NAME

SQL::Interp - Interpolate Perl variables into SQL statements

SYNOPSIS

  use SQL::Interp ':all';

  my ($sql, @bind) = sql_interp 'INSERT INTO table', \%item;
  my ($sql, @bind) = sql_interp 'UPDATE table SET',  \%item, 'WHERE y <> ', \2;
  my ($sql, @bind) = sql_interp 'DELETE FROM table WHERE y = ', \2;

  # These two select syntax produce the same result
  my ($sql, @bind) = sql_interp 'SELECT * FROM table WHERE x = ', \$s, 'AND y IN', \@v;
  my ($sql, @bind) = sql_interp 'SELECT * FROM table WHERE', {x => $s, y => \@v};

DESCRIPTION

SQL::Interp converts a list of intermixed SQL fragments and variable references into a conventional SQL string and list of bind values suitable for passing onto DBI. This simple technique creates database calls that are simpler to create and easier to read, while still giving you full access to custom SQL.

SQL::Interp properly binds or escapes variables. This recommended practice safeguards against "SQL injection" attacks. The DBI documentation has several links on the topic.

Besides the simple techniques shown above, The SQL::Interp integrates directly with DBIx::Simple for an excellent alternative to raw DBI access:

  use DBIx::Simple;

  ...

  my $rows = $db->iquery("
      SELECT title
          FROM threads
          WHERE date > ",\$x," AND subject IN ",\@subjects
   )->arrays;

Since DBIx::Simple still allows you complete access to the DBI API, using it as wrapper is recommended for most applications.

The One Function You Really Need

"sql_interp"

  ($sql, @bind) = sql_interp @params;

"sql_interp()" is the one central function you need to know. "sql_interp()" strings together the given list of elements and returns both an SQL string ($sql) with placeholders ("?") and a corresponding list of bind values (@bind) suitable for passing to DBI.

The interpolation list can contain elements of these types:

* SQL - string containing a raw SQL fragment such as 'SELECT * FROM mytable WHERE'.

* variable reference - scalarref, arrayref, hashref, or A sql_type() object referring to data to interpolate between the SQL.

* other interpolation list - an interpolation list can be nested inside another interpolation list. This is possible with the sql() function.

Interpolation Examples

The following variable names will be used in the below examples:

 $sref  = \3;                      # scalarref
 $aref  = [1, 2];                  # arrayref
 $href  = {m => 1, n => undef};    # hashref
 $hv = {v => $v, s => $$s};        # hashref containing arrayref
 $vv = [$v, $v];                   # arrayref of arrayref
 $vh = [$h, $h];                   # arrayref of hashref

 Let $x stand for any of these.

Default scalarref behavior

A scalarref becomes a single bind value.

  IN:  'foo', $sref, 'bar'
  OUT: 'foo ? bar', $$sref

Default hashref behavior

A hashref becomes a logical AND

  IN:  'WHERE', $href
  OUT: 'WHERE (m=? AND n IS NULL)', $h->{m},

  IN:  'WHERE', $hv
  OUT: 'WHERE (v IN (?, ?) AND s = ?)', @$v, $$s

Default arrayref of (hashref or arrayref) behavior

This is not commonly used.

  IN:  $vv
  OUT: '(SELECT ?, ? UNION ALL SELECT ?, ?)',
          map {@$_} @$v

  IN:  $vh
  OUT: '(SELECT ? as m, ? as n UNION ALL
            SELECT ?, ?)',
          $vh->[0]->{m}, $vh->[0]->{n},
          $vh->[1]->{m}, $vh->[1]->{n}

  # Typical usage:
  IN: $x
  IN: $x, 'UNION [ALL|DISTINCT]', $x
  IN: 'INSERT INTO mytable', $x
  IN: 'SELECT * FROM mytable WHERE x IN', $x

Context ('IN', $x)

A scalarref or arrayref can used to form an "IN" clause. As a convenience, a reference to an arrayref is also accepted. This way, you can simply provide a reference to a value which may be a single-valued scalar or a multi-valued arrayref.

