lib/dml: new compatibility method sql_cast_2signed for when MySQL thinks that 1 * -1 = 18446744073709551615

lib/dml/moodle_database.php

@@ -1518,6 +1518,19 @@ abstract class moodle_database {
         return ' ' . $fieldname . ' ';
     }
 
+    /**
+     * Returns the SQL to be used in order to an UNSIGNED INTEGER column to SIGNED.
+     *
+     * (Only MySQL needs this. MySQL things that 1 * -1 = 18446744073709551615
+     * if the 1 comes from an unsigned column).
+     *
+     * @param string fieldname the name of the field to be cast
+     * @return string the piece of SQL code to be used in your statement.
+     */
+    public function sql_cast_2signed($fieldname) {
+        return ' ' . $fieldname . ' ';
+    }
+
     /**
      * Returns the SQL text to be used to compare one TEXT (clob) column with
      * one varchar column, because some RDBMS doesn't support such direct

lib/dml/mysqli_adodb_moodle_database.php

@@ -291,6 +291,10 @@ class mysqli_adodb_moodle_database extends adodb_moodle_database {
         return $positivematch ? 'REGEXP' : 'NOT REGEXP';
     }
 
+    public function sql_cast_2signed($fieldname) {
+        return ' CAST(' . $fieldname . ' AS SIGNED) ';
+    }
+
     /**
      * Import a record into a table, id field is required.
      * Basic safety checks only. Lobs are supported.

lib/dml/mysqli_native_moodle_database.php

@@ -905,6 +905,10 @@ class mysqli_native_moodle_database extends moodle_database {
         return $positivematch ? 'REGEXP' : 'NOT REGEXP';
     }
 
+    public function sql_cast_2signed($fieldname) {
+        return ' CAST(' . $fieldname . ' AS SIGNED) ';
+    }
+
 /// session locking
     public function session_lock_supported() {
         return true;