Sql查询从表中检索数据

只需测试最小日期就足够了

drop table if exists t;
create table t(c_id int,cr_id int,dt date);
insert into t values
(1,56,'2020-12-17'),(56,56,'2020-12-17'),
(1,8,'2020-12-17'),(56,8,'2020-12-17'),
(123,78,'2020-12-17'),(1,78,'2020-12-18');

select c_id,cr_id,dt
from t
where c_id = 1 and
        dt = (select min(dt) from t t1 where t1.cr_id = t.cr_id);

+------+-------+------------+
| c_id | cr_id | dt         |
+------+-------+------------+
|    1 |    56 | 2020-12-17 |
|    1 |     8 | 2020-12-17 |
+------+-------+------------+
2 rows in set (0.002 sec)