Database abstraction layer

Allow the use of different database servers using the same code base.

Drupal provides a database abstraction layer to provide developers with the ability to support multiple database servers easily. The intent of this layer is to preserve the syntax and power of SQL as much as possible, but also allow developers a way to leverage more complex functionality in a unified way. It also provides a structured interface for dynamically constructing queries when appropriate, and enforcing security checks and similar good practices.

The system is built atop PHP's PDO (PHP Data Objects) database API and inherits much of its syntax and semantics.

Most Drupal database SELECT queries are performed by a call to db_query() or db_query_range(). Module authors should also consider using the PagerDefault Extender for queries that return results that need to be presented on multiple pages (see https://drupal.org/node/508796), and the TableSort Extender for generating appropriate queries for sortable tables (see https://drupal.org/node/1848372).

For example, one might wish to return a list of the most recent 10 nodes authored by a given user. Instead of directly issuing the SQL query

SELECT n.nid, n.title, n.created FROM node n WHERE n.uid = $uid
  ORDER BY n.created DESC LIMIT 0, 10;

one would instead call the Drupal functions:

$result = db_query_range('SELECT n.nid, n.title, n.created
  FROM {node} n WHERE n.uid = :uid
  ORDER BY n.created DESC', 0, 10, array(
  ':uid' => $uid,
));
foreach ($result as $record) {

  // Perform operations on $record->title, etc. here.
}

Curly braces are used around "node" to provide table prefixing via DatabaseConnection::prefixTables(). The explicit use of a user ID is pulled out into an argument passed to db_query() so that SQL injection attacks from user input can be caught and nullified. The LIMIT syntax varies between database servers, so that is abstracted into db_query_range() arguments. Finally, note the PDO-based ability to iterate over the result set using foreach ().

All queries are passed as a prepared statement string. A prepared statement is a "template" of a query that omits literal or variable values in favor of placeholders. The values to place into those placeholders are passed separately, and the database driver handles inserting the values into the query in a secure fashion. That means you should never quote or string-escape a value to be inserted into the query.

There are two formats for placeholders: named and unnamed. Named placeholders are strongly preferred in all cases as they are more flexible and self-documenting. Named placeholders should start with a colon ":" and can be followed by one or more letters, numbers or underscores.

Named placeholders begin with a colon followed by a unique string. Example:

SELECT nid, title FROM {node} WHERE uid=:uid;

":uid" is a placeholder that will be replaced with a literal value when the query is executed. A given placeholder label cannot be repeated in a given query, even if the value should be the same. When using named placeholders, the array of arguments to the query must be an associative array where keys are a placeholder label (e.g., :uid) and the value is the corresponding value to use. The array may be in any order.

Unnamed placeholders are simply a question mark. Example:

SELECT nid, title FROM {node} WHERE uid=?;

In this case, the array of arguments must be an indexed array of values to use in the exact same order as the placeholders in the query.

Note that placeholders should be a "complete" value. For example, when running a LIKE query the SQL wildcard character, %, should be part of the value, not the query itself. Thus, the following is incorrect:

SELECT nid, title FROM {node} WHERE title LIKE :title%;

It should instead read:

SELECT nid, title FROM {node} WHERE title LIKE :title;

and the value for :title should include a % as appropriate. Again, note the lack of quotation marks around :title. Because the value is not inserted into the query as one big string but as an explicitly separate value, the database server knows where the query ends and a value begins. That is considerably more secure against SQL injection than trying to remember which values need quotation marks and string escaping and which don't.

INSERT, UPDATE, and DELETE queries need special care in order to behave consistently across all different databases. Therefore, they use a special object-oriented API for defining a query structurally. For example, rather than:

INSERT INTO node (nid, title, body) VALUES (1, 'my title', 'my body');

one would instead write:

$fields = array(
  'nid' => 1,
  'title' => 'my title',
  'body' => 'my body',
);
db_insert('node')
  ->fields($fields)
  ->execute();

This method allows databases that need special data type handling to do so, while also allowing optimizations such as multi-insert queries. UPDATE and DELETE queries have a similar pattern.

Drupal also supports transactions, including a transparent fallback for databases that do not support transactions. To start a new transaction, simply call $txn = db_transaction(); in your own code. The transaction will remain open for as long as the variable $txn remains in scope. When $txn is destroyed, the transaction will be committed. If your transaction is nested inside of another then Drupal will track each transaction and only commit the outer-most transaction when the last transaction object goes out out of scope, that is, all relevant queries completed successfully.

Example:

function my_transaction_function() {

  // The transaction opens here.
  $txn = db_transaction();
  try {
    $id = db_insert('example')
      ->fields(array(
      'field1' => 'mystring',
      'field2' => 5,
    ))
      ->execute();
    my_other_function($id);
    return $id;
  } catch (Exception $e) {

    // Something went wrong somewhere, so roll back now.
    $txn
      ->rollback();

    // Log the exception to watchdog.
    watchdog_exception('type', $e);
  }

  // $txn goes out of scope here.  Unless the transaction was rolled back, it
  // gets automatically committed here.
}
function my_other_function($id) {

  // The transaction is still open here.
  if ($id % 2 == 0) {
    db_update('example')
      ->condition('id', $id)
      ->fields(array(
      'field2' => 10,
    ))
      ->execute();
  }
}

See also

http://drupal.org/developing/api/database

File

includes/database/database.inc, line 12

Core systems for the database layer.

