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Development:DML functions

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Revision as of 15:09, 4 May 2011 by Tim Hunt (talk | contribs) (Correct error in the last changes.)

Template:Moodle 2.0This page describes the functions available to access data in the Moodle database. You should exclusively use these functions in order to retrieve or modify database content because these functions provide a high level of abstraction and guarantee that your database manipulation will work against different RDBMSes.

Where possible, tricks and examples will be documented here in order to make developers' lives a bit easier. Of course, feel free to clarify, complete and add more information to this documentation. It will be welcome, absolutely!

Main info

Important note: All the functions shown on this page are for use in Moodle 2.0 upwards, where we changed the DB layer to support some new features. If you need information for previous Moodle version, take a look to the DML functions - pre 2.0 page. For a detailed reference of changes, see the migration docs.

  • All the function calls on this page are public methods of the $DB global object, so you'll need to "import" it within your functions (not needed in global scripts) with one simple:

global $DB;

  • All the $table parameters in the functions are meant to be the table name without prefixes.

$user = $DB->get_record('user', array('id'=>'1');

  • When using the xxx_sql() functions, table names must be enclosed between curly braces.

$user = $DB->get_record_sql('SELECT * FROM {user} WHERE id = ?', array(1));

  • All the $conditions parameters in the functions are arrays of fieldname=>fieldvalue elements.

$user = $DB->get_record('user', array('firstname'=>'Martin', 'lastname'=>'Dougiamas'));

  • All the $params parameters in the functions are arrays of values used to fill placeholders in SQL statements. Both the question mark and named placeholders can be used. Note that named params must be unique even if the value passed is the same.

/// Question mark placeholders: 

  $DB->get_record_sql('SELECT * FROM {user} WHERE firstname = ? AND lastname = ?', 
                      array('Martin', 'Dougiamas'));

/// Named placeholders: 

  $DB->get_record_sql('SELECT * FROM {user} WHERE firstname = :firstname AND lastname = :lastname',
                      array('firstname'=>'Martin', 'lastname'=>'Dougiamas'));

The functions

Seeing how many records match a given criteria

o $DB->count_records($table, array $conditions=null) 

 /// Count the records in a table where all the given conditions met.

o $DB->count_records_select($table, $select, array $params=null, $countitem="COUNT('x')") 

 /// Count the records in a table which match a particular WHERE clause.

o $DB->count_records_sql($sql, array $params=null) 

 /// Get the result of a SQL SELECT COUNT(...) query.

Seeing if one record exists

o $DB->record_exists($table, array $conditions=null) 

 /// Test whether a record exists in a table where all the given conditions met.

o $DB->record_exists_select($table, $select, array $params=null) 

 /// Test whether any records exists in a table which match a particular WHERE clause.

o $DB->record_exists_sql($sql, array $params=null) 

 /// Test whether a SQL SELECT statement returns any records.

Getting a single record

o $DB->get_record($table, array $conditions, $fields='*', $ignoremultiple=false) 

 /// Get a single database record as an object where all the given conditions met.

o $DB->get_record_select($table, $select, array $params=null, $fields='*', $ignoremultiple=false) 

 /// Get a single database record as an object which match a particular WHERE clause.

o $DB->get_record_sql($sql, array $params=null) 

 /// Get a single database record as an object using a SQL statement.

Getting an array of records

o $DB->get_records($table, array $conditions=null, $sort=, $fields='*', $limitfrom=0, $limitnum=0) 

 /// Get a number of records as an array of objects where all the given conditions met.

o $DB->get_record_select($table, $select, array $params=null, $sort=, $fields='*', $limitfrom=0, $limitnum=0) 

 /// Get a number of records as an array of objects which match a particular WHERE clause.

o $DB->get_records_sql($sql, array $params=null, $limitfrom=0, $limitnum=0) 

 /// Get a number of records as an array of objects using a SQL statement.

o $DB->get_records_menu($table, array $conditions=null, $sort=, $fields='*', $limitfrom=0, $limitnum=0) 

 /// Get the first two columns from a number of records as an associative array where all the given conditions met.

o $DB->get_records_select_menu($table, $select, array $params=null, $sort=, $fields='*', $limitfrom=0, $limitnum=0) 

 /// Get the first two columns from a number of records as an associative array which match a particular WHERE clause.

o $DB->get_records_sql_menu($sql, array $params=null, $limitfrom=0, $limitnum=0) 

 /// Get the first two columns from a number of records as an associative array using a SQL statement.

o $DB->get_records_list($table, $field, array $values, $sort=, $fields='*', $limitfrom=, $limitnum=) 

 /// Get a number of records as an array of objects where one field match one list of values.

