how to compare two table in sql

DECLARE @time1 as datetime;

SELECT TOP 1 @time1 = time FROM table1 ORDER BY ID DESC;

IF ((SELECT COUNT(*) FROM table2 WHERE time = @time1) = 0)
BEGIN
	INSERT INTO table2 VALUES (@time1);
END
ELSE
BEGIN
  UPDATE table2 SET time = time1 WHERE time = @time1;
END

SUBSTRING('19:09:00',1,5)

MERGE Table2 USING Table1
ON LEFT(Table2.Time,LEN(Table2.[Time])-3) = LEFT(Table1.[Time],LEN(Table1.[Time])-3)
WHEN MATCHED THEN UPDATE SET Table2.[Time] = Table1.[Time]
WHEN NOT MATCHED THEN INSERT([Time])
VALUES(Table1.[Time])