问如何使用MySQL连接四个表

EN

CREATE TABLE IF NOT EXISTS `omc_projects` (
  `id` int(10) NOT NULL AUTO_INCREMENT,
  ...
  PRIMARY KEY (`id`)
) ... ;

CREATE TABLE IF NOT EXISTS `omc_files` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `project_id` int(11) NOT NULL,
 ...
  PRIMARY KEY (`id`)
) ... ;


CREATE TABLE IF NOT EXISTS `omc_logs` (
  `id` int(10) NOT NULL AUTO_INCREMENT,
  `project_id` int(10) NOT NULL,
  ...
  PRIMARY KEY (`id`)
) ... ;


CREATE TABLE IF NOT EXISTS `omc_specs` (
  `id` int(10) NOT NULL AUTO_INCREMENT,
  `project_id` int(10) NOT NULL,
  ...
  ...
  PRIMARY KEY (`id`)
) ... ;

function checkproject($id){
// if there are any data in one of table, returns TRUE 

$query = 'SELECT omc_projects.id, omc_specs.id, omc_logs.id, omc_files.id
FROM omc_projects
JOIN omc_specs
         ON omc_specs.project_id = omc_projects.id
JOIN omc_logs
        ON omc_logs.project_id = omc_projects.id
JOIN omc_files
        ON omc_files.project_id = omc_projects.id
WHERE omc_projects.id = $id'

        $query = $this->db->get();
        if ($query->num_rows() > 0){              
                return TRUE;
        }else{
            return FALSE;
        }
        $query->free_result();

     }