有没有一种方法可以在删除行之前检查MySql中的约束违规？

Question

I am writing a PHP application in which a user can select an item from a list for deletion, but I want to check the constraints BEFORE even attempting the delete. Then I can potentially highlight these items ahead of time alerting the user of the constraint. I could make a programatic list of all of the constraints and do the checks through PHP, but what if down the line, other constraints are added on the database side? I would then need to define these new constraints in the program.

Basically, is there a constraint check query that can be run without actually trying the delete and catching the error?

Answer 1

As far as I know, there's no "DELETE CHECK" command, per se.

My approach would be to scan the INFORMATION_SCHEMA tables in MySQL, where the foreign key relationships are recorded. From those, one could generate the SQL to check if a delete would violate any constraints.

Answer 2

Does anyone see any problem with a solution like this? I built this off of schtever's suggestion of using the information_schema data. I have added this method to my database class, and from some simple tests, it seems to work. It will tell me the first constraint that fails if the key passed to it is found in any of the database tables matching a constraint. If there is no key match in any of the tables, it returns false indicating no constraints.

/**
 * hasDeleteConstraints - This method uses the MySQL information_schema information to check for
 *                        constraint violations. If violations are found, the method returns the
 *                        constraint name and the table name of where matching data is found.
 *
 * @param string $table The database table being checked
 * @param string $field The key field to check
 * @param string $value The value to search for
 *
 * @return mixed - The matching constraint data if found, false if no constraints found
 */
public static function hasDeleteConstraints($table, $field, $value) {
    $sql = "SELECT KCU.CONSTRAINT_NAME, KCU.TABLE_NAME, KCU.COLUMN_NAME
            FROM information_schema.KEY_COLUMN_USAGE KCU
            LEFT JOIN information_schema.REFERENTIAL_CONSTRAINTS REFC USING(CONSTRAINT_NAME)
            WHERE KCU.REFERENCED_TABLE_NAME = " . self::quote($table) . "
              AND KCU.REFERENCED_COLUMN_NAME = " . self::quote($field) . "
              AND REFC.DELETE_RULE = 'RESTRICT'
            GROUP BY KCU.CONSTRAINT_NAME";
    $result = self::dbQueryAll($sql);

    if ($result) {
        foreach ($result as $row) {
            $sql = "SELECT {$row['COLUMN_NAME']}
                    FROM {$row['TABLE_NAME']}
                    WHERE {$row['COLUMN_NAME']} = " . self::quote($value);
            if (self::dbQueryRow($sql) !== false) {
                return $row;
            }
        }
    }
    return false;
} //End public static function hasDeleteConstraints