Service pack 3 not updating
The build number for SP3 is 12.0.6024.0 and there are 31 fixes documented in KB #4022619.Many of these improvements are updates that have been back-ported after being implemented in SQL Server 2016, 2017, and 2019.The service pack includes fixes from the SP2 branch up to and including Cumulative Update #13 (the fixes from CU #14 will follow).Microsoft has changed their servicing model slightly, in that for any particular branch, the CU download page only offers the most recent Cumulative Update.
⚠This GDR update addresses an issue described in KB #3067257: a wrong results bug with Columnstore indexes.
One of the more important fixes is one that involves partial (and thus incorrect) results from a clustered columnstore index with a corrupted rowgroup (more gory details in this blog post and KB #3067257).
However, issues were discovered shortly after release: The Analysis Services fixes documented in the KB aren't actually included (in fact SSAS is not touched at all), and slipstreaming RTM SP1 CU1 was not possible.
And of course, please always perform full regression testing before installing any SQL Server update in production. Service Pack 2 (12.0.5000) was released on July 11, 2016, and includes 133 fixes and enhancements - including all of the fixes up to and including SP1 CU7.
Those from SP1 CU8 should all be found in SP2 CU1, released August 25th.
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This is true no matter which KB article you came from.