Functions

Name	Location	Description
db_and	includes/database/database.inc	Returns a new DatabaseCondition, set to "AND" all conditions together.
db_close	includes/database/database.inc	Closes the active database connection.
db_condition	includes/database/database.inc	Returns a new DatabaseCondition, set to the specified conjunction.
db_delete	includes/database/database.inc	Returns a new DeleteQuery object for the active database.
db_driver	includes/database/database.inc	Retrieves the name of the currently active database driver.
db_escape_field	includes/database/database.inc	Restricts a dynamic column or constraint name to safe characters.
db_escape_table	includes/database/database.inc	Restricts a dynamic table name to safe characters.
db_insert	includes/database/database.inc	Returns a new InsertQuery object for the active database.
db_like	includes/database/database.inc	Escapes characters that work as wildcard characters in a LIKE pattern.
db_merge	includes/database/database.inc	Returns a new MergeQuery object for the active database.
db_next_id	includes/database/database.inc	Retrieves a unique id.
db_or	includes/database/database.inc	Returns a new DatabaseCondition, set to "OR" all conditions together.
db_query	includes/database/database.inc	Executes an arbitrary query string against the active database.
db_query_range	includes/database/database.inc	Executes a query against the active database, restricted to a range.
db_query_temporary	includes/database/database.inc	Executes a SELECT query string and saves the result set to a temporary table.
db_select	includes/database/database.inc	Returns a new SelectQuery object for the active database.
db_set_active	includes/database/database.inc	Sets a new active database.
db_transaction	includes/database/database.inc	Returns a new transaction object for the active database.
db_truncate	includes/database/database.inc	Returns a new TruncateQuery object for the active database.
db_update	includes/database/database.inc	Returns a new UpdateQuery object for the active database.
db_xor	includes/database/database.inc	Returns a new DatabaseCondition, set to "XOR" all conditions together.

Constants

Name	Location	Description
POSTGRESQL_NEXTID_LOCK	includes/database/pgsql/database.inc	The name by which to obtain a lock for retrieving the next insert id.

Classes

Name	Location	Description
Database	includes/database/database.inc	Primary front-controller for the database system.
DatabaseCondition	includes/database/query.inc	Generic class for a series of conditions in a query.
DatabaseConnection	includes/database/database.inc	Base Database API class.
DatabaseConnectionNotDefinedException	includes/database/database.inc	Exception thrown if an undefined database connection is requested.
DatabaseConnection_mysql	includes/database/mysql/database.inc
DatabaseConnection_pgsql	includes/database/pgsql/database.inc
DatabaseDriverNotSpecifiedException	includes/database/database.inc	Exception thrown if no driver is specified for a database connection.
DatabaseStatementBase	includes/database/database.inc	Default implementation of DatabaseStatementInterface.
DatabaseStatementEmpty	includes/database/database.inc	Empty implementation of a database statement.
DatabaseStatementPrefetch	includes/database/prefetch.inc	An implementation of DatabaseStatementInterface that prefetches all data.
DatabaseTransaction	includes/database/database.inc	A wrapper class for creating and managing database transactions.
DatabaseTransactionCommitFailedException	includes/database/database.inc	Exception thrown when a commit() function fails.
DatabaseTransactionExplicitCommitNotAllowedException	includes/database/database.inc	Exception to deny attempts to explicitly manage transactions.
DatabaseTransactionNameNonUniqueException	includes/database/database.inc	Exception thrown when a savepoint or transaction name occurs twice.
DatabaseTransactionNoActiveException	includes/database/database.inc	Exception for when popTransaction() is called with no active transaction.
DatabaseTransactionOutOfOrderException	includes/database/database.inc	Exception thrown when a rollback() resulted in other active transactions being rolled-back.
DeleteQuery	includes/database/query.inc	General class for an abstracted DELETE operation.
DeleteQuery_sqlite	includes/database/sqlite/query.inc	SQLite specific implementation of DeleteQuery.
FieldsOverlapException	includes/database/database.inc	Exception thrown if an insert query specifies a field twice.
InsertQuery	includes/database/query.inc	General class for an abstracted INSERT query.
InsertQuery_mysql	includes/database/mysql/query.inc	@file Query code for MySQL embedded database engine.
InsertQuery_pgsql	includes/database/pgsql/query.inc	@file Query code for PostgreSQL embedded database engine.
InsertQuery_sqlite	includes/database/sqlite/query.inc	SQLite specific implementation of InsertQuery.
InvalidMergeQueryException	includes/database/database.inc	Exception thrown for merge queries that do not make semantic sense.
InvalidQueryConditionOperatorException	includes/database/database.inc	Exception thrown if an invalid query condition is specified.
MergeQuery	includes/database/query.inc	General class for an abstracted MERGE query operation.
NoFieldsException	includes/database/database.inc	Exception thrown if an insert query doesn't specify insert or default fields.
Query	includes/database/query.inc	Base class for query builders.
SelectQuery_pgsql	includes/database/pgsql/select.inc
SelectQuery_sqlite	includes/database/sqlite/select.inc	SQLite specific query builder for SELECT statements.
TruncateQuery	includes/database/query.inc	General class for an abstracted TRUNCATE operation.
TruncateQuery_mysql	includes/database/mysql/query.inc
TruncateQuery_sqlite	includes/database/sqlite/query.inc	SQLite specific implementation of TruncateQuery.
UpdateQuery	includes/database/query.inc	General class for an abstracted UPDATE operation.
UpdateQuery_mysql	includes/database/mysql/query.inc
UpdateQuery_pgsql	includes/database/pgsql/query.inc
UpdateQuery_sqlite	includes/database/sqlite/query.inc	SQLite specific implementation of UpdateQuery.

Interfaces

Name	Location	Description
DatabaseStatementInterface	includes/database/database.inc	Represents a prepared statement.
QueryAlterableInterface	includes/database/query.inc	Interface for a query that can be manipulated via an alter hook.
QueryConditionInterface	includes/database/query.inc	Interface for a conditional clause in a query.
QueryPlaceholderInterface	includes/database/query.inc	Interface for a query that accepts placeholders.