It is essential to understand the details of the databases underlying the applications you work with; to know what happens to which tables when you perform an action in your application. No matter how slick your SQL skills are, walking into a new situation with unfamiliar databases is only going to make that task trickier; and most large companies - and many small ones, too - will have a heterogeneous database environment, unless you are very fortunate. Mixtures of SQL Server and Oracle are fairly common, and every database type will have different structures and different syntax to remember.