Data manipulation API: Difference between revisions

Revision as of 09:32, 18 September 2020

Moodle 2.0

This 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!

General concepts

DB object

The data manipulation API is exposed via public methods of the $DB object.
Moodle core takes care of setting up the connection to the database according to values specified in the main config.php file.
The $DB global object is an instance of the moodle_database class. It is instantiated automatically during the bootstrap setup, i.e. as a part of including the main config.php file.
The DB object is available in the global scope right after including the config.php file:

<?php

require(__DIR__.'/../../../config.php');

// You can access the database via the $DB method calls here.

To make the DB object available in your local scope, such as within a function:

<?php

defined('MOODLE_INTERNAL') || die();

function my_function_making_use_of_database() {

   global $DB;

   // You can access the database via the $DB method calls here.

}

Table prefix

Most Moodle installations use a prefix for all the database tables, such as mdl_. This prefix is NOT to be used in the code in the code itself.
All the $table parameters in the functions are meant to be the table name without prefixes:

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

In custom SQL queries, table names must be enclosed between curly braces. They will be then automatically converted to the real prefixed table name. There is no need to access $CFG->prefix

$user = $DB->get_record_sql('SELECT COUNT(*) FROM {user} WHERE deleted = 1 OR suspended = 1;');

Conditions

All the $conditions parameters in the functions are arrays of fieldname=>fieldvalue elements.
They all must be fulfilled - i.e. logical AND is used to populate the actual WHERE statement.

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

Placeholders

All the $params parameters in the functions are arrays of values used to fill placeholders in SQL statements.
Placeholders help to avoid problems with SQL-injection and/or invalid quotes in SQL queries. They facilitate secure and cross-db compatible code.
Two types of placeholders are supported - question marks (SQL_PARAMS_QM) and named placeholders (SQL_PARAMS_NAMED).
Named params must be unique even if the value passed is the same. If you need to pass the same value multiple times, you need to have multiple distinct named parameters.

// Example of using question mark placeholders. $DB->get_record_sql('SELECT * FROM {user} WHERE firstname = ? AND lastname = ?',

   ['Martin', 'Dougiamas']);

// Example of using named placeholders.
$DB->get_record_sql('SELECT * FROM {user} WHERE firstname = :firstname AND lastname = :lastname',

   ['firstname' => 'Martin', 'lastname' => 'Dougiamas']);

Strictness

Some methods accept the $strictness parameter affecting the method behaviour. Supported modes are specified using the constants:

MUST_EXIST - In this mode, the requested record must exist and must be unique. An exception will be thrown if no record is found or multiple matching records are found.
IGNORE_MISSING - In this mode, a missing record is not an error. False boolean is returned if the requested record is not found. If more records are found, a debugging message is displayed.
IGNORE_MULTIPLE - This is not a recommended mode. The function will silently ignore multiple records found and will return just the first one of them.

Getting a single record

get_record

Return a single database record as an object where all the given conditions are met. $DB->get_record($table, array $conditions, $fields='*', $strictness=IGNORE_MISSING)

get_record_select

Return a single database record as an object where the given conditions are used in the WHERE clause. $DB->get_record_select($table, $select, array $params=null, $fields='*', $strictness=IGNORE_MISSING)

get_record_sql

Return a single database record as an object using a custom SELECT query. $DB->get_record_sql($sql, array $params=null, $strictness=IGNORE_MISSING)

Getting a hashed array of records

Each of the following methods return an array of objects. The array is indexed by the first column of the fields returned by the query. To assure consistency, it is a good practice to ensure that your query include an "id column" as the first field. When designing custom tables, make id their first column and primary key.

get_records

Return a list of records as an array of objects where all the given conditions are met. $DB->get_records($table, array $conditions=null, $sort=, $fields='*', $limitfrom=0, $limitnum=0)

get_records_select

Return a list of records as an array of objects where the given conditions are used in the WHERE clause. $DB->get_records_select($table, $select, array $params=null, $sort=, $fields='*', $limitfrom=0, $limitnum=0) $fields is a comma separated list of fields to return (optional, by default all fields are returned).

get_records_sql

Return a list of records as an array of objects using a custom SELECT query. $DB->get_records_sql($sql, array $params=null, $limitfrom=0, $limitnum=0)

get_records_list

Return a list of records as an array of objects where the given field matches one of the possible values. $DB->get_records_list($table, $field, array $values, $sort=, $fields='*', $limitfrom=, $limitnum=)