Getting a particular field value from one record

o $DB->get_field($table, $return, array $conditions) 

 /// Get a single field value from a table record where all the given conditions met.

o $DB->get_field_select($table, $return, $select, array $params=null) 

 /// Get a single field value from a table record which match a particular WHERE clause.

o $DB->get_field_sql($sql, array $params=null) 

 /// Get a single field value (first field) using a SQL statement.

Getting a particular field value from various records

o $DB->get_fieldset_select($table, $return, $select, array $params=null) 

 /// Selects records and return values of chosen field as an array which match a particular WHERE clause.

o $DB->get_fieldset_sql($sql, array $params=null) 

 /// Selects records and return values (first field) as an array using a SQL statement.

Setting a particular field in the database

o $DB->set_field($table, $newfield, $newvalue, array $conditions=null) 

 /// Set a single field in every table record where all the given conditions met.

o $DB->set_field_select($table, $newfield, $newvalue, $select, array $params=null) 

 /// Set a single field in every table record which match a particular WHERE clause.

Deleting Records

o $DB->delete_records($table, array $conditions=null) 

 /// Delete the records from a table where all the given conditions met.

o $DB->delete_records_select($table, $select, array $params=null) 

 /// Delete one or more records from a table which match a particular WHERE clause.

Inserting Records

o $DB->insert_record($table, $dataobject, $returnid=true, $bulk=false) 

 /// Insert a record into a table and return the "id" field if required.

Example(s)

$record = new stdClass(); $record->name = 'overview'; $record->displayorder = '10000'; $DB->insert_record('quiz_report', $record, false);

$record = new stdClass(); $record->name = 'overview'; $record->displayorder = '10000'; $lastinsertid = $DB->insert_record('quiz_report', $record);

Updating Records

o $DB->update_record($table, $dataobject, $bulk=false) 

 /// Update a record in a table.

Using Recordsets

Where the number of records to be retrieved from DB is high, the get_records_xxx() functions above are far from optimal, because they load all the records in memory at the same time. Under those circumstances, it is highly recommended to use these get_recordset_xxx() functions instead, which use one nice mechanism to iterate over all the target records and save a lot of memory.

Only one thing is absolutely important: Don't forget to close the recordsets after using them! (This will free up a lot of resources in the RDBMS).

Here is the general way to iterate over records using the get_recordset_xxx() functions:

if ($rs = $DB->get_recordset(....) { 

   foreach ($rs as $record) {
       // Do whatever you want with this record
   }
   $rs->close(); // Don't forget to close the recordset!

}

And this is the list of available functions (100% paired with the get_records_xxx() above): o $DB->get_recordset($table, array $conditions=null, $sort=, $fields='*', $limitfrom=0, $limitnum=0) 

 /// Get a number of records as a moodle_recordset where all the given conditions met.

o $DB->get_recordset_select($table, $select, array $params=null, $sort=, $fields='*', $limitfrom=0, $limitnum=0) 

 /// Get a number of records as a moodle_recordset which match a particular WHERE clause.

o $DB->get_recordset_sql($sql, array $params=null, $limitfrom=0, $limitnum=0); 

 /// Get a number of records as a moodle_recordset using a SQL statement.

o $DB->get_recordset_list($table, $field=, $values=, $sort=, $fields='*', $limitfrom=, $limitnum=) 

 /// Get a number of records as a moodle_recordset where one field matches one list of values.

Delegated transactions

  • Please note some databases do not support transactions (such as the MyISAM MySQL database engine), however all server administrators are strongly encouraged to migrate to databases that support transactions (such as the InnoDB MySQL database engine).
  • Previous versions supported only one level of transaction. Since Moodle 2.0, the DML layer emulates delegated transactions that allow nesting of transactions.
  • Transactions should not be used much in Moodle core; they are intended for various plugins such as web services, enrol and auth plugins.
  • Some subsystems (such as messaging) do not support transactions because is it is not possible to rollback in external systems.

