Postgres 14

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CREATE TABLE t1
AS
SELECT 1 AS id, 'a' AS c
UNION ALL
SELECT 2 , 'b'
UNION ALL
SELECT NULL, 'c';
3 rows affected

CREATE TABLE t2
AS
SELECT 2 AS id
UNION
SELECT 3 
UNION 
SELECT NULL
3 rows affected

SELECT * FROM T1;
SELECT * FROM T2


id
c


1
a

2
b

null
c




id


2

3

null


SELECT *
FROM T1
JOIN T2
  ON T1.ID = T2.ID


id
c
id


2
b
2


SELECT *
FROM T1
JOIN T2
  ON T1.ID IS NOT DISTINCT FROM T2.ID


id
c
id


2
b
2

null
c
null

id	c
1	a
2	b
null	c

id
2
3
null

id	c	id
2	b	2
null	c	null