From the SOQL reference guide.....
Alias Notation
You can use alias notation in SELECT queries: SELECT count() FROM Contact c, c.Account a WHERE a.name = 'MyriadPubs'
To establish the alias, first identify the object, in this example a contact, and then specify the alias, in this case “c.” For the rest of the SELECT statement, you can use the alias in place of the object or field name.
The following are SOQL keywords that can’t be used as alias names: AND, ASC, DESC, EXCLUDES, FIRST, FROM, GROUP, HAVING, IN, INCLUDES, LAST, LIKE, LIMIT, NOT, NULL, NULLS, OR, SELECT, WHERE, WITH
See page 2 of the reference guide here