INSERT public.myTable (field1_value, field2_value, field3_value, isActive) 
SELECT DISTINCT
	'21',
	'Some Text',
	'',
	1 
FROM
	public.myTable MT WITH (NOLOCK) 
WHERE
	NOT EXISTS 
	(
		SELECT
			field1_value,
			field2_value,
			field3_value,
			isActive 
		FROM
			public.myTable 
		WHERE
			field1_value = '21' 
			AND field2_value = 'Some Text' 
			AND field3_value = '' 
			AND isActive = 1 
	)