为什么会存在select in (1,2,3)比select in (1,2,3,4,5)查询速度要快的情况
时间: 2023-10-08 17:04:07 浏览: 36
`SELECT IN (1,2,3)` and `SELECT IN (1,2,3,4,5)` are both queries that check whether a value exists in a list.
In general, `SELECT IN (1,2,3)` may be faster than `SELECT IN (1,2,3,4,5)` because it has a smaller list of values to check. This can result in fewer comparisons and faster query execution.
However, the performance of these queries can also depend on factors such as the size of the data set, the indexing strategy used by the database, the complexity of the query, and the hardware and resources available. Therefore, it's important to benchmark and optimize queries based on the specific use case.
相关问题
select id from a not in (1,2,3,4,5)优化一下这个sql
可以尝试用`not exists`或`left join`的方式代替`not in`,因为`not in`的查询效率比较低,而且当被比较的列表比较大时,效率会更低。以下是两种优化方式的示例代码:
使用not exists:
```
select id from a where not exists (select 1 from b where a.id = b.id and b.id in (1,2,3,4,5));
```
使用left join:
```
select a.id from a left join b on a.id = b.id and b.id in (1,2,3,4,5) where b.id is null;
```
这里假设表b中存储了需要排除的id,如果需要排除的id比较少,那么使用`not in`也可以,但是如果需要排除的id比较多,那么使用上述两种方式会更有效率。
select id from a id not in (1,2,3,4,5)优化一下这个sql
可以使用NOT EXISTS子查询来优化该SQL语句,如下所示:
```
SELECT id FROM a WHERE NOT EXISTS (SELECT 1 FROM (SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5) b WHERE a.id = b.1)
```
该SQL语句使用子查询的方式,将(1,2,3,4,5)转换成一个临时表b,然后使用NOT EXISTS关键字查询a表中不在临时表b中的记录,从而达到优化的目的。
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