Identifying tables and their columns

The FROM clause can contain one table only (that is, joins are not supported). The table is specified by its name, which may be followed by an optional alias. The table can be referenced in the other clauses either by its name or its alias. As we will see later, sometimes the use of the table name or alias is mandatory. However, for table columns, the use of the table name or alias is optional. For example, here are 3 ways of writing the same query:

sql-> SELECT Users.lastname, age FROM Users;
 +----------+-----+
 | lastname | age |
 +----------+-----+
 | Scully   |  47 |
 | Smith    |  38 |
 | Morgan   |  38 |
 | Anderson |  35 |
 | Morrison |  25 |
 +----------+-----+

5 rows returned 

To identify the table Users with the alias u:

sql-> SELECT lastname, u.age FROM Users u ; 

The keyword AS can optionally be used before an alias. For example, to identify the table Users with the alias People:

sql-> SELECT People.lastname, People.age FROM Users AS People;