NAME
SQL::Statement - SQL parsing and processing engine
SYNOPSIS
require SQL::Statement;
# Create a parser
my($parser) = SQL::Parser->new('Ansi');
# Parse an SQL statement
$@ = '';
my ($stmt) = eval {
SQL::Statement->new("SELECT id, name FROM foo WHERE id > 1",
$parser);
};
if ($@) {
die "Cannot parse statement: $@";
}
# Query the list of result columns;
my $numColums = $stmt->columns(); # Scalar context
my @columns = $stmt->columns(); # Array context
# @columns now contains SQL::Statement::Column instances
# Likewise, query the tables being used in the statement:
my $numTables = $stmt->tables(); # Scalar context
my @tables = $stmt->tables(); # Array context
# @tables now contains SQL::Statement::Table instances
# Query the WHERE clause; this will retrieve an
# SQL::Statement::Op instance
my $where = $stmt->where();
# Evaluate the WHERE clause with concrete data, represented
# by an SQL::Eval object
my $result = $stmt->eval_where($eval);
# Execute a statement:
$stmt->execute($data, $params);
DESCRIPTION
For installing the module, see the section on "INSTALLATION"
below.
The SQL::Statement module implements a small, abstract SQL
engine. This module is not usefull itself, but as a base class
for deriving concrete SQL engines. The implementation is
designed to work fine with the DBI driver DBD::CSV, thus
probably not so well suited for a larger environment, but I'd
hope it is extendable without too much problems.
By parsing an SQL query you create an SQL::Statement instance.
This instance offers methods for retrieving syntax, for WHERE
clause and statement evaluation.
Creating a parser object
What's accepted as valid SQL, depends on the parser object.
There is a set of so-called features that the parsers may have
or not. Usually you start with a builtin parser:
my $parser = SQL::Parser->new($name, [ \%attr ]);
Currently two parsers are builtin: The *Ansi* parser implements
a proper subset of ANSI SQL. (At least I hope so. :-) The
*SQL::Statement* parser is used by the DBD:CSV driver.
You can query or set individual features. Currently available
are:
create.type_blob
create.type_real
create.type_text
These enable the respective column types in a *CREATE
TABLE* clause. They are all disabled in the *Ansi*
parser, but enabled in the *SQL::Statement* parser.
Example:
select.join
This enables the use of multiple tables in a SELECT
statement, for example
SELECT a.id, b.name FROM a, b WHERE a.id = b.id AND a.id = 2
To enable or disable a feature, for example *select.join*, use
the following:
# Enable feature
$parser->feature("select", "join", 1);
# Disable feature
$parser->feature("select", "join", 0);
Of course you can query features:
# Query feature
my $haveSelectJoin = $parser->feature("select", "join");
The `new' method allows a shorthand for setting features. For
example, the following is equivalent to the *SQL::Statement*
parser:
$parser = SQL::Statement->new('Ansi',
{ 'create' => { 'type_text' => 1,
'type_real' => 1,
'type_blob' => 1 },
'select' => { 'join' => 0 }});
Parsing a query
A statement can be parsed with
my $stmt = SQL::Statement->new($query, $parser);
In case of syntax errors or other problems, the method throws a
Perl exception. Thus, if you want to catch exceptions, the above
becomes
$@ = '';
my $stmt = eval { SQL::Statement->new($query, $parser) };
if ($@) { print "An error occurred: $@"; }
The accepted SQL syntax is restricted, though easily extendable.
See the section on "SQL syntax" below. See the section on
"Creating a parser object" above.
Retrieving query information
The following methods can be used to obtain information about a
query:
command Returns the SQL command, currently one of *SELECT*,
*INSERT*, *UPDATE*, *DELETE*, *CREATE* or *DROP*, the
last two referring to *CREATE TABLE* and *DROP TABLE*.
See the section on "SQL syntax" below. Example:
my $command = $stmt->command();
columns my $numColumns = $stmt->columns(); # Scalar context my
@columnList = $stmt->columns(); # Array context
my($col1, $col2) = ($stmt->columns(0), $stmt-
>columns(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, like in
INSERT INTO $table VALUES (...)
and in *CREATE* or *DROP* statements.
