NAME
SQL::Statement - SQL parsing and processing engine
SYNOPSIS
require SQL::Statement;
# Create a parser
my($parser) = SQL::Statement->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.
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
