# phpman > perldoc > SQL::Statement::Structure

## NAME
    [SQL::Statement::Structure](https://www.chedong.com/phpMan.php/perldoc/SQL%3A%3AStatement%3A%3AStructure/markdown) - parse and examine structure of SQL queries

## SYNOPSIS
        use [SQL::Statement](https://www.chedong.com/phpMan.php/perldoc/SQL%3A%3AStatement/markdown);
        my $sql    = "SELECT a FROM b JOIN c WHERE c=? AND e=7 ORDER BY f DESC LIMIT 5,2";
        my $parser = [SQL::Parser](https://www.chedong.com/phpMan.php/perldoc/SQL%3A%3AParser/markdown)->new();
        $parser->{RaiseError}=1;
        $parser->{PrintError}=0;
        $parser->parse("LOAD '[MyLib::MySyntax](https://www.chedong.com/phpMan.php/perldoc/MyLib%3A%3AMySyntax/markdown)' ");
        my $stmt = [SQL::Statement](https://www.chedong.com/phpMan.php/perldoc/SQL%3A%3AStatement/markdown)->new($sql,$parser);
        printf "Command             %s\n",$stmt->command;
        printf "Num of Placeholders %s\n",scalar $stmt->params;
        printf "Columns             %s\n",join( ',', map {$_->name} $stmt->column_defs() );
        printf "Tables              %s\n",join( ',', map {$_->name} $stmt->tables() );
        printf "Where operator      %s\n",join( ',', $stmt->where->op() );
        printf "Limit               %s\n",$stmt->limit();
        printf "Offset              %s\n",$stmt->offset();

        # these will work not before $stmt->execute()
        printf "Order Columns       %s\n",join(',', map {$_->column} $stmt->order() );

## DESCRIPTION
    The [SQL::Statement](https://www.chedong.com/phpMan.php/perldoc/SQL%3A%3AStatement/markdown) module can be used by itself, without DBI and without a subclass to parse SQL
    statements and to allow you to examine the structure of the statement (table names, column
    names, where clause predicates, etc.). It will also execute statements using in-memory tables.
    That means that you can create and populate some tables, then query them and fetch the results
    of the queries as well as examine the differences between statement metadata during different
    phases of prepare, execute, fetch. See the remainder of this document for a description of how
    to create and modify a parser object and how to use it to parse and examine SQL statements. See
    [SQL::Statement](https://www.chedong.com/phpMan.php/perldoc/SQL%3A%3AStatement/markdown) for other uses of the module.

## Creating a parser object
    The parser object only needs to be created once per script. It can then be reused to parse any
    number of SQL statements. The basic creation of a parser is this:

        my $parser = [SQL::Parser](https://www.chedong.com/phpMan.php/perldoc/SQL%3A%3AParser/markdown)->new();

    You can set the error-reporting for the parser the same way you do in DBI:

        $parser->{RaiseError}=1;   # turn on die-on-error behaviour
        $parser->{PrinteError}=1;  # turn on warnings-on-error behaviour

    As with DBI, RaiseError defaults to 0 (off) and PrintError defaults to 1 (on).

    For many purposes, the built-in SQL syntax should be sufficient. However, if you need to, you
    can change the behaviour of the parser by extending the supported SQL syntax either by loading a
    file containing definitions; or by issuing SQL commands that modify the way the parser treats
    types, keywords, functions, and operators.

        $parser->parse("LOAD [MyLib::MySyntax](https://www.chedong.com/phpMan.php/perldoc/MyLib%3A%3AMySyntax/markdown)");
        $parser->parse("CREATE TYPE myDataType");

    See [SQL::Statement::Syntax](https://www.chedong.com/phpMan.php/perldoc/SQL%3A%3AStatement%3A%3ASyntax/markdown) for details of the supported SQL syntax and for methods of extending
    the syntax.

## Parsing SQL statements
    While you only need to define a new [SQL::Parser](https://www.chedong.com/phpMan.php/perldoc/SQL%3A%3AParser/markdown) object once per script, you need to define a new
    [SQL::Statment](https://www.chedong.com/phpMan.php/perldoc/SQL%3A%3AStatment/markdown) object once for each statement you want to parse.

        my $stmt = [SQL::Statement](https://www.chedong.com/phpMan.php/perldoc/SQL%3A%3AStatement/markdown)->new($sql, $parser);

    The call to new() takes two arguments - the SQL string you want to parse, and the [SQL::Parser](https://www.chedong.com/phpMan.php/perldoc/SQL%3A%3AParser/markdown)
    object you previously created. The call to new is the equivalent of a DBI call to prepare() - it
    parses the SQL into a structure but does not attempt to execute the SQL unless you explicitly
    call execute().