But what does "returning a column" mean? It is returning
an SQL::Statement::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 usefull ... :-)
row_values
my $rowValueNum = $stmt->row_values(); # Scalar context
my @rowValues = $stmt->row_values(); # Array context
my($rval1, $rval2) = ($stmt->row_values(0),
$stmt->row_values(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)
to read the values $val1, $val2, ... $valN. It returns
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()', `$o->column()' and `$o->desc()' to examine
the order object.
limit my $l = $stmt->limit(); if ($l) { my $offset = $l->offset();
my $limit = $l->limit(); }
In a SELECT statement you can use a `LIMIT' clause to
implement cursoring:
SELECT * FROM FOO LIMIT 5
SELECT * FROM FOO LIMIT 5, 5
SELECT * FROM FOO LIMIT 10, 5
These three statements would retrieve the rows 0..4,
5..9, 10..14 of the table FOO, respectively. If no
`LIMIT' clause is used, then the method `$stmt->limit'
returns undef. Otherwise it returns an instance of
SQL::Statement::Limit. This object has the methods
`offset' and `limit' to retrieve the index of the first
row and the maximum number of rows, respectively.
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::Op. The Op
instance offers 4 methods:
op returns the operator, one of `AND', `OR', `=',
`<>', `>=', `>', `<=', `<', `LIKE', `CLIKE'
or `IS'.
arg1
arg2 returns the left-hand and right-hand sides of
the operator. This can be a scalar value, an
SQL::Statement::Param object or yet another
SQL::Statement::Op instance.
neg returns a TRUE value, if the operation result
must be negated after evalution.
To evaluate the *WHERE* clause, fetch the topmost Op
instance with the `where' 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.
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:
Evaluating a WHERE clause
The WHERE clause evaluation depends on an object being used for
fetching parameter and column values. Usually this can be an
SQL::Eval object, but in fact it can be any object that supplies
the methods
$val = $eval->param($paramNum);
$val = $eval->column($table, $column);
See the SQL::Eval manpage for a detailed description of these
methods. Once you have such an object, you can call a
$match = $stmt->eval_where($eval);
Evaluating queries
So far all methods have been concrete. However, the interface
for executing and evaluating queries is abstract. That means,
for using them you have to derive a subclass from SQL::Statement
that implements at least certain missing methods and/or
overwrites others. See the `test.pl' script for an example
subclass.
Something that all methods have in common is that they simply
throw a Perl exception in case of errors.
execute After creating a statement, you must execute it by calling
the `execute' method. Usually you put an eval statement
around this call:
$@ = '';
my $rows = eval { $self->execute($data); };
if ($@) { die "An error occurred!"; }
In case of success the method returns the number of
affected rows or -1, if unknown. Additionally it sets
the attributes
$self->{'NUM_OF_FIELDS'}
$self->{'NUM_OF_ROWS'}
$self->{'data'}
the latter being an array ref of result rows. The
argument $data is for private use by concrete subclasses
and will be passed through to all methods. (It is
intentionally not implemented as attribute: Otherwise we
might well become self referencing data structures which
could prevent garbage collection.)
CREATE
DROP
INSERT
UPDATE
DELETE
SELECT Called by `execute' for doing the real work. Usually they
create an SQL::Eval object by calling `$self-
>open_tables()', call `$self->verify_columns()' and then
do their job. Finally they return the triple
($self->{'NUM_OF_ROWS'}, $self->{'NUM_OF_FIELDS'},
$self->{'data'})
so that execute can setup these attributes. Example:
($self->{'NUM_OF_ROWS'}, $self->{'NUM_OF_FIELDS'},
$self->{'data'}) = $self->SELECT($data);
verify_columns
Called for verifying the row names that are used in the
statement. Example:
$self->verify_columns($eval, $data);
open_tables
Called for creating an SQL::Eval object. In fact what it
returns doesn't need to be derived from SQL::Eval, it's
completely sufficient to implement the same interface of
methods. See the SQL::Eval manpage for details. The
arguments `$data', `$createMode' and `$lockMode' are
corresponding to those of SQL::Eval::Table::open_table
and usually passed through. Example:
my $eval = $self->open_tables($data, $createMode, $lockMode);
The eval object can be used for calling `$self-
'verify_columns> or `$self-'eval_where>.
open_table
This method is completely abstract and *must* be
implemented by subclasses. The default implementation of
`$self-'open_tables> calls this method for any table
used by the statement. See the `test.pl' script for an
example of imlplementing a subclass.
