创建示例表

/*B表列类型为VARCHAR*/
create table B(DATE1 VARCHAR(20),TIME1 VARCHAR(20),DATE2 VARCHAR(20),TIME2 VARCHAR(20));
insert into B VALUES('2021-01-01','09:12:12','2021-01-02','19:02:01');
insert into B VALUES('2021-01-02','12:12:12','2021-01-05','20:02:01');
insert into B VALUES('2021-01-02','11:12:12','2021-01-04','09:02:01');
insert into B VALUES('2021-01-03','15:12:12','2021-01-06','10:02:01');
insert into B VALUES('2021-01-03','15:12:12','2021-01-06','10:02:01');
insert into B VALUES('2021-01-04','15:00:12','2021-01-06','15:00:12');
insert into B VALUES('2021-01-06','09:00:12','2021-01-06','10:10:15');
COMMIT;

/*BB表列类型为DATETIME*/
create table bb (d1 DATETIME,d2 DATETIME);
insert into BB VALUES(TO_DATE('2021-01-01 09:12:12','yyyy-mm-dd hh24:mi:ss'),TO_DATE('2021-01-02 19:02:01','yyyy-mm-dd hh24:mi:ss'));
insert into BB VALUES(TO_DATE('2021-01-02 12:12:12','yyyy-mm-dd hh24:mi:ss'),TO_DATE('2021-01-05 20:02:01','yyyy-mm-dd hh24:mi:ss'));
insert into BB VALUES(TO_DATE('2021-01-02 11:12:12','yyyy-mm-dd hh24:mi:ss'),TO_DATE('2021-01-04 09:02:01','yyyy-mm-dd hh24:mi:ss'));
insert into BB VALUES(TO_DATE('2021-01-03 15:12:12','yyyy-mm-dd hh24:mi:ss'),TO_DATE('2021-01-06 19:02:01','yyyy-mm-dd hh24:mi:ss'));
insert into BB VALUES(TO_DATE('2021-01-06 09:00:12','yyyy-mm-dd hh24:mi:ss'),TO_DATE('2021-01-06 10:02:01','yyyy-mm-dd hh24:mi:ss'));
insert into BB VALUES(TO_DATE('2021-01-04 15:00:12','yyyy-mm-dd hh24:mi:ss'),TO_DATE('2021-01-06 15:02:01','yyyy-mm-dd hh24:mi:ss'));
COMMIT;

计算两个日期时间间隔以"x天x时x分x秒"显示

使用EXTRACT函数，用法与Oracle基本一致。
这里使用到的语法：

SELECT EXTRACT(DAY FROM (DATE2 - DATE1) DAY TO SECOND) ,
EXTRACT(HOUR FROM (DATE2 - DATE1) DAY TO SECOND),
EXTRACT(MINUTE FROM (DATE2 - DATE1) DAY TO SECOND),
EXTRACT(SECOND FROM (DATE2 - DATE1) DAY TO SECOND)
FROM TABLE;
/*
DATE2、DATE1均为日期时间类型，如果数据类型为VARCHAR需要使用to_date等函数转成日期时间类型
DATE2-DATE1得到的是天数（带小数），需要DAY TO SECOND进行转换后才能识别
*/

/*BB表查询*/
select extract(DAY FROM (D2-D1) DAY TO SECOND)||'天'||
extract(HOUR FROM (D2-D1) DAY TO SECOND)||'时'||
extract(MINUTE FROM (D2-D1) DAY TO SECOND)||'分'||
cast(extract(SECOND FROM (D2-D1) DAY TO SECOND)as int)||'秒' AS "时间间隔"
,BB.*
from BB;

/*B表查询*/
select extract(DAY FROM (TO_DATE(date2||time2,'YYYY-MM-DDHH24:MI:SS')-TO_DATE(date1||time1,'YYYY-MM-DDHH24:MI:SS')) DAY TO SECOND)||'天'||
extract(HOUR FROM (TO_DATE(date2||time2,'YYYY-MM-DDHH24:MI:SS')-TO_DATE(date1||time1,'YYYY-MM-DDHH24:MI:SS')) DAY TO SECOND)||'时'||
extract(MINUTE FROM (TO_DATE(date2||time2,'YYYY-MM-DDHH24:MI:SS')-TO_DATE(date1||time1,'YYYY-MM-DDHH24:MI:SS')) DAY TO SECOND)||'分'||
cast(extract(SECOND FROM (TO_DATE(date2||time2,'YYYY-MM-DDHH24:MI:SS')-TO_DATE(date1||time1,'YYYY-MM-DDHH24:MI:SS')) DAY TO SECOND)as int)||'秒' AS "时间间隔"
,B.*
from B;

---格式化显示
select
        extract(DAY FROM (TO_DATE(date2
        ||time2, 'YYYY-MM-DDHH24:MI:SS')-TO_DATE(date1
        ||time1, 'YYYY-MM-DDHH24:MI:SS')) DAY TO SECOND)
        ||'天'
        || extract(HOUR FROM (TO_DATE(date2
        ||time2, 'YYYY-MM-DDHH24:MI:SS')-TO_DATE(date1
        ||time1, 'YYYY-MM-DDHH24:MI:SS')) DAY TO SECOND)
        ||'时'
        || extract(MINUTE FROM (TO_DATE(date2
        ||time2, 'YYYY-MM-DDHH24:MI:SS')-TO_DATE(date1
        ||time1, 'YYYY-MM-DDHH24:MI:SS')) DAY TO SECOND)
        ||'分'
        || cast(extract(SECOND FROM (TO_DATE(date2
        ||time2, 'YYYY-MM-DDHH24:MI:SS')-TO_DATE(date1
        ||time1, 'YYYY-MM-DDHH24:MI:SS')) DAY TO SECOND)as int)
        ||'秒' AS "时间间隔",
        B.*
from
        B;

B表查询显示结果：

   需要注意的是，达梦数据库中，EXTRACT函数对于SECOND之外的任何域，函数返回整数，对于SECOND返回小数。所以在最后获取秒的部分会存在小数，返回的类型为DOUBLE类型，DOUBLE类型与字符串拼接时会发生转换（转换为varchar），结果会显示为"4.9E1秒"这样的格式，而不"49秒"。所以需要使用cast将extract返回的值类型转为整数类型然后再拼接，这样会显示为"49秒"。而Oracle中返回的都是number，所以不需要使用cast进行类型转换。

不转换为int，查询结果如下：