materials 
  mat_id int(11)
  mat_name varchar(512)
  mat_tch_test_folder_id varchar(1024)
  mat_tch_test_type varchar(3)
  mat_tch_test_name varchar(1024)
  
mat_details (
  mthd_id int(11)
  mthd_file_id varchar(255)
  mthd_mat_id int(11) - это mat_id из родительской записи
  mthd_test int(11)   - уровень теста, может быть от 1 до 5

  SELECT mat_tch_test_type
  FROM materials
    JOIN mat_details ON mat_id = mthd_mat_id
  WHERE mthd_mat_id = mat_id AND mthd_test = 1 LIMIT 1;

  SELECT mat_tch_test_type 
  FROM mat_details
    JOIN materials ON mthd_mat_id = mat_id
  WHERE mthd_mat_id = mat_id AND mthd_test = 1 LIMIT 1;

FUNCTION is_tch_mat_test_exists(matID int)
  RETURNS varchar(3) CHARSET utf8
BEGIN

  DECLARE Test varchar(3) DEFAULT '';

  SELECT
    mat_tch_test_type INTO Test
  FROM materials
    JOIN mat_tch_details ON mat_id = mthd_mat_id
  WHERE mthd_mat_id = matID
  AND mthd_test = 1 
  ORDER BY mthd_id LIMIT 1;

  RETURN Test;

END

SELECT mat_id, mat_name, mat_tch_test_name, is_tch_mat_test_exists(mat_id)
FROM materials;