distinct

I'm trying to write a mysql statement that pulls only unique messages from my chat system. There is a to_id and a from_id. I want to pull one row from every conversation. The problem is if there is a row with to_id = 1 from_id = 2 and another row with to_id = 2 from_id = 1 I want both of these rows to be treated as the same. The statement I came up with is "SELECT * FROM chat_messages GROUP BY to_id,from_id ". This works except for the situation I mentioned above. Another attempt I made was using "SELECT DISTINCT least(to_id,from_id) as value1, greatest(to_id,from_id) as value2 from chat

#########sample 1 mysql中去重 distinct 用法 在使用 MySQL 时，有时需要查询出某个字段不重复的记录，这时可以使用mysql提供的distinct这个关键字来过滤重复的记录，但是实际中我们往往用distinct来返回不重复字段的条数（count(distinct id)）,其原因是distinct只能返回他的目标字段，而无法返回其他字段，例如有如下表user: 用distinct来返回不重复的用户名：select distinct name from user;，结果为： 这样只把不重复的用户名查询出来了，但是用户的id，并没有被查询出来：select distinct name,id from user;，这样的结果为： distinct name,id 这样的mysql 会认为要过滤掉name和id两个字段都重复的记录，如果sql这样写：select id,distinct name from user，这样mysql会报错，因为distinct必须放在要查询字段的开头。 所以一般distinct用来查询不重复记录的条数。 如果要查询不重复的记录，有时候可以用group by ： select id,name from user group by name; https://www.cnblogs.com/shiluoliming/p

I'm stuck on a problem with sql. I have a table with many duplicate entries with column names like:- eventnumber housenumber value1 value2 None of these column names are the primary key as there are many duplicates. What I would like to do is select into another table by distinct housenumber but I seem to get the whole table copied across, I'm using:- Select * into temp_table from schedule_temp where housenumber in (select distinct housenumerb from schedule_temp) Now I broke it down a bit and if I do:- Select distinct housenumber into temp from schedule_temp group by housenumber I get a table

原文: 性能调优4：统计信息 SQL Server优化器基于开销（Cost）评估执行计划，选择开销最小的作为“最优化”的执行计划。计算开销的根据是索引及其统计信息，因此，索引和统计数据是非常重要的。查询优化器（Query Optimizer）使用统计信息对查询的开销进行评估（Estimate），选择开销最小的查询计划，作为最终的、“最优的”的执行计划。SQL Server自动为索引列或查询的数据列创建统计信息，统计信息包括三部分：头部（Header），密度向量（Density Vector） 和 分布直方图（Distribution Histogram）。 统计信息是数据分布的反馈，SQL Server根据数据更新的数量和特定的规则自动更新统计信息，一般情况下，表的数据量越大，SQL Server更新统计信息需要的数据更新量越大，随着数据的更新，有些表的数据不会及时更新，以至于统计信息过时，不能真实反映数据的分布情况，用户可以通过命令手动更新统计信息，但是更新统计信息需要扫描数据表，这可能是一个非常耗时的IO密集型操作，用户需要权衡性能的提升和资源的消耗。 一，查看统计信息 统计信息不是实时更新的，如果统计信息过期，查询优化器（Query optimizer）可能不能生成高质量的查询计划，必须有必要的调度程序，自动更新统计数据。数据库管理员（DBA）可以使用DBCC SHOW

I think many similar questions have already been asked, but I could not make up a copy-paste solution. So here is the problem: I periodically compute analysis on my user data for reporting. I made up a model to store these computations in my sqlite database. class report_data(models.Model): name = models.TextField() matrix = PickledObjectField() creation_date = models.DateTimeField('date of creation') I am using an append only scheme, to persist historical data, so there are many records in database, that have same name different matrix different creation date My report view now needs a set,