What's the purpose of SQL keyword “AS”?

Question

You can set table aliases in SQL typing the identifier right after the table name.

SELECT * FROM table t1;

You can even use the keyword

Answer 1

There is no difference between both statements above. AS is just a more explicit way of mentioning the alias

Answer 2

The use is more obvious if you don't use 'SELECT *' (which is a bad habit you should get out of):

SELECT t1.colA, t2.colB, t3.colC FROM alongtablename AS t1, anotherlongtablename AS t2, yetanotherlongtablename AS t3 WHERE t1.colD = t2.colE...

Answer 3

The AS in this case is an optional keyword defined in ANSI SQL 92 to define a <<correlation name> ,commonly known as alias for a table.

<table reference> ::=
            <table name> [ [ AS ] <correlation name>
                [ <left paren> <derived column list> <right paren> ] ]
          | <derived table> [ AS ] <correlation name>
                [ <left paren> <derived column list> <right paren> ]
          | <joined table>

     <derived table> ::= <table subquery>

     <derived column list> ::= <column name list>

     <column name list> ::=
          <column name> [ { <comma> <column name> }... ]


     Syntax Rules

     1) A <correlation name> immediately contained in a <table refer-
        ence> TR is exposed by TR. A <table name> immediately contained
        in a <table reference> TR is exposed by TR if and only if TR
        does not specify a <correlation name>.

It seems a best practice NOT to use the AS keyword for table aliases as it is not supported by a number of commonly used databases.