MySQL 8.0

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CREATE TABLE Table_2 
(friendship_id INT,user_id INT,user_id_request INT,friend_status INT);
INSERT INTO Table_2 VALUES
(1,16,26,0),
(2,16,25,0),
(3,16,27,1),
(4,21,27,1);

CREATE TABLE Table_1 (user_id INT, signupdate DATE);

INSERT INTO Table_1 VALUES
(16,'2020-11-30'),
(23,'2020-11-30'),(24,'2020-11-30'),
(25,'2020-11-30'),(26,'2020-11-30'),
(27,'2020-11-30'),(28,'2020-11-30');
SELECT *,COALESCE(t2.user_id,t1.user_id) AS user_id,
      t1.signupdate,
      CASE 
           WHEN t2.user_id='16'/*($_SESSION[ 'SESS_USER_ID' ])*/ THEN 0
           WHEN t1.user_id='16'/*($_SESSION[ 'SESS_USER_ID' ])*/ THEN 0
           ELSE 1 END AS Chk
FROM Table_1 AS t1 LEFT JOIN Table_2 AS t2 
ON t1.user_id = t2.user_id;


SELECT *,COALESCE(t2.user_id,t1.user_id) AS user_id,
      t1.signupdate,
      CASE 
           WHEN t2.user_id='16'/*($_SESSION[ 'SESS_USER_ID' ])*/ THEN 0
           WHEN t1.user_id='16'/*($_SESSION[ 'SESS_USER_ID' ])*/ THEN 0
           ELSE 1 END AS Chk
FROM Table_1 AS t1 LEFT JOIN Table_2 AS t2 
ON t1.user_id = t2.user_id_request;





user_id
signupdate
friendship_id
user_id
user_id_request
friend_status
user_id
signupdate
Chk


16
2020-11-30
3
16
27
1
16
2020-11-30
0

16
2020-11-30
2
16
25
0
16
2020-11-30
0

16
2020-11-30
1
16
26
0
16
2020-11-30
0

23
2020-11-30
null
null
null
null
23
2020-11-30
1

24
2020-11-30
null
null
null
null
24
2020-11-30
1

25
2020-11-30
null
null
null
null
25
2020-11-30
1

26
2020-11-30
null
null
null
null
26
2020-11-30
1

27
2020-11-30
null
null
null
null
27
2020-11-30
1

28
2020-11-30
null
null
null
null
28
2020-11-30
1




user_id
signupdate
friendship_id
user_id
user_id_request
friend_status
user_id
signupdate
Chk


16
2020-11-30
null
null
null
null
16
2020-11-30
0

23
2020-11-30
null
null
null
null
23
2020-11-30
1

24
2020-11-30
null
null
null
null
24
2020-11-30
1

25
2020-11-30
2
16
25
0
16
2020-11-30
0

26
2020-11-30
1
16
26
0
16
2020-11-30
0

27
2020-11-30
4
21
27
1
21
2020-11-30
1

27
2020-11-30
3
16
27
1
16
2020-11-30
0

28
2020-11-30
null
null
null
null
28
2020-11-30
1


SELECT *,COALESCE(t2.user_id,t1.user_id) AS user_id,
      t1.signupdate,
      CASE 
           WHEN t2.user_id='16'/*($_SESSION[ 'SESS_USER_ID' ])*/ THEN 0
           WHEN t1.user_id='16'/*($_SESSION[ 'SESS_USER_ID' ])*/ THEN 0
           ELSE 1 END AS Chk
FROM Table_1 AS t1 LEFT JOIN Table_2 AS t2 
ON t1.user_id = t2.user_id_request
HAVING Chk=1;


user_id
signupdate
friendship_id
user_id
user_id_request
friend_status
user_id
signupdate
Chk


23
2020-11-30
null
null
null
null
23
2020-11-30
1

24
2020-11-30
null
null
null
null
24
2020-11-30
1

27
2020-11-30
4
21
27
1
21
2020-11-30
1

28
2020-11-30
null
null
null
null
28
2020-11-30
1


SELECT COALESCE(t2.user_id,t1.user_id) AS user_id,
      t1.signupdate,
      CASE 
           WHEN t2.user_id='16'/*($_SESSION[ 'SESS_USER_ID' ])*/ THEN 0
           WHEN t1.user_id='16'/*($_SESSION[ 'SESS_USER_ID' ])*/ THEN 0
           ELSE 1 END AS Chk
FROM Table_1 AS t1 LEFT JOIN Table_2 AS t2 
ON t1.user_id = t2.user_id_request
HAVING Chk=1;


user_id
signupdate
Chk


23
2020-11-30
1

24
2020-11-30
1

21
2020-11-30
1

28
2020-11-30
1

user_id	signupdate	friendship_id	user_id	user_id_request	friend_status	user_id	signupdate	Chk
16	2020-11-30	3	16	27	1	16	2020-11-30	0
16	2020-11-30	2	16	25	0	16	2020-11-30	0
16	2020-11-30	1	16	26	0	16	2020-11-30	0
23	2020-11-30	null	null	null	null	23	2020-11-30	1
24	2020-11-30	null	null	null	null	24	2020-11-30	1
25	2020-11-30	null	null	null	null	25	2020-11-30	1
26	2020-11-30	null	null	null	null	26	2020-11-30	1
27	2020-11-30	null	null	null	null	27	2020-11-30	1
28	2020-11-30	null	null	null	null	28	2020-11-30	1