Development:DML functions: Difference between revisions

Revision as of 19:20, 20 October 2010

Template:Moodle 2.0In this page you'll access to the available functions under Moodle to be able to access to DB data. You should use exclusively these functions in order to retrieve or modify DB contents because these functions provide an high level of abstraction and guarantee that your DB manipulation will work against different RDBMS.

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 info to all 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.

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

 /// 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 $DB->get_records_select($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=, $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.

Updating Records

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

 /// Update a record in a table.

Using Recordsets

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

Only one thing is absolutely important. Don't forget to close the record sets after using them! (that will freed a lot of resources in the RDBMS).

Here it's 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 match one list of values.

Delegated transactions

Please note some databases do not support transactions (such as MyISAM MySQL database engine), however all server administrators are strongly encouraged to migrate to databases that support transactions.
Previous version supported only one level of transaction. Since Moodle 2.0 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, the reason is it is not possible to rollback in external systems.

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

and finished by $transaction->allow_commit();

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

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

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, having 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.

@@ Line 9: / Line 9: @@
 * 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:
 <code php>global $DB;</code>
-* All the $table parameters in the functions are mean to be the table name '''without'' prefixes.
+* All the $table parameters in the functions are meant to be the table name '''without'' prefixes.
 <code php>$user = $DB->get_record('user', array('id'=>'1');</code>
 * When using the xxx_sql() functions, table names must be enclosed between curly braces.

Documentation