Normally, Microsoft SQL Express edition is the default database that ships with Lync.
For high availability reasons, sometimes a Microsoft SQL server cluster is necessary. For this, you'll need a Standard or Enterprise edition.
The choice depends on the number of windows failover clustering nodes you'll install. The Standard version supports clusters with up to 2 nodes and maximum 64 GB RAM. The Enterprise version is capable of addressing even more memory and supports huge clusters.
I assume that you've choosen for a SQL Server 2008 Standard cluster in this case.
SQL edition | HA | CAL’s | CPU licenses | extra OS licenses | SQL Server 2008 licenses |
---|---|---|---|---|---|
SQL Express |
no |
0 |
0 |
0 |
|
SQL Standard |
yes |
0 / #users * |
0 |
2 |
1 STD license *** |
SQL Enterprise |
yes |
0 ** |
#CPU’s |
+2 |
+1 ENT license *** |
* Lync Standard edition does not require any CAL's. Lync Enterprise does require CAL's.
** CAL's for an unlimited amount of users included.
*** SQL server licenses needed for every active cluster node. Passive nodes don't need a license. After a failover, passive node can take over the load and run for 30 days without a license. In other words: You should failback to an active node within a timeframe of 30 days.