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Intro to parsing

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Slideshow:

Parsing
 
Parsing: Converting an (SQL) query into a query parse tree

Structure of the SQL query parser
 
We will discuss the details of the SQL language parser in this webpage
(SQL pre-proprecessor will be discussed in next webpage)
Terminology: parser and parse tree
 
Terminology: parser and parse tree
Example of a parse tree (annotated with atoms and syntactic categories)

Atoms (a.ka.: terminals in compilers)
 
Examples:
Keywords:   SELECT, FROM, ...
Identifiers:   employee, address, ...
Constants:   3.14, "John", ...
Operators:   +, >=, ...
Separators:   (, ;, ...
Syntactic Categories (a.ka.: non-terminals in compilers)
 
Examples:
<QUERY>,   <Arithmetic-Expression>,  <Boolean-Expression>,...
Structure of a Parse Tree

Structure of a Parse Tree
   
Structure of a Parse Tree
   
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• The (SQL) Query parser
  • The SQL query parser consists of 2 parts:

    The SQL language parser Parses an SQL command into a parse tree The SQL pre-processor Checks for some semantic consistencies Replaces virtual tables (views) by the corresponding SQL query used to obtain the virtual tables (views)

    Graphically:

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• Parser and parse tree
  • Parser:

    Parser = a computer program that translate statements ("sentences") in a programming language (e.g., SQL) into a parse tree

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  • Parse tree:

    Parse tree = a tree whose nodes corresponds to atoms of the programming language        or          syntactic categories of the programming language

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  • Example:

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• Atoms and Syntactic Categories
  • Atom:

    Atom = a lexical element in a (programming) language that cannot be expressed in more elementary lexical elements I.e.: Atoms can not be divided any further .... Atoms is a.k.a. terminals in a compiler course....

    Examples of atoms:

    keywords: SELECT,   FROM,   WHERE, etc identifiers employee,   name, .... Constants: 3,   3.14,   'April', ... Operators: +,   >= ,   LIKE, .... Tokens: (,   ; ,   , , ....

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  • Syntactic category:

    Syntactic category = a lexical construct in a (programming) language that is built up with other lexical elements following some syntactic rules I.e.: Syntactic categories can be divided further .... A syntactic category is denoted as follows: < Name-of-a-Syntactic-category > Syntactic categories is a.k.a. non-terminals in a compiler course....

    Examples of syntactic categories:

    <Query> <Arithmetic expression> <Condition> (or Boolean expression)

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• Properties of a parse tree
  • Parse tree:

    A node in the parse tree is either: An atom        or        A syntactic category Example: If a node is an atom, then: that node does not have any children (i.e.: atoms are always leaf nodes) If a node is a syntactic category, then: the subtree of the node is the instantiation of one of the syntax rules of the grammar