  IN:  'WHERE x IN', $aref
  OUT: 'WHERE x IN (?, ?)', @$aref

  IN:  'WHERE x IN', $sref
  OUT: 'WHERE x IN (?)', $$sref

  IN:  'WHERE x IN', []
  OUT: 'WHERE 1=0'

  IN:  'WHERE x NOT IN', []
  OUT: 'WHERE 1=1'

Context ('INSERT INTO tablename', $x)

  IN:  'INSERT INTO mytable', $href
  OUT: 'INSERT INTO mytable (m, n) VALUES(?, ?)', $href->{m}, $href->{n}

  IN:  'INSERT INTO mytable', $aref
  OUT: 'INSERT INTO mytable VALUES(?, ?)', @$aref;

  IN:  'INSERT INTO mytable', $sref
  OUT: 'INSERT INTO mytable VALUES(?)', $$sref;

MySQL's "REPLACE INTO" is supported the same way.

Context ('SET', $x)

  IN:  'UPDATE mytable SET', $href
  OUT: 'UPDATE mytable SET m = ?, n = ?', $href->{m}, $href->{n}

MySQL's "ON DUPLICATE KEY UPDATE" is supported the same way.

Context ('FROM | JOIN', $x)

This is not commonly used.

  IN:  'SELECT * FROM', $vv
  OUT: 'SELECT * FROM
       (SELECT ?, ? UNION ALL SELECT ?, ?) as t001',
       map {@$_} @$v

  IN:  'SELECT * FROM', $vh
  OUT: 'SELECT * FROM
       (SELECT ? as m, ? as n UNION ALL SELECT ?, ?) as temp001',
       $vh->[0]->{m}, $vh->[0]->{n},
       $vh->[1]->{m}, $vh->[1]->{n}

  IN:  'SELECT * FROM', $vv, 'AS t'
  OUT: 'SELECT * FROM
       (SELECT ?, ? UNION ALL SELECT ?, ?) AS t',
       map {@$_} @$v

  # Example usage (where $x and $y are table references):
  'SELECT * FROM', $x, 'JOIN', $y

Context ('ARRAY', $x)

A scalarref or arrayref can be turned into an array value. Such values are supported by PostgreSQL.

  IN:  'SELECT ARRAY', $aref
  OUT: 'SELECT ARRAY[?, ?]', @$aref

  IN:  'SELECT ARRAY', $sref
  OUT: 'SELECT ARRAY[?]', $$sref

  IN:  'SELECT ARRAY', []
  OUT: 'SELECT ARRAY[]'

Other Rules

Whitespace is automatically added between parameters:

 IN:  'UPDATE', 'mytable SET', {x => 2}, 'WHERE y IN', \@colors;
 OUT: 'UPDATE mytable SET x = ? WHERE y in (?, ?)', 2, @colors

Variables must be passed as references; otherwise, they will processed as SQL fragments and interpolated verbatim into the result SQL string, negating the security and performance benefits of binding values.

In contrast, any scalar values inside an arrayref or hashref are by default treated as binding variables, not SQL. The contained elements may be also be sql_type() or sql().

Security: sql_interp_strict

The "sql_interp" function has a security weakness. Consider these two statements, one easily a typo of the other:

    sql_interp("SELECT * FROM foo WHERE a = ",\$b)
    sql_interp("SELECT * FROM foo WHERE a = ",$b)

Both would produce valid SQL, but the first would be secure due to use of bind variables, while the second is potentially insecure, because $b is added directly to the SQL statement. If $b contains a malicious value, it could be used for a SQL injection attack.

To prevent this accident, we also supply "sql_interp_strict()", which works exactly the same as sql_interp(), but with an additional check that two non-references never appear in a row . If they do, an exception will be thrown.

This does mean some previously safe-but-valid SQL be need to be rewritten, such as when you are building a complex query from pieces. Here's a contrived example:

    sql_interp("SELECT * FROM ","foo","WHERE a = ",\$b);

To work under strict mode, you need to concatenate the strings instead:

    sql_interp("SELECT * FROM "."foo"."WHERE a = ",\$b);

Note that strict mode only checks the immediate arguments of "sql_interp_strict()", it does not check nested interpolation using the "sql()" function (described below). Thus, the previous example can also be written as:

    sql_interp(sql("SELECT * FROM ","foo","WHERE a = "),\$b);

A Couple Helper Functions You Sometimes Need

"sql()"

  sql_interp 'INSERT INTO mytable',
      {x => $x, y => sql('CURRENT_TIMESTAMP')};
  # OUT: 'INSERT INTO mytable (x, y) VALUES(?, CURRENT_TIMESTAMP)', $x

sql() is useful if you want insert raw SQL as a value in an arrayref or hashref.

