Join on multiple tables using distinct on - postgresql

Question

create table emp
(
emp_id serial primary key,
emp_no integer,
emp_ref_no character varying(15),
emp_class character varying(15)
);
create table emp_detail
(
emp_detail_id serial primary key,
emp_id integer,
class_no integer,
created_at timestamp without time zone,
constraint con_fk foreign key(emp_id) references emp(emp_id)
);
create table class_detail
(
class_id serial primary key,
emp_id integer,
class_no integer,
col1 JSONB,
created_at timestamp without time zone default now(),
constraint cd_fk foreign key(emp_id) references emp(emp_id)
);
INSERT INTO emp(
emp_no, emp_ref_no, emp_class)
VALUES ('548251', '2QcW', 'abc' );
INSERT INTO emp(
emp_no, emp_ref_no, emp_class)
VALUES ('548251', '2FQx', 'abc');
INSERT INTO emp(
emp_no, emp_ref_no, emp_class)
VALUES ('548251', '2yz', 'abc');
INSERT INTO emp_detail(
emp_id, class_no, created_at
)
VALUES ( 1, 2, '2018-05-04 11:00:00'
);
INSERT INTO emp_detail(
emp_id, class_no, created_at
)
VALUES ( 1, 1, '2018-04-04 11:00:00'
);
INSERT INTO emp_detail(
emp_id, class_no, created_at
)
VALUES ( 2, 1, '2018-05-10 11:00:00'
);
INSERT INTO emp_detail(
emp_id, class_no, created_at
)
VALUES ( 2, 2, '2018-02-01 11:00:00'
);
INSERT INTO emp_detail(
emp_id, class_no, created_at
)
VALUES ( 3, 2, '2018-02-01 11:00:00'
);
insert into class_detail(emp_id, class_no, col1, created_at) values(1,1,'{"Name":"Nik"}', '2018-02-01 10:00:00');
insert into class_detail(emp_id, class_no, col1, created_at) values(1,1,'{"Name":"Nik Anderson"}', '2018-03-01 10:00:00');
insert into class_detail(emp_id, class_no, col1, created_at) values(1,2,'{"Name":"James Anderson TST"}', '2018-03-15 10:00:00');
insert into class_detail(emp_id, class_no, col1, created_at) values(1,2,'{"Name":"Tim Paine ST"}', '2018-04-01 10:00:00');
I want to display corresponding emp_id, emp_no, emp_ref_no, class_no(the latest one from emp_detail table based on created at)along with all the columns of class_detail table. Class_detail table should show the latest corresponding record of the class no
The expected output which I would like to see is something like below :-
emp id | emp_no | emp_ref_no | class_no | class_id | class.col1 | class.created_at | class.created_by
1 | 548251 | 2QcW | 2 | 4 |{"Name":"Tim Paine ST"}|2018-04-01 10:00:00| NUlL
2 | 548251 | 2FQx | 1 | 2 |{"Name":"Nik Anderson"}|2018-03-01 10:00:00| NULL
3 | 548251 | 2yz | 2 | 4 |{"Name":"Tim Paine ST"}|2018-04-01 10:00:00| NULL

Answer 1

As I stated in the comments: It is exactly the same thing as in Inner join using distinct on. You simply have to add another join and another ORDER BY group (cd.created_at DESC)
demo:db<>fiddle
SELECT DISTINCT ON (ed.emp_id)
e.emp_id, e.emp_no, e.emp_ref_no, ed.class_no, cd.*
FROM
emp_detail ed
JOIN emp e ON e.emp_id = ed.emp_id
JOIN class_detail cd ON ed.class_no = cd.class_no
ORDER BY ed.emp_id, ed.created_at DESC, cd.created_at DESC
Note: I am not sure what the emp_id column in class_detail is for. It seems not well designed (this is also because it is always 1 in your example.) You should check whether you really need it.