NOT EXISTS

Example:

DECLARE @tab1 TABLE(number1 TINYINT);

INSERT INTO @tab1(number1)
VALUES(1), (2), (3);

SELECT * FROM @tab1;

DECLARE @tab2 TABLE(number2 TINYINT);

INSERT INTO @tab2(number2)
VALUES(3), (5), (6);

SELECT *
FROM @tab2;

INSERT INTO @tab1(number1)
    SELECT number2 FROM @tab2 WHERE NOT EXISTS
    (
        SELECT * FROM @tab1 WHERE number1 = number2
    );

SELECT * FROM @tab1;

SELECT * FROM @tab2;