如何在Oracle表中的列中移動值？

Question

考慮下面的Oracle數據庫表：

 
group revision comment 
1  1   1    
1  2   2 
1  null  null 
2  1   1    
2  2   2 
2  3   3 
2  4   4 
2  null  null 
3  1   1    
3  2   2 
3  3   3 
3  null  null 

我要評論欄就該一步向下調節，版本，其組內，使我得到如下表：

 
group revision comment 
1  1   null    
1  2   1 
1  null  2 
2  1   null    
2  2   1 
2  3   2 
2  4   3 
2  null  4 
3  1   null    
3  2   1 
3  3   2 
3  null  3 

我有以下查詢：

    

    MERGE INTO example_table t1 
    USING example_table t2 
    ON (
     (t1.revision = t2.revision+1 OR 
     (t2.revision = (
      SELECT MAX(t3.revision) 
      FROM example_table t3 
      WHERE t3.group = t1.group 
     ) AND t1.revision IS NULL) 
    ) 
    AND t1.group = t2.group) 
    WHEN MATCHED THEN UPDATE SET t1.comment = t2.comment; 

這樣做大部分（仍然需要一個單獨的查詢來覆蓋修訂= 1），但它是非常緩慢。

所以我的問題是，我如何儘可能有效地使用Max來爲每個組拉出最高版本？

Answer 1

我會用lag不max

create table example_table(group_id number, revision number, comments varchar2(40)); 
insert into example_table values (1,1,1); 
insert into example_table values (1,2,2); 
insert into example_table values (1,3,null); 
insert into example_table values (2,1,1); 
insert into example_table values (2,2,2); 
insert into example_table values (2,3,3); 
insert into example_table values (2,4,null); 

select * from example_table; 

merge into example_table e 
using (select group_id, revision, comments, lag(comments, 1) over (partition by group_id order by revision nulls last) comments1 from example_table) u 
on (u.group_id = e.group_id and nvl(u.revision,0) = nvl(e.revision,0)) 
when matched then update set comments = u.comments1; 

select * from example_table;