distinct | 易学教程

Android: Distinct and GroupBy in ContentResolver

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问题 What would be the correct way to add DISTINCT and/or GROUPBY to ContentResolver -based queries? Right now I have to create custom URI for each special case. Is there a better way? (I still program for 1.5 as lowest common denominator) 回答1: You can do nice hack when querying contentResolver, use: String selection = Models.SOMETHING + "=" + something + ") GROUP BY (" + Models.TYPE; 回答2: Since no one came to answer I'm just going to tell how I solved this. Basically I would create custom URI for

How does Distinct() function work in Spark?

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问题 I'm a newbie to Apache Spark and was learning basic functionalities. Had a small doubt.Suppose I have an RDD of tuples (key, value) and wanted to obtain some unique ones out of them. I use distinct() function. I'm wondering on what basis does the function consider that tuples as disparate..? Is it based on the keys, or values, or both? 回答1: .distinct() is definitely doing a shuffle across partitions. To see more of what's happening, run a .toDebugString on your RDD. val hashPart = new

Postgres: Distinct but only for one column

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问题 I have a table on pgsql with names (having more than 1 mio. rows), but I have also many duplicates. I select 3 fields: id , name , metadata . I want to select them randomly with ORDER BY RANDOM() and LIMIT 1000 , so I do this is many steps to save some memory in my PHP script. But how can I do that so it only gives me a list having no duplicates in names. For example [1,"Michael Fox","2003-03-03,34,M,4545"] will be returned but not [2,"Michael Fox","1989-02-23,M,5633"] . The name field is the

Delete duplicate rows from a BigQuery table

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问题 I have a table with >1M rows of data and 20+ columns. Within my table (tableX) I have identified duplicate records (~80k) in one particular column (troubleColumn). If possible I would like to retain the original table name and remove the duplicate records from my problematic column otherwise I could create a new table (tableXfinal) with the same schema but without the duplicates. I am not proficient in SQL or any other programming language so please excuse my ignorance. delete from Accidents

MySQL SELECT DISTINCT multiple columns

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问题 Let's say I have columns a, b c, d in a table in a MySQL database. What I'm trying to do is to select the distinct values of ALL of these 4 columns in my table (only the distinct values). I tried stuff like: SELECT DISTINCT a,b,c,d FROM my_table; SELECT DISTINCT a,b,c,d FROM my_table GROUP BY a,b,c,d; None of those worked. Can anybody help out here? Thank you NOTE I want the distinct values of the columns a, b, c d separately. Not the distinct combination of values 回答1: can this help? select

MySQL SELECT DISTINCT multiple columns

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How do you create a Distinct query in HQL

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问题 Is there a way to create a Distinct query in HQL. Either by using the "distinct" keyword or some other method. I am not sure if distinct is a valid keywork for HQL, but I am looking for the HQL equivalent of the SQL keyword "distinct". 回答1: Here's a snippet of hql that we use. (Names have been changed to protect identities) String queryString = "select distinct f from Foo f inner join foo.bars as b" + " where f.creationDate >= ? and f.creationDate < ? and b.bar = ?"; return

MySQL学习之单表查询

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目录语法优先级 distinct where子句 like子句通配符 group by子句 having子句 order by子句 limit子句语法语法： SELECT distinct 字段 1 , 字段 2. . . FROM 表名 WHERE 条件 GROUP BY field HAVING 筛选 ORDER BY field LIMIT 限制条数 ; 优先级优先级： from where group by having select distinct order by limit 1. 找到表: from 2. 拿着 where 指定的约束条件，去文件 / 表中取出一条条记录 3. 将取出的一条条记录进行分组 group by ，如果没有 group by ，则整体作为一组 4. 将分组的结果进行 having 过滤 5. 执行 select 6. distinct 去重 7. 将结果按条件排序： order by 8. limit 限制结果的显示条数 distinct # DISTINCT: 放在字段的前面，对其后所有的字段都生效，记录重复的只显示一次（记录去重） # distinct 效率不高，如果能用其他方式实现，首选其他 # =============================================================

MySQL - Conditional MIN MAX to return distinct record

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问题 I have a database dump from the geonames website for Great Britain. It consists of approx 60000 records. example data is as follows: id | name | admin1 | admin2 | admin3 | feature_class | feature_code ------------------------------------------------------------------------------------------- 2652355 | Cornwall | ENG | C6 | | A | ADM2 11609029 | Cornwall | ENG | | | L | RGN 6269131 | England | ENG | | | A | ADM1 The first record with feature code ADM2 means it is administrative level 2 The

LINQ distinct on class item?

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问题 Note this question is similar this one except I'm not working with linq-to-sql, so the "let" is not usable. Basically I have a select of the type ... .Select(c => new SampleClass { Id = c.Location.Name, Name = c.Location.Name }).Distinct().ToList() which used to work when I just had ... .Select(c => c.Location.Name).Distinct().ToList() How would I make a distinct call on one of the items within the SampleClass? 回答1: You can group items by the key, and then select what item from the group you