SQL is traditionally one-dimensional in that you can access values in a row, in the horizontal dimension. SQL Server 2012's support of LAG and LEAD and other so-called window functions open up the vertical dimension, giving access to all values in a column.
The key to getting accurate information from your database is to write good SELECT statements. In fact, it's the key to just about everything you do with your data. Even the tasks of updating and deleting data depend on your ability to select the relevant data in the first place. Because selecting the correct data is the foundation for much of what you do with your database, it's worth taking some time to think about how you write SELECT statements.