Microsoft® SQL Server 2012 Unleashed

Creating and Using CLR Functions

Prior to SQL Server 2005, the only way to extend the functionality of SQL Server beyond what was available using the T-SQL language was to create extended stored procedures or Component Object Model (COM) components. The main problem with these types of extensions was that if not written very carefully, they could have an adverse impact on the reliability and security of SQL Server. For example, extended stored procedures are implemented as DLLs that run in the same memory space as SQL Server. An access violation raised in a poorly written extended stored procedure could crash SQL Server itself.

In addition, neither extended stored procedures nor COM components allow you to create custom user-defined functions ...