SQL Server 2017

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CREATE TABLE base (
  id INT
);
CREATE TABLE A (
  id INT,
  base_id INT
);
CREATE TABLE B (
  id INT,
  base_id INT
);
INSERT INTO base VALUES
  (1), (2), (3), (4), (5);
INSERT INTO A VALUES
  (1, 2), (2, 4), (3, 6);
INSERT INTO B VALUES
  (1, 2), (1, 3);
10 rows affected

SELECT base.*, A.id, B.id, 
  CAST(CASE WHEN [A].id IS NULL THEN 0 ELSE 1 END AS BIT) AS isA,
  CAST(CASE WHEN [B].id IS NULL THEN 0 ELSE 1 END AS BIT) AS isB
FROM [base] LEFT JOIN [A] ON [base].id = [A].base_id 
  LEFT JOIN B ON [base].id = [B].base_id


id
id
id
isA
isB


1
null
null
False
False

2
1
1
True
True

3
null
1
False
True

4
2
null
True
False

5
null
null
False
False

id	id	id	isA	isB
1	null	null	False	False
2	1	1	True	True
3	null	1	False	True
4	2	null	True	False
5	null	null	False	False