實現前後行相減的SQL該如何寫？有兩種方法！

針對這類場景，sql語句改如何寫呢？

以統計前後行的訂單號是否連續為例，

表結構：

create table `t_test3` (

`id` int(11) not null auto_increment comment '自增id',
`order_id` int(11) default '0' comment '訂單號',
primary key (`id`),
key (`order_id`)
) engine=innodb default charset=utf8mb4 comment='';

insert into t_test3(order_id) values(1),(2),(3),(5),(6),(4),(2);

mysql>select *, (select t1.order_id-order_id from t_test3 where order_id=t1.order_id-1 order by order_id desc limit 1) diff from t_test3 t1;

+----+----------+------+
| id | order_id | diff |
+----+----------+------+
| 1 | 1 | null |
| 2 | 2 | 1 |
| 7 | 2 | 1 |
| 3 | 3 | 1 |
| 4 | 5 | null |
| 6 | 5 | null |
| 5 | 6 | 1 |
+----+----------+------+
7 rows in set (0.00 sec)

mysql> select *, (select order_id-t1.order_id from t_test3 where order_id=t1.order_id-1 order by order_id desc limit 1) diff from t_test3 t1 where diff is null;

error 1054 (42s22): unknown column 'diff' in 'where clause'

mysql> select * from t_test3 t1 left join (select * from t_test3  ) t2  on t1.order_id = t2.order_id-1 ;

+----+----------+------+----------+
| id | order_id | id | order_id |
+----+----------+------+----------+
| 1 | 1 | 2 | 2 |
| 1 | 1 | 7 | 2 |
| 2 | 2 | 3 | 3 |
| 7 | 2 | 3 | 3 |
| 4 | 5 | 5 | 6 |
| 6 | 5 | 5 | 6 |
| 3 | 3 | null | null |
| 5 | 6 | null | null |
+----+----------+------+----------+
8 rows in set (0.00 sec)

mysql> select * from t_test3 t1 left join (select * from t_test3  ) t2  on t1.order_id = t2.order_id-1 where t2.order_id is null;

+----+----------+------+----------+
| id | order_id | id | order_id |
+----+----------+------+----------+
| 3 | 3 | null | null |
| 5 | 6 | null | null |
+----+----------+------+----------+
2 rows in set (0.00 sec)

一定要注意欄位的型別，如果是字串型別，又涉及到運算的則可能不會使用到索引。需要強轉回char比較

select u1.id,u1.detail_id,(select detail_id from t_order_detail where status=1 and create_time>='2019-01-09 16:00:00' 

and create_time<='2019-01-09 16:01:00' 
-- 型別轉換
and convert((cast(detail_id as signed)-1),char(100))=u1.detail_id
) as diff
from t_order_detail u1 where status=1 
and create_time>='2019-01-09 16:00:00' 
and create_time<='2019-01-09 16:10:00';

select u1.id,u2.id,u1.detail_id,u2.detail_id  from t_order_detail u1 right join   

(select id,convert((cast(detail_id as signed)-1),char(100)) detail_id from t_order_detail where status=1 
and create_time>='2019-01-09 16:00:00' 
and create_time<='2019-01-09 16:01:00') u2 
on u1.detail_id=u2.detail_id where u1.detail_id is null limit 100;

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