I work in an environment where databases (DB) are a way of life yet many people fear the DB. This should not be the case, if you are using Oracle or mySQL the basics remain the same.
A database is a collection of objects not tables!
People immediately think about database as tables of data, this is not the full story. A database is a collection of objects, common object are tables, procedures, constraints, index's and jobs.
Objects are arranged/grouped in "schemas"!
^--get this and you are a database guru! A database has one or many schema's, a schema is normally associated with a user i.e you would log onto DBx.SchemaY where DBx is your main DB name and SchemaY is the name of a user. Within this SchemaY (or user) you have its associated objects(tables, procs ect. as mentioned above)
Each Schema is private!
The owner in possession of the schema password can "grant" other uses(i.e. their schema) access to his objects.
In 99% of cases anything before the where clause will not affect query speed! it is what comes [b]after[/b] the "where" clause that affects speed.