SQL syntax
The SQL::Statement module is far away from ANSI SQL or something
similar, it is designed for implementing the DBD::CSV module.
See the DBD::CSV(3) manpage.
I do not want to give a formal grammar here, more an informal
description: Read the statement definition in sql_yacc.y, if you
need something precise.
The main lexical elements of the grammar are:
Integers
Reals Syntax obvious
Strings Surrounded by either single or double quotes; some
characters need to be escaped with a backslash, in
particular the backslash itself (\\), the NUL byte (\0),
Line feeds (\n), Carriage return (\r), and the quotes
(\' or \").
Parameters
Parameters represent scalar values, like Integers, Reals
and Strings do. However, their values are read inside
Execute() and not inside Prepare(). Parameters are
represented by question marks (?).
Identifiers
Identifiers are table or column names. Syntactically
they consist of alphabetic characters, followed by an
arbitrary number of alphanumeric characters. Identifiers
like SELECT, INSERT, INTO, ORDER, BY, WHERE, ... are
forbidden and reserved for other tokens.
What it offers is the following:
CREATE
This is the CREATE TABLE command:
CREATE TABLE $table ( $col1 $type1, ..., $colN $typeN,
[ PRIMARY KEY ($col1, ... $colM) ] )
The column names are $col1, ... $colN. The column types can be
`INTEGER', `CHAR(n)', `VARCHAR(n)', `REAL' or `BLOB'. These
types are currently completely ignored. So is the (optional)
`PRIMARY KEY' clause.
DROP
Very simple:
DROP TABLE $table
INSERT
This can be
INSERT INTO $table [ ( $col1, ..., $colN ) ]
VALUES ( $val1, ... $valN )
DELETE
DELETE FROM $table [ WHERE $where_clause ]
See the SELECT manpage below for a decsription of $where_clause
UPDATE
UPDATE $table SET $col1 = $val1, ... $colN = $valN
[ WHERE $where_clause ]
See the SELECT manpage below for a decsription of $where_clause
SELECT
SELECT [DISTINCT] $col1, ... $colN FROM $table
[ WHERE $where_clause ] [ ORDER BY $ocol1, ... $ocolM ]
The $where_clause is based on boolean expressions of the form
$val1 $op $val2, with $op being one of '=', '<>', '>', '<',
'>=', '<=', 'LIKE', 'CLIKE' or IS. You may use OR, AND and
brackets to combine such boolean expressions or NOT to negate
them.
INSTALLATION
Like most other Perl modules, you simply do a
perl Makefile.PL
make (nmake or dmake, if you are using Win32)
make test (Let me know, if any tests fail)
make install
Known problems are:
* Some flavours of SCO Unix don't seem to have alloca() or
something similar. I recommend using gcc or egcs for
compiling Perl and the SQL::Statement module: Both
compilers have a builtin alloca().
Another option could be to use external alloca.c, for
example
http://www.pu.informatik.th-darmstadt.de/FTP/pub/pu/alloca.c
http://www.cs.purdue.edu/homes/young/src2www-example/alloca.c.html
I did test neither of them and cannot give detailed
instructions for including them into the SQL::Statement
module. However, it should be sufficient to compile
alloca.c with the same instructions than, for example,
sql_yacc.c and finally repeat the linker command by
inserting alloca.o after sql_yacc.o.
Note that I cannot modify the sources to work without
alloca(), as it is the bison parser that's using
alloca() and I don't have the bison generated code in my
hands.
My thanks to Theo Petersen, <
[email protected]>, for
pointing out this problem and the possible workarounds.
INTERNALS
Internally the module is splitted into three parts:
Perl-independent C part
This part, contained in the files `sql_yacc.y', `sql_data.h',
`sql_data.c' and `sql_op.c', is completely independent from
Perl. It might well be used from within another script language,
Tcl say, or from a true C application.