Getting data as key/value pairs in an associative array

get_records_menu

Return the first two columns from a list of records as an associative array where all the given conditions are met. $DB->get_records_menu($table, array $conditions=null, $sort=, $fields='*', $limitfrom=0, $limitnum=0)

get_records_select_menu

Return the first two columns from a list of records as an associative array where the given conditions are used in the WHERE clause. $DB->get_records_select_menu($table, $select, array $params=null, $sort=, $fields='*', $limitfrom=0, $limitnum=0)

get_records_sql_menu

Return the first two columns from a number of records as an associative array using a custom SELECT query. $DB->get_records_sql_menu($sql, array $params=null, $limitfrom=0, $limitnum=0)

Counting records that match the given criteria

count_records

Count the records in a table where all the given conditions are met. $DB->count_records($table, array $conditions=null)

count_records_select

Count the records in a table where the given conditions are used in the WHERE clause. $DB->count_records_select($table, $select, array $params=null, $countitem="COUNT('x')")

count_records_sql

Counting the records using a custom SELECT COUNT(...) query. $DB->count_records_sql($sql, array $params=null)

Checking if a given record exists

record_exists

Test whether a record exists in a table where all the given conditions are met. $DB->record_exists($table, array $conditions=null)

record_exists_select

Test whether any records exists in a table where the given conditions are used in the WHERE clause. $DB->record_exists_select($table, $select, array $params=null)

record_exists_sql

Test whether the given SELECT query would return any record. $DB->record_exists_sql($sql, array $params=null)

Getting a particular field value from one record

get_field

Get a single field value from a table record where all the given conditions are met. $DB->get_field($table, $return, array $conditions, $strictness=IGNORE_MISSING)

get_field_select

Get a single field value from a table record where the given conditions are used in the WHERE clause. $DB->get_field_select($table, $return, $select, array $params=null, $strictness=IGNORE_MISSING)

get_field_sql

Get a single field value (first field) using a custom SELECT query. $DB->get_field_sql($sql, array $params=null, $strictness=IGNORE_MISSING)

Getting field values from multiple records

get_fieldset_select

Return values of the given field as an array where the given conditions are used in the WHERE clause. $DB->get_fieldset_select($table, $return, $select, array $params=null)

get_fieldset_sql

Return values of the first column as an array using a custom SELECT field FROM ... query. $DB->get_fieldset_sql($sql, array $params=null)

Setting a field value

set_field

Set a single field in every record where all the given conditions are met. $DB->set_field($table, $newfield, $newvalue, array $conditions=null)

set_field_select

Set a single field in every table record where the given conditions are used in the WHERE clause. $DB->set_field_select($table, $newfield, $newvalue, $select, array $params=null)

Deleting records

delete_records

Delete records from the table where all the given conditions are met. $DB->delete_records($table, array $conditions=null)

delete_records_select

Delete records from the table where the given conditions are used in the WHERE clause. $DB->delete_records_select($table, $select, array $params=null)

Inserting records

insert_record

Insert the given data object into the table and return the "id" of the newly created record. $DB->insert_record($table, $dataobject, $returnid=true, $bulk=false)

insert_records

Moodle 2.7 Insert multiple records into the table as fast as possible. Records are inserted in the given order, but the operation is not atomic. Use transactions if necessary. $DB->insert_records($table, $dataobjects)

insert_record_raw

For rare cases when you also need to specify the ID of the record to be inserted.

Updating records

update_record

Update a record in the table. The data object must have the property "id" set. $DB->update_record($table, $dataobject, $bulk=false)

Executing a custom query

execute

If you need to perform a complex update using arbitrary SQL, you can use the low level "execute" method. Only use this when no specialised method exists.
Do NOT use this to make changes in database structure, use database_manager methods instead!

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

Using recordsets

If 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 into the memory via the returned array. Under those circumstances, it is highly recommended to use these get_recordset_xxx() functions instead. They return an iterator to iterate over all the found records and save a lot of memory.

It is absolutely important to not forget to close the returned recordset iterator after using it. This is to free up a lot of resources in the RDBMS.

