CREATE TABLE test_dates (
    date DATE
);

insert into test_dates values('2018-06-28'),('2018-06-21'),('2014-05-19'),('2018-05-02');

SELECT 
    date,
    CASE
        WHEN date BETWEEN DATE_ADD(NOW(), INTERVAL - 30 DAY) AND NOW() THEN 'Last Month'
        WHEN date BETWEEN DATE_ADD(NOW(), INTERVAL - 90 DAY) AND NOW() THEN 'Last 3 Months'
        WHEN date < DATE_ADD(NOW(), INTERVAL - 30 DAY) THEN 'Over 3 Months'
    END AS DateTag
FROM
    test_dates;

你会使用 case 表达式：

select date,
       (case when date <= curdate() and date > curdate() - interval 1 month
             then 'within 1 month'
             when date <= curdate() - interval 1 month and date > curdate() - interval 2 month
             then '2-3 months ago'
             when date <= curdate() - interval 3 month
             then '3+ months ago'
        end)
from t;