## Examining the structure of SQL statements
    The following methods can be used to obtain information about a query:

  command
    Returns the SQL command. See [SQL::Statement::Syntax](https://www.chedong.com/phpMan.php/perldoc/SQL%3A%3AStatement%3A%3ASyntax/markdown) for supported command. Example:

        my $command = $stmt->command();

  column definitions
        my $numColumns = $stmt->column_defs();  # Scalar context
        my @columnList = $stmt->column_defs();  # Array context
        my($col1, $col2) = ($stmt->[column_defs(0)](https://www.chedong.com/phpMan.php/man/columndefs/0/markdown), $stmt->[column_defs(1)](https://www.chedong.com/phpMan.php/man/columndefs/1/markdown));

    This method is used to retrieve column lists. The meaning depends on the query command:

        SELECT $col1, $col2, ... $colN FROM $table WHERE ...
        UPDATE $table SET $col1 = $val1, $col2 = $val2, ...
            $colN = $valN WHERE ...
        INSERT INTO $table ($col1, $col2, ..., $colN) VALUES (...)

    When used without arguments, the method returns a list of the columns $col1, $col2, ..., $colN,
    you may alternatively use a column number as argument. Note that the column list may be empty as
    in

        INSERT INTO $table VALUES (...)

    and in *CREATE* or *DROP* statements.

    But what does "returning a column" mean? It is returning an "[SQL::Statement::Util::Column](https://www.chedong.com/phpMan.php/perldoc/SQL%3A%3AStatement%3A%3AUtil%3A%3AColumn/markdown)"
    instance, a class that implements the methods "table" and "name", both returning the respective
    scalar. For example, consider the following statements:

        INSERT INTO foo (bar) VALUES (1)
        SELECT bar FROM foo WHERE ...
        SELECT foo.bar FROM foo WHERE ...

    In all these cases exactly one column instance would be returned with

        $col->name() eq 'bar'
        $col->table() eq 'foo'

  tables
        my $tableNum = $stmt->tables();  # Scalar context
        my @tables = $stmt->tables();    # Array context
        my($table1, $table2) = ($stmt->[tables(0)](https://www.chedong.com/phpMan.php/man/tables/0/markdown), $stmt->[tables(1)](https://www.chedong.com/phpMan.php/man/tables/1/markdown));

    Similar to "columns", this method returns instances of "[SQL::Statement::Table](https://www.chedong.com/phpMan.php/perldoc/SQL%3A%3AStatement%3A%3ATable/markdown)". For *UPDATE*,
    *DELETE*, *INSERT*, *CREATE* and *DROP*, a single table will always be returned. *SELECT*
    statements can return more than one table, in case of joins. Table objects offer a single
    method, "name" which returns the table name.

  params
        my $paramNum = $stmt->params();  # Scalar context
        my @params = $stmt->params();    # Array context
        my($p1, $p2) = ($stmt->[params(0)](https://www.chedong.com/phpMan.php/man/params/0/markdown), $stmt->[params(1)](https://www.chedong.com/phpMan.php/man/params/1/markdown));

    The "params" method returns information about the input parameters used in a statement. For
    example, consider the following:

        INSERT INTO foo VALUES (?, ?)

    This would return two instances of "[SQL::Statement::Param](https://www.chedong.com/phpMan.php/perldoc/SQL%3A%3AStatement%3A%3AParam/markdown)". Param objects implement a single
    method, "$param-"num()>, which retrieves the parameter number. (0 and 1, in the above example).
    As of now, not very useful ... :-)

  row_values
        my $rowValueNum = $stmt->row_values(); # Scalar context
        my @rowValues = $stmt->[row_values(0)](https://www.chedong.com/phpMan.php/man/rowvalues/0/markdown);  # Array context
        my($rval1, $rval2) = ($stmt->row_values(0,0),
                              $stmt->row_values(0,1));

