distinct

distinct的用法

由 巧了我就是萌 提交于 2020-01-01 21:19:38
distinct的用法： select distinct expression[,expression...] from tables [where conditions]; 在使用distinct的过程中主要注意一下几点： 在对字段进行去重的时候，要保证distinct在所有字段的最前面 如果distinct关键字后面有多个字段时，则会对多个字段进行组合去重，只有多个字段组合起来的值是相等的才会被去重 distinct的原理： distinct进行去重的主要原理是通过先对要进行去重的数据进行分组操作，然后从分组后的每组数据中去一条返回给客户端，在这个分组的过程可能会出现两种不同的情况： distinct 依赖的字段全部包含索引： 该情况mysql直接通过操作索引对满足条件的数据进行分组，然后从分组后的每组数据中去一条数据。 distinct 依赖的字段未全部包含索引： 该情况由于索引不能满足整个去重分组的过程，所以需要用到临时表，mysql首先需要将满足条件的数据放到临时表中，然后在临时表中对该部分数据进行分组，然后从临时表中每个分组的数据中去一条数据，在临时表中进行分组的过程中不会对数据进行排序。 来源： https://www.cnblogs.com/Mr-Echo/p/12129919.html

Retrieving distinct records based on a column on Django

由 和自甴很熟 提交于 2020-01-01 09:17:13
问题 I need to retrieve a list of records for the following table with distinct values in regards to name: Class C: name value A ------------------ 10 A ------------------ 20 A ------------------ 20 B ------------------ 50 C ------------------ 20 D ------------------ 10 B ------------------ 10 A ------------------ 30 I need to get rid of all the duplicate values for name and only show the following: name value A ------------------ 30 B ------------------ 10 C ------------------ 20 D --------------

Retrieving distinct records based on a column on Django

由 折月煮酒 提交于 2020-01-01 09:17:13
问题 I need to retrieve a list of records for the following table with distinct values in regards to name: Class C: name value A ------------------ 10 A ------------------ 20 A ------------------ 20 B ------------------ 50 C ------------------ 20 D ------------------ 10 B ------------------ 10 A ------------------ 30 I need to get rid of all the duplicate values for name and only show the following: name value A ------------------ 30 B ------------------ 10 C ------------------ 20 D --------------

Does SQL Server support IS DISTINCT FROM clause?

由 不打扰是莪最后的温柔 提交于 2020-01-01 07:37:12
问题 Does SQL Server support IS DISTINCT FROM statement which is SQL:1999 standard? E.g. the query SELECT * FROM Bugs WHERE assigned_to IS NULL OR assigned_to <> 1; can be rewritten using IS DISTINCT FROM SELECT * FROM Bugs WHERE assigned_to IS DISTINCT FROM 1; 回答1: No, it doesn't. The following SO question explains how to rewrite them into equivalent (but more verbose) SQL Server expressions: How to rewrite IS DISTINCT FROM and IS NOT DISTINCT FROM? There's also a Uservoice entry for this issue,

Linq to SQL: DISTINCT with Anonymous Types

由 非 Y 不嫁゛ 提交于 2020-01-01 07:28:51
问题 Given this code: dgIPs.DataSource = from act in Master.dc.Activities where act.Session.UID == Master.u.ID select new { Address = act.Session.IP.Address, Domain = act.Session.IP.Domain, FirstAccess = act.Session.IP.FirstAccess, LastAccess = act.Session.IP.LastAccess, IsSpider = act.Session.IP.isSpider, NumberProblems = act.Session.IP.NumProblems, NumberSessions = act.Session.IP.Sessions.Count() }; How do I pull the Distinct() based on distinct Address only? That is, if I simply add Distinct(),

Oracle SQL - How to get distinct rows using RANK() or DENSE_RANK() or ROW_NUMBER() analytic function?

