Nick Says

For my parsing project, I decided to write an LL (top down left recursive) parser for a simplified version of SQL. Apparently, the specification for SQL is not freely available, but a professor at another university uses a simplified example of the grammar to demonstrate parsing, so I have used that example, which can be found here.

I'll copy the grammar here, too:

   <Query>     ::=  SELECT <SelList>
                    FROM   <FromList>
            WHERE  <Condition>

   <SelList>   ::=  <Attribute> |
                    <Attribute> , <SelList>

   <FromList>  ::=  <Relation>  |
                    <Relation>  , <FromList>

   <Condition> ::=  <Condition> AND <Condition>   |            
                    <Attribute> IN ( <Query> )    |
            <Attribute> = <Attribute>     |
            <Attribute> LIKE <Pattern>

While not explicitly stated in the grammar, I inferred that tokens like could be broken down further into more broadly applicable tokens, like NAME, which matches any string that begins with a letter and contains any number of alphanumeric characters following the initial letter.

Because I have worked with SLY previously, I decided to use it for this project as well. SLY has a builtin Lexer and Parser class, but I decided only to use the builtin Lexer class, and to write the parser by hand. We have to eat our vegetables sometimes...

I'm a little stuck in the implementation of the subroutine that is responsible for parsing conditions: I probably need to rewrite the grammar so that the AND case is handled without causing an infinite loop. I think I can do so if I use lookahead to determine which of the terminal cases applies without too much trouble...I'm going to keep hacking at it but upload what I have so far just in case. (I need to update the readme to indicate that this program is both a lexer AND a parser, too...)

attachments

		last modified	size
	create.sql	Fri Oct 31 2025 05:53 am	397B
	nick.md	Fri Oct 31 2025 05:53 am	1.2K
	populate.sql	Fri Oct 31 2025 05:53 am	1.4K
	sqllexer.py	Fri Oct 31 2025 05:53 am	5.4K