    This method is used for statements like

        UPDATE $table SET $col1 = $val1, $col2 = $val2, ...
            $colN = $valN WHERE ...
        INSERT INTO $table (...) VALUES ($val1, $val2, ..., $valN),
                                        ($val1, $val2, ..., $valN)

    to read the values $val1, $val2, ... $valN. It returns (lists of) scalar values or
    "[SQL::Statement::Param](https://www.chedong.com/phpMan.php/perldoc/SQL%3A%3AStatement%3A%3AParam/markdown)" instances.

  order
        my $orderNum = $stmt->order();   # Scalar context
        my @order = $stmt->order();      # Array context
        my($o1, $o2) = ($stmt->[order(0)](https://www.chedong.com/phpMan.php/man/order/0/markdown), $stmt->[order(1)](https://www.chedong.com/phpMan.php/man/order/1/markdown));

    In *SELECT* statements you can use this for looking at the ORDER clause. Example:

        SELECT * FROM FOO ORDER BY id DESC, name

    In this case, "order" could return 2 instances of "[SQL::Statement::Order](https://www.chedong.com/phpMan.php/perldoc/SQL%3A%3AStatement%3A%3AOrder/markdown)". You can use the
    methods "$o->table()", "$o->column()", "$o->direction()" and "$o->desc()" to examine the order
    object.

  limit
        my $limit = $stmt->limit();

    In a SELECT statement you can use a "LIMIT" clause to implement cursoring:

        SELECT * FROM FOO LIMIT 5
        SELECT * FROM FOO LIMIT 5, 5
        SELECT * FROM FOO LIMIT 10, 5

    These three statements would retrieve the rows 0..4, 5..9, 10..14 of the table FOO,
    respectively. If no "LIMIT" clause is used, then the method "$stmt->limit" returns undef.
    Otherwise it returns the limit number (the maximum number of rows) from the statement (5 or 10
    for the statements above).

  offset
        my $offset = $stmt->offset();

    If no "LIMIT" clause is used, then the method "$stmt->limit" returns *undef*. Otherwise it
    returns the offset number (the index of the first row to be included in the limit clause).

  where_hash
        my $where_hash = $stmt->where_hash();

    To manually evaluate the *WHERE* clause, fetch the topmost where clause node with the
    "where_hash" method. Then evaluate the left-hand and right-hand side of the operation, perhaps
    recursively. Once that is done, apply the operator and finally negate the result, if required.

    The where clause nodes have (up to) 4 attributes:

    op          contains the operator, one of "AND", "OR", "=", "<>", ">=", ">", "<=", "<", "LIKE",
                "CLIKE", "IS", "IN", "BETWEEN" or a user defined operator, if any.

    arg1        contains the left-hand side of the operator. This can be a scalar value, a hash
                containing column or function definition, a parameter definition (hash has attribute
                "type" defined) or another operation (hash has attribute "op" defined).

    arg2        contains the right-hand side of the operator. This can be a scalar value, a hash
                containing column or function definition, a parameter definition (hash has attribute
                "type" defined) or another operation (hash has attribute "op" defined).

    neg         contains a TRUE value, if the operation result must be negated after evaluation.

    To illustrate the above, consider the following WHERE clause:

        WHERE NOT (id > 2 AND name = 'joe') OR name IS NULL

    We can represent this clause by the following tree:

                  (id > 2)   (name = 'joe')
                         \   /
              NOT         AND
                             \      (name IS NULL)
                              \    /
                                OR

    Thus the WHERE clause would return an [SQL::Statement::Op](https://www.chedong.com/phpMan.php/perldoc/SQL%3A%3AStatement%3A%3AOp/markdown) instance with the op() field set to
    'OR'. The arg2() field would return another [SQL::Statement::Op](https://www.chedong.com/phpMan.php/perldoc/SQL%3A%3AStatement%3A%3AOp/markdown) instance with arg1() being the
    [SQL::Statement::Column](https://www.chedong.com/phpMan.php/perldoc/SQL%3A%3AStatement%3A%3AColumn/markdown) instance representing id, the arg2() field containing the value undef
    (NULL) and the op() field being 'IS'.

    The arg1() field of the topmost Op instance would return an Op instance with op() eq 'AND' and
### neg
    = 'joe'".