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

$rs = $DB->get_recordset(....) { foreach ($rs as $record) {

   // Do whatever you want with this record

} $rs->close();

Unlike get_record functions, you cannot check if $rs == true or !empty($rs) to determine if any records were found. Instead, if you need to, you can use:

if ($rs->valid()) {

   // The recordset contains some records.

}

get_recordset

Return a list of records as a moodle_recordset where all the given conditions are met. $DB->get_recordset($table, array $conditions=null, $sort=, $fields='*', $limitfrom=0, $limitnum=0)

get_recordset_select

Return a list of records as a moodle_recordset where the given conditions are used in the WHERE clause. $DB->get_recordset_select($table, $select, array $params=null, $sort=, $fields='*', $limitfrom=0, $limitnum=0)

get_recordset_sql

Return a list of records as a moodle_recordset using a custom SELECT query. $DB->get_recordset_sql($sql, array $params=null, $limitfrom=0, $limitnum=0)

get_recordset_list

Return a list of records as a moodle_recordset where the given field matches one of the possible values. $DB->get_recordset_list($table, $field, array $values, $sort=, $fields='*', $limitfrom=, $limitnum=)

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.
Some subsystems (such as messaging) do not support transactions because 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);

which 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 DB layer 2.0 delegated transactions or MDL-20625.

Example

global $DB; try {

    $transaction = $DB->start_delegated_transaction();
    $DB->insert_record('foo', $object);
    $DB->insert_record('bar', $otherobject);

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

} catch(Exception $e) {

    $transaction->rollback($e);

}

Cross-DB compatibility

Moodle supports several SQL servers (MySQL, PostgreSQL, MS-SQL and Oracle). Each of them have some specific syntax in certain cases. In order to achieve cross-db compatibility of the code, following functions must be used to generate the fragments of the query valid for the actual SQL server.

sql_bitand

Return the SQL text to be used in order to perform a bitwise AND operation between 2 integers. $DB->sql_bitand($int1, $int2)

sql_bitnot

Return the SQL text to be used in order to perform a bitwise NOT operation on the given integer. $DB->sql_bitnot($int1)

sql_bitor

Return the SQL text to be used in order to perform a bitwise OR operation between 2 integers. $DB->sql_bitor($int1, $int2)

sql_bitxor

Return the SQL text to be used in order to perform a bitwise XOR operation between 2 integers. $DB->sql_bitxor($int1, $int2)

sql_null_from_clause

Return an empty FROM clause required by some DBs in all SELECT statements. $DB->sql_null_from_clause()

sql_ceil

Return the correct CEIL expression applied to the given fieldname. $DB->sql_ceil($fieldname)

sql_equal

Moodle 3.2 Return the query fragment to perform cross-db varchar comparisons when case-sensitiveness is important. $DB->sql_equal($fieldname, $param, $casesensitive = true, $accentsensitive = true, $notequal = false)

sql_like

Return the query fragment to perform the LIKE comparison. $DB->sql_like($fieldname, $param, $casesensitive = true, $accentsensitive = true, $notlike = false, $escapechar = ' \\ ')

Example: Searching for records partially matching the given hard-coded literal. $DB->get_records_sql('SELECT id,fullname FROM {course} WHERE '.$DB->sql_like('idnumber', ':idnum'), ['idnum' => 'DEMO-%')); See below if you need to compare with a value submitted by the user.

sql_like_escape

Escape the value submitted by the user so that it can be used for partial comparison and the special characters like '_' or '%' behave as literal characters, not wildcards. $DB->sql_like_escape($text, $escapechar = '\\')

Example: If you need to perform a partial comparison with a value that has been submitted by the user. $search = required_param('search', PARAM_RAW); $DB->get_records_sql('SELECT id,fullname FROM {course} WHERE '.$DB->sql_like('fullname', ':fullname'), ['fullname' => '%'.$DB->sql_like_escape($search).'%']);

sql_length

Return the query fragment to be used to calculate the length of the expression in characters. $DB->sql_length($fieldname)

sql_modulo

Return the query fragment to be used to calculate the remainder after division. $DB->sql_modulo($int1, $int2)

sql_position

Return the query fragment for searching a string for the location of a substring. If both needle and haystack use placeholders, you must use named placeholders. $DB->sql_position($needle, $haystack)

sql_substr

Return the query fragment for extracting a substring from the given expression. $DB->sql_substr($expr, $start, $length=false)

sql_cast_char2int

Return the query fragment to cast a CHAR column to INTEGER $DB->sql_cast_char2int($fieldname, $text=false)

sql_cast_char2real

Return the query fragment to cast a CHAR column to REAL (float) number $DB->sql_cast_char2real($fieldname, $text=false)

sql_compare_text

Return the query fragment to be used when comparing a TEXT (clob) column with a given string or a VARCHAR field (some RDBMs do not allow for direct comparison). $DB->sql_compare_text($fieldname, $numchars=32)

