore ends at the end of the "product's support lifecycle" which is 12 April 2011 for mainstream support, and 12 April 2016 for extended support.

It's MS's way of saying if you have a problem on SP3, then try SP4 before whinging!

From the Support Lifecycle Policy, http://support.microsoft.com/gp/lifepolicy:

21. How can customers and partners estimate the support lifecycle for products that are not yet available or that are still in the Mainstream Support phase?

The Support Lifecycle policy is designed to help take the guesswork out of the length of time Microsoft provides support for a product. For Business and Developer products, Microsoft standardizes a minimum of 10 years of support. The lifecycle combines 5 years of Mainstream Support or 2 years after the successor product (N+1) is released, whichever is longer and 5 years Extended Support or 2 years after the second successor product (N+2) is released, whichever is longer. For Consumer, Hardware, and Multimedia products, Microsoft standardizes a minimum of 5 years of Mainstream Support or 2 years after the successor product (N+1) is released, whichever is longer. Products that release new versions annually will receive 3 years of Mainstream Support. Extended Support is not offered for Consumer, Hardware, and Multimedia products, and for products that release new versions annually. For information about end-of-support dates and any Extended support options (if applicable) for all products, visit the Select a Product for Lifecycle Information site (products listed by Product Family) or the Support Lifecycle Index site.

So you've either got Release date of SQL2008 + 2 years, or release date of SQL 2005 + 5 years, whichever is later. Given SQL Server 2008 was generally available in Nov 2008, and SQL Server 2005 was generally available in Jan 2006, it seems as though they've gone for a "neither of the above" option. Typical. However, we have got a better deal than either of those blind calculations would give, as shown by the Product Lifecycle website.

Given that everything on the Lifecycle search results at http://support.microsoft.com/lifecycle/search/default.aspx?sort=PN&alpha=SQL+Server+2005&Filter=FilterNO shows mainstream support ending for the main product (not for the specific service packs) on 2011-04-12, then that's when mainstream support ended for SQL Server 2005.

In addition to @Kev Riley 's answer, just to clear the ambiguity that on 10/Jul/2007, the mainstream support for only RTM version (SP0) expired. But since SP1 was there, so if someone needed mainstream support then had to be on SP1 at that time. Same is the case now, if someone wants mainstream support for SQL Server 2005, then the user must be running instance with SP4.

Now what does the mainstream support for SP4 ends mean?

1. You cannot call or open a case with CSS for technical support if you are running these versions after the respective dates above. There are only 2 exceptions to this:

   . You are contacting CSS to get help with an upgrade to a supported version i.e. SQL Server 2008 OR higher versions
   . You have a Premier Support Agreement and have purchased something called a Custom Support Agreement (CSA) 
   

2. Microsoft would produce hotfixes or security updates only for SQL Server 2008 OR Higher versions (Custom Support Agreement customers are the exception).