这里使用CASE WHEN THEN最合适不过了！
SELECT (CASE WHEN RW >=3 THEN 3 WHEN RW >= 1 AND RW < 3 THEN 2 WHEN RW < 1 THEN 1 ELSE 0 END) FLAG FROM TABLE_NAME WHERE TO_CHAR(TJYF,'YYYY-MM-DD') >= '2020-01-01' GROUP BY (CASE WHEN RW >=3 THEN 3 WHEN RW >= 1 AND RW < 3 THEN 2 WHEN RW < 1 THEN 1 ELSE 0 END)
