问Mysql内连接具有匹配键列但为null的表

EN

CREATE TABLE `registration_mt` (
  `registration_id` int(11) NOT NULL AUTO_INCREMENT,
  `student_type` varchar(45) NOT NULL,
  PRIMARY KEY (`registration_id`)
) ;

CREATE TABLE `admission_mt` (
  `admission_id` int(11) NOT NULL AUTO_INCREMENT,
  `registration_id` int(11) NOT NULL,
  `isComplete` bit(1) NOT NULL DEFAULT b'0',
  `completion_date` datetime DEFAULT NULL,
  PRIMARY KEY (`admission_id`),
  UNIQUE KEY `registration_id_UNIQUE` (`registration_id`),
  CONSTRAINT `fk_admission_mtTABLE_registration_idCOL` FOREIGN KEY (`registration_id`) REFERENCES `registration_mt` (`registration_id`) ON DELETE NO ACTION ON UPDATE NO ACTION
) ;

CREATE TABLE `schoolyear_student_lt` (
  `schoolyear_id` int(11) NOT NULL,
  `student_id` int(11) NOT NULL,
  `gradelevel_id` int(11) NOT NULL,
  `section_id` int(11) DEFAULT NULL,
  `passed` bit(1) DEFAULT b'0',
  UNIQUE KEY `uk_schoolyear_idCOL_student_idCOL` (`schoolyear_id`,`student_id`)
);

CREATE TABLE `student_mt` (
  `student_id` int(11) NOT NULL AUTO_INCREMENT,
  `registration_id` int(11) NOT NULL,
  `entry_date` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP,
  `isGraduated` bit(1) NOT NULL DEFAULT b'0',
  `date_graduated` datetime DEFAULT NULL,
  `isActive` bit(1) DEFAULT b'0' ,
  PRIMARY KEY (`student_id`),
  UNIQUE KEY `registration_id_UNIQUE` (`registration_id`),
  KEY `fk_student_mtTABLE_registration_idCOL_idx` (`registration_id`),
  CONSTRAINT `fk_student_mtTABLE_registration_idCOL` FOREIGN KEY (`registration_id`) REFERENCES `registration_mt` (`registration_id`) ON DELETE NO ACTION ON UPDATE NO ACTION
);

SELECT 

a.admission_id, 
a.isComplete, 
a.completion_date,
s.student_id,
s.entry_date,
s.isGraduated, 
s.date_graduated, 
s.isActive

FROM admission_mt a 
INNER JOIN registration_mt r ON a.registration_id = r.registration_id
INNER JOIN student_mt s ON s.registration_id = a.registration_id
-- INNER JOIN schoolyear_student_lt sslt ON s.student_id = sslt.student_id
-- LEFT JOIN schoolyear_student_lt sslt ON s.student_id = sslt.student_id 


 WHERE 

 s.isActive = 0 ;

START TRANSACTION;

INSERT INTO registration_mt(student_type) VALUES('New'); -- insert to registration_mt

INSERT INTO admission_mt(registration_id)
VALUES(LAST_INSERT_ID()); --insert to admissiont_mt

COMMIT;

START TRANSACTION;

UPDATE admission_mt 
SET isComplete = 1 
WHERE registration_id = 1;

INSERT INTO student_mt(registration_id)
VALUES(1); --insert to student_mt

COMMIT;