You probably ask, why Perl independence? Well, first of all, I
think this is a valuable target in itself. But the main reason
was the impossibility to use the Perl headers inside bison
generated code. The Perl headers export almost the complete Yacc
interface to XS, for whatever reason, thus redefining constants
and structures created by your own bison code. :-(
Perl-dependent C part
This is contained in `Statement.xs'. The both C parts
communicate via a C structure sql_stmt_t. In fact, an
SQL::Statement object is nothing else than a pointer to such a
structure. The XS calls columns(), Table(), where(), ... do
nothing more than fetching data from this structure and
converting it to Perl objects. See the section on "The
sql_stmt_t structure" below for details on the structure.
Perl part
Besides some stub functions for retrieving statement data, this
is mainly the query processing with the exception of WHERE
clause evaluation.
The sql_stmt_t structure
This structure is designed for optimal performance. A typical
query will be parsed with only 4 or 5 malloc() calls; in
particular no memory will be aquired for storing strings; only
pointers into the query string are used.
The statement stores its tokens in the values array. The array
elements are of type sql_val_t, a union, that can represent the
most interesting tokens; for example integers and reals are
stored in the data.i and data.d parts of the union, strings are
stored in the data.str part, columns in the data.col part and so
on. Arrays are allocated in chunks of 64 elements, thus a single
malloc() will be usually sufficient for allocating the complete
array. Some types use pointers into the values array: For
example, operations are stored in an sql_op_t structure that
containes elements arg1 and arg2 which are pointers into the
value table, pointing to other operations or scalars. These
pointers are stored as indices, so that the array can be
extended using realloc().
The sql_stmt_t structure contains other arrays: columns, tables,
rowvals, order, ... representing the data returned by the
columns(), tables(), row_values() and order() methods. All of
these contain pointers into the values array, again stored as
integers.
Arrays are initialized with the _InitArray call in
SQL_Statement_Prepare and deallocated with _DestroyArray in
SQL_Statement_Destroy. Array elements are obtained by calling
_AllocData, which returns an index. The number -1 is used for
errors or as a NULL value.
The WHERE clause evaluation
A WHERE clause is evaluated by calling
SQL_Statement_EvalWhere(). This function is in the Perl
independent part, but it needs the possibility to retrieve data
from the Perl part, for example column or parameter values.
These values are retrieved via callbacks, stored in the
sql_eval_t structure. The field stmt->evalData points to such a
structure. Of course the calling method can extend the
sql_eval_t structure (like eval_where in Statement.xs does) to
include private data not used by SQL_Statement_EvalWhere.
Features
Different parsers are implemented via the sql_parser_t
structure. This is mainly a set of yes/no flags. If you'd like
to add features, do the following:
First of all, extend the sql_parser_t structure. If your feature
is part of a certain statement, place it into the statements
section, for example "select.join". Otherwise choose a section
like "misc" or "general". (There's no particular for the section
design, but structure never hurts.)
Second, add your feature to sql_yacc.y. If your feature needs to
extend the lexer, do it like this:
if (FEATURE(misc, myfeature) {
/* Scan your new symbols */
...
}
See the *BOOL* symbol as an example.
If you need to extend the parser, do it like this:
my_new_rule:
/* NULL, old behaviour, doesn't use my feature */
| my_feature
{ YFEATURE(misc, myfeature); }
;
Thus all parsers not having FEATURE(misc, myfeature) set will
produce a parse error here. Again, see the BOOL symbol for an
example.
Third thing is to extend the builtin parsers. If they support
your feature, add a 1, otherwise a 0. Currently there are two
builtin parsers: The *ansiParser* in sql_yacc.y and the
sqlEvalParser in Statement.xs.
Finally add support for your feature to the `feature' method in
Statement.xs. That's it!
MULTITHREADING
The complete module code is reentrant. In particular the parser
is created with `%pure_parser'. See the bison(1) manpage for
details on reentrant parsers. That means, the module is ready
for multithreading, as long as you don't share handles between
threads. Read-only handles, for example parsers, can even be
shared.
Statement handles cannot be shared among threads, at least not,
if you don't grant serialized access. Per-thread handles are
always safe.
AUTHOR AND COPYRIGHT
This module is Copyright (C) 1998 by
Jochen Wiedmann
Am Eisteich 9
72555 Metzingen
Germany
Email:
[email protected]
Phone: +49 7123 14887
All rights reserved.
You may distribute this module under the terms of either the GNU
General Public License or the Artistic License, as specified in
the Perl README file.
SEE ALSO
the DBI(3) manpage, the DBD::CSV(3) manpage
