Question

我有一个包含4列的表格：

ID，GUID，Binary，Timestamp。

我的目标是将二进制文件的最后10个修改保存到数据库中。如果插入了第11个修改，则应删除最旧的修改。

我目前的方法是分两步完成（伪mssql）：

1) DELETE FROM mytable WHERE GUID = 'XXX' AND 
   ID NOT IN (SELECT TOP 9 ID FROM mytable WHERE GUID = 'XXX' ORDER BY Timestamp)

2) INSERT new binary ...

有没有办法更有效率，也许有一个声明？有没有办法使它兼容，mssql和postgresql兼容（没有TOP / Limit）？

Answer 1

您可以使用cte来兼容：

with cte as (
   select
       row_number() over(order by Timestamp desc) as row_num
   from mytable
   where GUID = 'XXX'
)
delete from cte
where row_num > 10

编辑
请参阅Gordon Linoff的回答，我的语法在PostgreSQL中无效，只是在sqlfiddle中测试过。我在SQL Server上工作太多了......

<强> EDIT2
关于在一个查询中删除和插入，PostgreSQL允许：

with cte_del as (
   select
       id,
       row_number() over(order by id desc) as row_num
   from tbl
   where GUID = 'XXX'
), cte_d as (
   delete from tbl where id in (select id from cte_del where row_num > 10)
)
insert into ...
select id from cte_del where row_num <= 10;

sql fiddle demo

Answer 2

我认为以下内容适用于SQL Server和Postgres：

with todelete as (
      select id, row_number() over (partition by GUID order by timestamp) as seqnum
      from mytable
      where GUID = 'xxx'
     )
delete from mytable
    where id in (select id from todelete where seqnum > 10);

允许最多10个具有相同GUID的项目

2 个答案: