SQL Server is a highly performant resource manager for structured data in SharePoint Foundation. SQL Server provides a fast, efficient SQL query engine and exceptional query-based retrieval of structured data.
However, a large proportion of data in a typical enterprise deployment of SharePoint Foundation is stored as unstructured, binary data streams associated with SharePoint files. These binary streams, or BLOBs, which correspond with SharePoint files, often comprise a majority of your data.
The problem, however, is that SQL Server is not the best option for storing unstructured binary data streams because unstructured binary data does not expose itself to the rich SQL query environment. In fact, store and retrieve operations for BLOB data in SQL Server are little more than simple get and put statements. Therefore, because BLOB data is not able to leverage one of SQL Server's most useful features, its query engine, its value as a BLOB store is limited.