A transaction is started by: $transaction = $DB->start_delegated_transaction();

and finished by: $transaction->allow_commit();

Usually a transaction is rolled back when an exception is thrown. $transaction->rollback($ex); must be used very carefully because it might break compatibility with databases that do not support transactions. Transactions cannot be used as part of expected code flow; they can be used only as an emergency protection of data consistency.

See more details in Development:DB layer 2.0 delegated transactions or MDL-20625.

Example(s)

global $DB; try { 

    $transaction = $DB->start_delegated_transaction();
    // Insert a record
    $DB->insert('foo', $object);
    $DB->insert('bar', $otherobject);

    // Assuming the both inserts work, we get to the following line.
    $transaction->allow_commit();

} catch(Exception $e) { 

    $transaction->rollback($e);

}

Helper Functions

In order have real cross-db compatibility, there are some helper functions used to build SQL fragments based on the DB Moodle is running. Using them we'll avoid conditional queries here and there and have those "incompatibilities" fixed once and for ever. o $DB->sql_bitand($int1, $int2) 

 /// Returns the SQL text to be used in order to perform one bitwise AND 
 /// operation between 2 integers.

o $DB->sql_bitnot($int1) 

 /// Returns the SQL text to be used in order to perform one bitwise NOT 
 /// operation with 1 integer.

o $DB->sql_bitor($int1, $int2) 

 /// Returns the SQL text to be used in order to perform one bitwise OR 
 /// operation between 2 integers.

o $DB->sql_bitxor($int1, $int2) 

 /// Returns the SQL text to be used in order to perform one bitwise XOR 
 /// operation between 2 integers.

o $DB->sql_null_from_clause() 

 /// Returns the FROM clause required by some DBs in all SELECT statements.

o $DB->sql_ceil($fieldname) 

 /// Returns the correct CEIL expression applied to fieldname.

o $DB->sql_ilike() 

 /// Returns the proper SQL to do LIKE in a case-insensitive way.

o $DB->sql_length($fieldname) 

 /// Returns the SQL text to be used to calculate the length in characters of one expression.

o $DB->sql_modulo($int1, $int2) 

 /// Returns the SQL text to be used in order to calculate module - remainder after division

o $DB->sql_position($needle, $haystack) 

 /// Returns the SQL for returning searching one string for the location of another.
 /// Note: If using placeholders BOTH in $needle and $haystack, they MUST be named placeholders.

o $DB->sql_substr($expr, $start, $length=false) 

 /// Returns the proper substr() SQL text used to extract substrings from DB.
 /// Note: This fuction has changed in Moodle 2.0 and now at least 2 params are mandatory.
 /// Note: Now it returns the whole SQL text to be used instead of only the function name.

o $DB->sql_cast_char2int($fieldname, $text=false) 

 /// Returns the SQL to be used in order to CAST one CHAR column to INTEGER.

o $DB->sql_cast_char2real($fieldname, $text=false) 

 /// Returns the SQL to be used in order to CAST one CHAR column to REAL number.

o $DB->sql_compare_text($fieldname, $numchars=32) 

 /// Returns the SQL text to be used to compare one TEXT (clob) column.
 /// with one VARCHAR column.

o $DB->sql_order_by_text($fieldname, $numchars=32) 

 /// Returns the SQL text to be used to order by one TEXT (clob) column.

o $DB->sql_concat() 

 /// Returns the proper SQL to do CONCAT between the elements passed.

o $DB->sql_concat_join($separator="' '", $elements=array()) 

 /// Returns the proper SQL to do CONCAT between the elements passed using one separator.

o $DB->sql_fullname($first='firstname', $last='lastname') 

 /// Returns the proper SQL to concatenate $firstname and $lastname.

o $DB->sql_isempty($tablename, $fieldname, $nullablefield, $textfield) 

 /// Returns the proper SQL to know if one field is empty.

o $DB->sql_isnotempty($tablename, $fieldname, $nullablefield, $textfield) 

 /// Returns the proper SQL to know if one field is not empty.

o $DB->sql_empty() 

 /// Returns the empty string char used by every supported DB.

See also

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