User's Guide
PART 1. Working with Databases
CHAPTER 4. Queries: Selecting Data from a Table
The FROM clause is required in every SELECT statement involving data from tables or views.
The FROM clause can include JOIN conditions linking two or more tables, and can include joins to other queries (derived tables). For information on these features, see Joins: Retrieving Data from Several Tables.
In the FROM clause, the full naming syntax for tables and views is always permitted, such as:
SELECT select-list FROM owner.table_name
Qualifying table and view names is necessary only when there might be some confusion about the name.
You can give table names correlation names to save typing. You assign the correlation name in the FROM clause by entering it after the table name, like this:
SELECT d.dept_id, d.dept_name FROM Department d
All other references to the Department table, for example in a WHERE clause, must use the correlation name. Correlation names must conform to the rules for valid identifiers.