"sql_type()"

  my $sqlvar = sql_type($value_ref, type => $sql_type, %params);

"sql_type()" provides a general way to represent a binding variable along with metadata. It is necessary in rare applications which you need to explicity give the bind type of a SQL variable.

$value_ref - variable reference contained

$sql_type - any DBI SQL_DATA_TYPE (e.g. SQL_INTEGER). Optional. Default is undef.

Any other named parameters (%params) passed in will be saved into the object as attributes.

sql_type objects are useful only in special cases where additional information should be tagged onto the variable. For example, DBI allows bind variables to be given an explicit type:

  my ($sql, @bind) = sql_interp 'SELECT * FROM mytable WHERE',
      'x=', \$x, 'AND y=', sql_type(\$y, SQL_VARCHAR), 'AND z IN',
      sql_type([1, 2], SQL_INTEGER);
  # RESULT: @bind =
  #   ([$x, sql_type(\$x)], [$y, sql_type(\$y, type => SQL_VARCHAR)],
  #    [1, sql_type([1, 2], type => SQL_INTEGER)],
  #    [2, sql_type([1, 2], type => SQL_INTEGER)]);

  my $idx = 1;
  for my $var (@bind) {
      $sth->bind_param($idx++, $var->[0], $var->[1]->{type});
  }
  $sth->execute();
  my $ret = $sth->selectall_arrayref();

If the interpolation list contains at least one sql_type object, then all the variable references are transparently converted into sql_type objects, and the elements of @bind take a special form: an arrayref consisting of the bind value and the sql_type object that generated the bind value. Note that a single sql_type holding an aggregate (arrayref or hashref) may generate multiple bind values.

Enabling debugging output

To have the generated SQL and bind variables sent to STDOUT, you can set the environment variable "TRACE_SQL" to "1"

 TRACE_SQL=1 perl my_script.pl

Here's some example output:

 DEBUG:interp[sql=INSERT INTO mytable VALUES(?),bind=5]

Philosophy

The query language is SQL. There are other modules, such as SQL::Abstract, that hide SQL behind method calls and/or Perl data structures (hashes and arrays). The former may be undesirable in some cases since it replaces one language with another and hides the full capabilities and expressiveness of your database's native SQL language. The latter may load too much meaning into the syntax of "{, "[" and "\" thereby rendering the meaning less clear:

  SQL::Abstract example:
  %where = (lname => {like => '%son%'},
            age   => [-and => {'>=', 10}, {'<=', 20}])
  Plain SQL:
  "lname LIKE '%son' AND (age >= 10 AND age <= 20)"

In contrast, SQL::Interp does not abstract away your SQL but rather makes it easier to interpolate Perl variables into your SQL. Now, SQL::Interp does load some meaning into "{, "[" and "\", but we try to limit its use to obvious cases. Since your raw SQL is exposed, you can use your particular dialect of SQL.

Limitations

Some types of interpolation are context-sensitive and involve examination of your SQL fragments. The examination could fail on obscure syntax, but it is generally robust. Look at the examples to see the types of interpolation that are accepted, and if doubt, examine the SQL output yourself with the TRACE_SQL environment variable set. If needed, you can disable context sensitivity by inserting a null-string before a variable.

 "SET", "", \$x

A few things are just not possible with the ('WHERE', \%hashref) syntax, so in such case, use a more direct syntax:

  # ok--direct syntax
  sql_interp '...WHERE', {x => $x, y => $y}, 'AND y = z';
  # bad--trying to impose a hashref but keys must be scalars and be unique
  sql_interp '...WHERE',
      {sql_type(\$x) => sql('x'), y => $y, y => sql('z')};

In the cases where this module parses or generates SQL fragments, this module should work for many databases, but its been tested mostly on MySQL and PostgreSQL. Please inform the author of any incompatibilities.

Contributor and Contributing

David Manura is the author of the original SQL::Interpolate, Mark Stosberg (<http://mark.stosberg.com/>) created the SQL::Interp fork. It is now maintained by Yoran Heling (<https://yorhel.nl/>).

Also thanks to: Mark Tiefenbruck (syntax), Wojciech Pietron (Oracle compat), Jim Chromie (DBIx::Interp idea), Juerd Waalboer, Terrence Brannon (early feedback), and others.

The SQL::Interp Git repository is hosted at <https://code.blicky.net/yorhel/SQL-Interp>.

License

This module is free software. It may be used, redistributed and/or modified under the same terms as Perl itself. See <http://www.perl.com/perl/misc/Artistic.html>.