coalesce | 易学教程

Spark Coalesce More Partitions

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问题 I have a spark job that processes a large amount of data and writes the results to S3. During processing I might have in excess of 5000 partitions. Before I write to S3 I want to reduce the number of partitions since each partition is written out as a file. In some other cases I may only have 50 partitions during processing. If I wanted to coalesce rather than repartition for performance reasons what would happen. From the docs it says coalesce should only be used if the number of output

SQL SERVER - Problems with COALESCE() function

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问题 I have this SQL function which is wrapped by a stored procedure: ALTER FUNCTION dbo.GetObjList ( @filterUID int = NULL, @filterSID varchar(32) = NULL ) RETURNS TABLE AS RETURN SELECT ROW_NUMBER() OVER (ORDER BY UID) AS [RowNumber], * FROM ObjTable WHERE (COALESCE(@filterUID, @filterSID) IS NULL) OR ( ((@filterUID IS NOT NULL) AND (UID = @filterUID)) OR ((@filterSID IS NOT NULL) AND (SID = @filterSID)) ) Why would I receive such an error: "Conversion failed when converting the varchar value

Using COALESCE with different data types?

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问题 I have a query using COALESCE(timestamp_type::date,charachter_varying) which fails due to the mismatched data types: ERROR: COALESCE types date and character varying cannot be matched It works if i cast the timestamp as text : COALESCE(timestamp_type::text,charachter_varying) However, now it is returning the full timestamp when I only want YYYY-MM-DD (instead of the full timestamp, YYYY-MM-DD HH:MM:SS.000000+00 ) How can I use COALESCE and return only the date portion of the timestamp? 回答1:

MySql coalesce函数

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语法 coalesce(str1,str2,str3,…,default) str1,str2,str3：参数，但是最终只会返回最左边那个非空参数的值。 default：当前面所有的表达式值都为NULL的时候，返回此参数的值。 MySql中的coalesce()函数用于返回参数中的第一个非空表达式，顺序为从左往右。与nvl()函数的对比这个函数其实和nvl()函数的作用基本是一样的，只不过nvl()函数不能在MySql中使用，因此用coalesce()函数来替代。相比于nvl()函数，coalesce中参数可以有多个，而nvl()函数只有两个参数。 SQL示例比如我们要获取form_xf这张表中XFCode这个字段的最大值，当此表无数据时，则直接返回默认值0。 SELECT coalesce(max(XFCode), 0) FROM form_xf 来源： CSDN 作者： wunianisme 链接： https://blog.csdn.net/qq_29645505/article/details/103263310

Coalesce vs empty string concatenation

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问题 My coworker is new to C# and didn't know about the coalesce operator. So, I saw him write a line of code like this: string foo = "" + str; The idea being that if str is null, this expression would return an empty string. Of course, that could be rewritten as this: string foo = str ?? ""; And I feel that would be more readable. But is it really that big a deal? Are the readability benefits enough to suggest going back and making those lines look like the second? Or is this one of those things

How do I count individual text count from a column from all records [duplicate]

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This question already has answers here : SQL String: Counting Words inside a String (3 answers) Closed 2 years ago . I need to display count of each text occurrence on all rows from a particular column. See result below: Here I have a set of Tags, I need to display count each tag as column in the name 'Total' What I have done is: DECLARE @tags VARCHAR(8000) DECLARE @tot INT select @tags = coalesce(@tags + ',' , ' ') + Labels from addNew select @tot = count(@tags) select a.Labels as Tags,@tot as Total from addNew a inner join addNew n on a.Labels = n.Labels group by a.Labels The result must be:

MySQL NULL的理解和应用

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在本教程中，您将学习如何使用MySQL NULL 值。此外，您将学习一些有用的函数来有效地处理 NULL 值。 MySQL NULL值简介在MySQL中， NULL 值意味着未知值。 NULL 值不是零或空字符''值。 NULL 值不等于其自身值。如果将 NULL 值与另一个 NULL 值或任何其他值进行比较，则结果为 NULL ，因为每个 NULL 值的值都是未知的。通常，使用 NULL 值来表示数据丢失，未知或不适用。例如，当客户的电话号码可设置为 NULL ，并且可以稍后添加。当创建表时，可以通过使用 NOT NULL 约束来指定列是否接受 NULL 值。例如，以下语句创建 leads 表： CREATE TABLE studymysql.leads ( id INT AUTO_INCREMENT PRIMARY KEY, customer_name VARCHAR(50) NOT NULL, source VARCHAR(255) NOT NULL, email VARCHAR(100), phone VARCHAR(25) ); 因此，id是主键列，因此它不接受任何 NULL 值。 customer_name 和 source 列使用 NOT NULL 约束，因此，不能在这些列中插入 NULL 值。 email 和 phone 列是可以接受

Spark RDD coalesce()方法和repartition()方法

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在Spark的RDD中，RDD是分区的。有时候需要重新设置RDD的分区数量，比如RDD的分区中，RDD分区比较多，但是每个RDD的数量比较小，需要设置一个比较合理的分区。或者需要把RDD的分区数量调大。还有就是通过设置一个RDD的分区来达到设置生成的文件的数量。有这两种方法是可以重设RDD分区：分别是coalesce()方法和repartition()。这两个方法有什么区别，看看源码就知道了： def coalesce(numPartitions: Int, shuffle: Boolean = false , partitionCoalescer: Option[PartitionCoalescer] = Option.empty) (implicit ord: Ordering[T] = null ) : RDD[T] = withScope { require(numPartitions > 0 , s "Number of partitions ($numPartitions) must be positive." ) if (shuffle) { /** Distributes elements evenly across output partitions, starting from a random partition. */ val

oracle的一些函数收集

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coalesce coalesce (expression_1, expression_2, ...,expression_n) 　　功能　　　　依次遍历括号内的表达式，遇到非null值的时候返回该值。　　可以解决的问题：　　　　处理多表查询的时候，处理空值。　　举例： select '','b','c' from dual; select coalesce('','b','c') from dual; --第一个字为空时，返回一个后面不为空的值 select to_number(''),0 from dual --所有的表达式的值类型必须为第一个值的类型 select coalesce(to_number(''),0) from dual; --让为空的值，默认为0 持续收集、更新... 来源： https://www.cnblogs.com/recurision/p/11988388.html

mysql subtract two rows from column and places into an alias

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问题 I have table meter_readings with columns: id , date_taken , kwh . I'm trying to subtract two rows in kwh column together and put the results into an alias called consumption . I'm using: SELECT id, kwh COALESCE(kwh-(SELECT kwh FROM meter_readings WHERE id= id+1), kwh) AS consumption FROM meter_readings; What I get back in consumption alias is simple the same as the original kwh : id date_taken kwh consumption 1 2013-01-01 4567.89 4567.89 2 2013-01-08 4596.71 4596.71 3 2013-01-15 4607.89 4607