Postgres 16

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

--Create two identically strctured tables
CREATE TABLE Table1(PK BIGINT NOT NULL
					, Col1 BIGINT
					, Col2 BIGINT
					)
;

CREATE TABLE Table2(PK BIGINT NOT NULL
					, Col1 BIGINT
					, Col2 BIGINT
					)
;

--insert some data into them
INSERT INTO Table1(PK
				, Col1
				, Col2 
				)
VALUES
	(1, 100, 1000) --Pk In both Table1 and Table2, and contents match
	,(2, 101, 1001) --Pk In both Table1 and Table2, and contents do not match
	,(3, 102, 1002) --Pk only in Table1
;

INSERT INTO Table2(PK
				, Col1
				, Col2 
				)
VALUES
	(1, 100, 1000) --Pk In both Table1 and Table2, and contents match
	,(2, 0, 0) --Pk In both Table1 and Table2, and contents do not match
	,(4, 103, 1003) --Pk only in Table2
;

--Compare them
SELECT CASE 
		WHEN t1.PK IS NULL --This assumes that PK being NULL is due to the JOIN only!
			THEN 'In Table2, and not in Table1'
		WHEN t2.pk IS NULL --This assumes that PK being NULL is due to the JOIN only!
			THEN 'In Table1, and not in Table2'
		ELSE 'In both tables, but contents differ'
	END AS ComparisonDescription
	, t1.*
	, t2.*
FROM Table1 t1
FULL JOIN Table2 t2
	ON t1.pk = t2.pk
WHERE NOT EXISTS(
	SELECT t1.* --IF not identically structured, select the common columns here
	INTERSECT
	SELECT t2.* --IF not identically structured, select the common columns here
	)
;

CREATE TABLE

CREATE TABLE

INSERT 0 3

INSERT 0 3

comparisondescription	pk	col1	col2	pk	col1	col2
In both tables, but contents differ	2	101	1001	2	0	0
In Table1, and not in Table2	3	102	1002	null	null	null
In Table2, and not in Table1	null	null	null	4	103	1003

SELECT 3