SQL Server 2017

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CREATE TABLE test (
    answer NVARCHAR(255),
    c_num_value INT
);
INSERT INTO test VALUES
('9.037' ,	null),
('9'     ,	9),
(N'Текст',	null);
3 rows affected

UPDATE test 
SET c_num_value = TRY_CONVERT(DECIMAL(18, 4), answer)
WHERE c_num_value IS NULL;
2 rows affected

SELECT * 
FROM test;


answer
c_num_value


9.037
9

9
9

Текст
null