SQL::Statement::Structure(3)
NAME
SQL::Statement::Structure - parse and examine structure of SQL queries
SYNOPSIS
use SQL::Statement;
my $sql = "SELECT a FROM b JOIN c WHERE c=? AND e=7 ORDER BY f DESC LIMIT 5,2";
my $parser = SQL::Parser->new();
$parser->{RaiseError}=1;
$parser->{PrintError}=0;
$parser->parse("LOAD 'MyLib::MySyntax' ");
my $stmt = SQL::Statement->new($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 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 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->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");
$parser->parse("CREATE TYPE myDataType");
See SQL::Statement::Syntax 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 object once per script,
you need to define a new SQL::Statment object once for each statement
you want to parse.
my $stmt = SQL::Statement->new($sql, $parser);
The call to new() takes two arguments - the SQL string you want to
parse, and the SQL::Parser 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 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), $stmt->column_defs(1));
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" 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), $stmt->tables(1));
Similar to "columns", this method returns instances of
"SQL::Statement::Table". 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), $stmt->params(1));
The "params" method returns information about the input parameters used
in a statement. For example, consider the following:
INSERT INTO foo VALUES (?, ?)
This would return two instances of "SQL::Statement::Param". Param
objects implement a single method, "$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 = $stmt->row_values(0); # Array context
my($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" instances.
order
my $orderNum = $stmt->order(); # Scalar context
my @order = $stmt->order(); # Array context
my($o1, $o2) = ($stmt->order(0), $stmt->order(1));
In SELECT statements you can use this for looking at the ORDER clause.
Example:
SELECT * FROM FOO ORDER BY id DESC, name
In this case, "order" could return 2 instances of
"SQL::Statement::Order". You can use the methods "$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 instance with
the op() field set to 'OR'. The arg2() field would return another
SQL::Statement::Op instance with arg1() being the
SQL::Statement::Column 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() returning TRUE. The arg1() and arg2()
fields would be Op's representing "id > 2" and "name = '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 object or SQL::Eval 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.
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" 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, 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, or BerkeleyDB for DBD::DBM). If you are using
SQL::Statement 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->new();
my $stmt = SQL::Statement->new('CREATE TABLE x (id INT)',$parser);
$stmt->execute( $cache );
If you are using a statement with placeholders, those can be passed to
execute after the $cache:
$stmt = SQL::Statement->new('INSERT INTO y VALUES(?,?)',$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;
my $cache={};
my $parser = SQL::Parser->new();
for my $sql(split /\n/,
" CREATE TABLE a (b INT)
INSERT INTO a VALUES(1)
INSERT INTO a VALUES(2)
SELECT MAX(b) FROM a "
)
{
$stmt = SQL::Statement->new($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.
perl v5.30.3 2020-09-25 SQL::Statement::Structure(3)
sql-statement 1.101.0 - Generated Sat Oct 24 15:30:38 CDT 2020