由 核能气质少年 提交于 2020-01-01 06:13:44
问题 I am looking to get the top 3 distinct salaries of each department. I was able to do it either using RANK() or DENSE_RANK() or ROW_NUMBER() but my table is having some records with same salaries. Mentioned below is my query and its result. The top 3 salaries of Dept 20 should be 6000, 3000, 2975. But there are 2 employees with salary 3000 and both of them have rank 2. So it is giving me 4 records for this department (1 for rank 1, 2 records for rank2 and 1 record for rank3). Please suggest

Hive性能优化（全面）

由 笑着哭i 提交于 2019-12-31 17:05:39
简介： Hadoop的计算框架特性下的HIve有效的优化手段 作者：浪尖 本文转载自公众号：Spark学习技巧 1.介绍 首先，我们来看看Hadoop的计算框架特性，在此特性下会衍生哪些问题？ 数据量大不是问题，数据倾斜是个问题。 jobs数比较多的作业运行效率相对比较低，比如即使有几百行的表，如果多次关联多次汇总，产生十几个jobs，耗时很长。原因是map reduce作业初始化的时间是比较长的。 sum,count,max,min等UDAF，不怕数据倾斜问题,hadoop在map端的汇总合并优化，使数据倾斜不成问题。 count(distinct ),在数据量大的情况下，效率较低，如果是多count(distinct )效率更低，因为count(distinct)是按group by 字段分组，按distinct字段排序，一般这种分布方式是很倾斜的。举个例子：比如男uv,女uv，像淘宝一天30亿的pv，如果按性别分组，分配2个reduce,每个reduce处理15亿数据。 面对这些问题，我们能有哪些有效的优化手段呢？下面列出一些在工作有效可行的优化手段： 好的模型设计事半功倍。 解决数据倾斜问题。 减少job数。 设置合理的map reduce的task数，能有效提升性能。(比如，10w+级别的计算，用160个reduce，那是相当的浪费，1个足够)。 了解数据分布

SQL: check insert successful (in a task to get 8 distinct random rows from a table with two columns)

由 纵然是瞬间 提交于 2019-12-31 05:59:09
问题 Update: I fixed the previous problems. Now the codes are up-dated. Results are unique and IDs are right. But new problem: The amount of result rows is often less than requirement (8). Because I added CREATE UNIQUE INDEX topicid on rands (topicid); to deny the repeated inserts in SQL layer; the loop - 1 regardless the insert is denied. I am now looking for a method like: IF insert successful THEN cnt-=1. Do you know any way to do this in SQL layer? Thanks. I have a table called topictable

Oracle get DISTINCT numeric with a CLOB in the query

由 依然范特西╮ 提交于 2019-12-31 04:15:10
问题 EDIT : I am looking for a DISTINCT NUMERIC while including a CLOB within the query. I have two relations. Relation One: LOGID_NBR NUMBER (12) APPID_NBR NUMBER (2) EVENTID_NBR NUMBER (10) KEYID_NBR NUMBER (8) KEYVALUE VARCHAR2 (100 Byte) ARGUMENTSXML VARCHAR2 (4000 Byte) SENTINDICATOR CHAR (5 Byte) RECEIVED_DATEDATE DATE sysdate LAST_UPDATED DATE sysdate TEXTINDICATOR VARCHAR2 (5 Byte) UPSELL_ID VARCHAR2 (5 Byte) GECKOIMAGEIND CHAR (1 Byte) DELIVERYTYPE VARCHAR2 (30 Byte) Relation Two: LOGID

How to get the distinct data from a list?

由 纵饮孤独 提交于 2019-12-30 10:52:40
问题 I want to get distinct list from list of persons . List<Person> plst = cl.PersonList; How to do this through LINQ . I want to store the result in List<Person> 回答1: Distinct() will give you distinct values - but unless you've overridden Equals / GetHashCode() you'll just get distinct references . For example, if you want two Person objects to be equal if their names are equal, you need to override Equals / GetHashCode to indicate that. (Ideally, implement IEquatable<Person> as well as just
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