Mysql Inner Join a table with matching key column but null - mysql

Question

I am trying to join 4 tables.
registration_mt, admission_mt, student_mt, and schoolyear_student_lt
Currently registration_mt, admission_mt and student_mt tables have 1 record each.
There are no problems inner-joining all three of them except for schoolyear_student_lt which has no records yet.
I want to be able to get the columns of schoolyear_student_lt and join it with the result set of the 3 other tables even when the matching key student_id HAS OR HAS-NO records
I want to add schoolyear_id, student_id, gradelevel_id, section_id, passedfrom schoolyear_student_lt to the result set I get from my join query where isActive = 0;
CREATE table statements
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
);
My INNER-JOIN statement
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 ;
INSERT statements to registration_mt, admission_mt, and student_mt tables
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;
I commented out the -- INNER JOIN schoolyear_student_lt sslt ON s.student_id = sslt.student_id because I get no results when I include it in my select query since there are no records yet in schoolyear_student_lt table.
This is the current resultset I get. There are no columns from schoolyear_student_lt table which I wish to add even when NULL and even when NOT NULL on schoolyear_student_lt.student_id
LEFT-JOIN doesn't display the columns from schoolyear_student_lt when NULL
Thanks.

Answer 1

You should try the LEFT OUTER JOIN
The idea of an left outer join is that it takes the rows of left relation and fills the fields of the right relation with NULL if there is no join
partner.
There is also a right outer join...