Example: $todogroups = $DB->get_records_sql('SELECT id FROM {group} WHERE ' . $DB->sql_compare_text('description') . ' = ' . $DB->sql_compare_text(':description'), ['description' => 'TODO']);

sql_order_by_text

Return the query fragment to be used to get records ordered by a TEXT (clob) column. Note this affects the performance badly and should be avoided if possible. $DB->sql_order_by_text($fieldname, $numchars=32)

sql_concat

Return the query fragment to concatenate all given paremeters into one string. $DB->sql_concat(...)

sql_concat_join

Return the query fragment to concatenate all given elements into one string using the given separator. $DB->sql_concat_join($separator="' '", $elements=array())

sql_fullname

Return the query fragment to concatenate the given $firstname and $lastname $DB->sql_fullname($first='firstname', $last='lastname')

sql_isempty

Return the query fragment to check if the field is empty $DB->sql_isempty($tablename, $fieldname, $nullablefield, $textfield)

sql_isnotempty

Return the query fragment to check if the field is not empty $DB->sql_isnotempty($tablename, $fieldname, $nullablefield, $textfield)

get_in_or_equal

Return the query fragment to check if a value is IN the given list of items (with a fallback to plain equal comparison if there is just one item) $DB->get_in_or_equal($items, $type=SQL_PARAMS_QM, $prefix='param', $equal=true, $onemptyitems=false)

Example: $statuses = ['todo', 'open', 'inprogress', 'intesting']; list($insql, $inparams) = $DB->get_in_or_equal($statuses); $sql = "SELECT * FROM {bugtracker_issues} WHERE status $insql"; $bugs = $DB->get_records_sql($sql, $inparams);

An example using named params:

... list($insql, $params) = $DB->get_in_or_equal($contexts, SQL_PARAMS_NAMED, 'ctx'); $contextsql = "AND rc.contextid $insql"; ...

sql_regex_supported

Does the current database driver support regex syntax when searching? $DB->sql_regex_supported()

sql_regex

Return the query fragment to perform a regex search. $DB->sql_regex($positivematch = true, $casesensitive = false)

Example: Searching for Page module instances containing links. if ($DB->sql_regex_supported()) {

   $select = 'content ' . $DB->sql_regex() . ' :pattern';
   $params = ['pattern' => "(src|data)\ *=\ *[\\\"\']https?://"]

} else {

   $select = $DB->sql_like('content', ':pattern', false);
   $params = ['pattern' => '%=%http%://%'];

$pages = $DB->get_records_select('page', $select, $params, 'course', 'id, course, name');

sql_intersect

Moodle 2.8 Return the query fragment that allows to find intersection of two or more queries $DB->sql_intersect($selects, $fields)

Debugging

set_debug

You can enable a debugging mode to make $DB output the SQL of every executed query, along with some timing information. This can be useful when debugging your code. Obviously, all such calls should be removed before code is submitted for integration. $DB->set_debug(true)

Special cases

get_course

From Moodle 2.5.1 onwards, you should use the get_course function instead of using get_record('course', ...) if you want to get a course record based on its ID, especially if there is a significant possibility that the course being retrieved is either the current course for the page, or the site course. Those two course records have probably already been loaded, and using this function will save a database query.

Additionally, the code is shorter and easier to read.

get_courses

If you want to get all the current courses in your Moodle, use get_courses() without parameter:

   $courses = get_courses();

@@ Line 248: / Line 248: @@
 === get_recordset_sql ===
-Return a list of records as an array of objects using a custom SELECT query.
+Return a list of records as a moodle_recordset using a custom SELECT query.
 <code php>$DB->get_recordset_sql($sql, array $params=null, $limitfrom=0, $limitnum=0)</code>

Documentation