    Of course there's a ready-for-use method for WHERE clause evaluation:

    The WHERE clause evaluation depends on an object being used for fetching parameter and column
    values. Usually this can be an [SQL::Statement::RAM::Table](https://www.chedong.com/phpMan.php/perldoc/SQL%3A%3AStatement%3A%3ARAM%3A%3ATable/markdown) object or [SQL::Eval](https://www.chedong.com/phpMan.php/perldoc/SQL%3A%3AEval/markdown) object, but in
    fact it can be any object that supplies the methods

        $val = $eval->param($paramNum);
        $val = $eval->column($table, $column);

    Once you have such an object, you can call eval_where;

        $match = $stmt->eval_where($eval);

  where
        my $where = $stmt->where();

    This method is used to examine the syntax tree of the "WHERE" clause. It returns *undef* (if no
    "WHERE" clause was used) or an instance of [SQL::Statement::Term](https://www.chedong.com/phpMan.php/perldoc/SQL%3A%3AStatement%3A%3ATerm/markdown).

    The where clause is evaluated automatically on the current selected row of the table currently
    worked on when it's "value()" method is invoked.

    "[SQL::Statement](https://www.chedong.com/phpMan.php/perldoc/SQL%3A%3AStatement/markdown)" creates the object tree for where clause evaluation directly after successfully
    parsing a statement from the given "where_clause", if any.

## Executing and fetching data from SQL statements
  execute
    When called from a DBD or other subclass of [SQL::Statement](https://www.chedong.com/phpMan.php/perldoc/SQL%3A%3AStatement/markdown), the execute() method will be
    executed against whatever data-source (persistent storage) is supplied by the DBD or the
    subclass (e.g. CSV files for [DBD::CSV](https://www.chedong.com/phpMan.php/perldoc/DBD%3A%3ACSV/markdown), or BerkeleyDB for [DBD::DBM](https://www.chedong.com/phpMan.php/perldoc/DBD%3A%3ADBM/markdown)). If you are using
    [SQL::Statement](https://www.chedong.com/phpMan.php/perldoc/SQL%3A%3AStatement/markdown) directly rather than as a subclass, you can call the execute() method and the
    statements will be executed() using temporary in-memory tables. When used directly, like that,
    you need to create a cache hashref and pass it as the first argument to execute:

      my $cache  = {};
      my $parser = [SQL::Parser](https://www.chedong.com/phpMan.php/perldoc/SQL%3A%3AParser/markdown)->new();
      my $stmt   = [SQL::Statement](https://www.chedong.com/phpMan.php/perldoc/SQL%3A%3AStatement/markdown)->new('CREATE TABLE x (id INT)',$parser);
      $stmt->execute( $cache );

    If you are using a statement with placeholders, those can be passed to execute after the $cache:

      $stmt      = [SQL::Statement](https://www.chedong.com/phpMan.php/perldoc/SQL%3A%3AStatement/markdown)->new('INSERT INTO y VALUES(?,?)',$parser);
      $stmt->execute( $cache, 7, 'foo' );

  fetch
    Only a single "fetch()" method is provided - it returns a single row of data as an arrayref. Use
    a loop to fetch all rows:

     while (my $row = $stmt->fetch()) {
         # ...
     }

  an example of executing and fetching
     #!/usr/bin/perl -w
     use strict;
     use [SQL::Statement](https://www.chedong.com/phpMan.php/perldoc/SQL%3A%3AStatement/markdown);

     my $cache={};
     my $parser = [SQL::Parser](https://www.chedong.com/phpMan.php/perldoc/SQL%3A%3AParser/markdown)->new();
     for my $sql(split /\n/,
     "  CREATE TABLE a (b INT)
        INSERT INTO a [VALUES(1)](https://www.chedong.com/phpMan.php/man/VALUES/1/markdown)
        INSERT INTO a [VALUES(2)](https://www.chedong.com/phpMan.php/man/VALUES/2/markdown)
        SELECT MAX(b) FROM a  "
     )
     {
        $stmt = [SQL::Statement](https://www.chedong.com/phpMan.php/perldoc/SQL%3A%3AStatement/markdown)->new($sql,$parser);
        $stmt->execute($cache);
        next unless $stmt->command eq 'SELECT';
        while (my $row=$stmt->fetch)
        {
            print "@$row\n";
        }
     }
     __END__

AUTHOR & COPYRIGHT
    Copyright (c) 2005, Jeff Zucker <jzuckerATcpan.org>, all rights reserved. Copyright (c)
    2009-2020, Jens Rehsack <rehsackATcpan.org>, all rights reserved.

    This document may be freely modified and distributed under the same terms as Perl itself.