SQL WHERE ID IN (id1, id2, ..., idn) [英] SQL WHERE ID IN (id1, id2, ..., idn)

问题描述

我需要编写一个查询来检索一个大的 id 列表.

I need to write a query to retrieve a big list of ids.

我们确实支持许多后端(MySQL、Firebird、SQLServer、Oracle、PostgreSQL ...)，所以我需要编写一个标准的 SQL.

We do support many backends (MySQL, Firebird, SQLServer, Oracle, PostgreSQL ...) so I need to write a standard SQL.

id 集的大小可能很大，查询将以编程方式生成.那么，最好的方法是什么?

The size of the id set could be big, the query would be generated programmatically. So, what is the best approach?

SELECT * FROM TABLE WHERE ID IN (id1, id2, ..., idn)

我的问题是.如果 n 很大会发生什么?另外，性能怎么样?

My question here is. What happens if n is very big? Also, what about performance?

SELECT * FROM TABLE WHERE ID = id1 OR ID = id2 OR ... OR ID = idn

我认为这种方法没有 n 限制，但如果 n 很大，性能如何?

I think that this approach does not have n limit, but what about performance if n is very big?

  foreach (var id in myIdList)
  {
      var item = GetItemByQuery("SELECT * FROM TABLE WHERE ID = " + id);
      myObjectList.Add(item);
  }

当通过网络查询数据库服务器时，我们遇到了这种方法的一些问题.通常，执行一次检索所有结果的查询比进行大量小查询更好.也许我错了.

We experienced some problems with this approach when the database server is queried over the network. Normally is better to do one query that retrieve all results versus making a lot of small queries. Maybe I'm wrong.

这个问题的正确解决方案是什么?

What would be a correct solution for this problem?

推荐答案

选项 1 是唯一好的解决方案.

Option 1 is the only good solution.

• 选项 2 的作用相同，但您多次重复列名；此外，SQL 引擎不会立即知道您要检查该值是否是固定列表中的值之一.然而，一个好的 SQL 引擎可以优化它以获得与 IN 相同的性能.仍然存在可读性问题...

• Option 2 does the same but you repeat the column name lots of times; additionally the SQL engine doesn't immediately know that you want to check if the value is one of the values in a fixed list. However, a good SQL engine could optimize it to have equal performance like with IN. There's still the readability issue though...

选项 3 在性能方面简直太糟糕了.它在每个循环中发送一个查询，并用小查询来冲击数据库.它还可以防止它对值是给定列表中的值之一"使用任何优化

Option 3 is simply horrible performance-wise. It sends a query every loop and hammers the database with small queries. It also prevents it from using any optimizations for "value is one of those in a given list"

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