Question

I stumbled across an interesting (to me) query, and I’m not really sure what it does.

SELECT SQL_CALC_FOUND_ROWS DISTINCT P.* FROM table P WHERE P.status = ...

The question is in regard to the P specifically FROM table P. I know in the FROM clause you can use an AS clause, but this particular query doesn’t. Is the word AS optional?

Answer 1

Definining aliases isn’t always optional, but when you do define an alias the keyword AS is always optional.

table_factor:
    tbl_name [PARTITION (partition_names)] 
        [[AS] alias] [index_hint_list]
  | table_subquery [AS] alias
  | ( table_references )
  | { OJ table_reference LEFT OUTER JOIN table_reference
        ON conditional_expr }

Source: JOIN Syntax

The AS keyword is also optional when defining column aliases:

The AS keyword is optional when aliasing a select_expr with an identifier

Source: SELECT Syntax