coalesce

Something I find myself doing more and more is checking a string for empty (as in "" or null) and a conditional operator. A current example: s.SiteNumber.IsNullOrEmpty() ? "No Number" : s.SiteNumber; This is just an extension method, it's equivalent to: string.IsNullOrEmpty(s.SiteNumber) ? "No Number" : s.SiteNumber; Since it's empty and not null, ?? won't do the trick. A string.IsNullOrEmpty() version of ?? would be the perfect solution. I'm thinking there has to be a cleaner way of doing this (I hope!), but I've been at a loss to find it. Does anyone know of a better way to do this, even if

ISNULL是判断是否为NULL 而NULLIF是把值换成NULL COALESCE是用别的来代替NULL SELECT employee_id,first_name,last_name,NULLIF (SALES_QUOTA,-1) as Quota FROM employees 就是把-1变成 NULL COALESCE(表达式1,表达式2,....表达式n) 从前到后,谁不是NULL就显示谁 Select employee_id,first_name,last_name, COALESCE ( appt_quota,(Select Min(appt_quota) From employees),0 ) AS quota From employees Where department = 'Marketing' 附: ISNULL(check_expression, replacement_value) check_expression 与 replacement_value 数据类型必须一致 如果 check_expression 为 NULL，则返回 replacement_value 如果 check_expression 不为 NULL，则返回 check_expression NULLIF 用于检查两个表达式，语法： NULLIF(expression,

简介 Spark中的OOM问题不外乎以下两种情况 map执行中内存溢出 shuffle后内存溢出 map执行中内存溢出代表了所有map类型的操作。包括：flatMap，filter，mapPatitions等。 shuffle后内存溢出的shuffle操作包括join，reduceByKey，repartition等操作。 后面先总结一下我对Spark内存模型的理解，再总结各种OOM的情况相对应的解决办法和性能优化方面的总结。如果理解有错，希望在评论中指出。 Spark 内存模型 Spark在一个Executor中的内存分为三块，一块是execution内存，一块是storage内存，一块是other内存。 execution内存是执行内存，文档中说join，aggregate都在这部分内存中执行，shuffle的数据也会先缓存在这个内存中，满了再写入磁盘，能够减少IO。其实map过程也是在这个内存中执行的。 storage内存是存储broadcast，cache，persist数据的地方。 other内存是程序执行时预留给自己的内存。 execution和storage是Spark Executor中内存的大户，other占用内存相对少很多，这里就不说了。 在spark-1.6.0以前的版本，execution和storage的内存分配是固定的，使用的参数配置

I want to display a default message when there is no data obtained from a query. For example Let us take a query select empname from employee where id = 100 If no data matches this search i want to get Unavailable as a result or the required result should display. So how should i write a SQL query to achieve this. I am using Oracle 10g. SELECT COALESCE((SELECT empname FROM employee WHERE id = 100), 'Unavailable') FROM DUAL; You have to wrap the SELECT into another SELECT or no row will be returned. And where there is no row, there cannot be a value. 来源： https://stackoverflow.com/questions

Given the following: SELECT ISNULL('XY' + NULL, 'ABCDEFGHIJ') -- Outputs ABC (Why?) SELECT COALESCE('XY' + NULL, 'ABCDEFGHIJ') -- Outputs ABCDEFGHIJ Why are these statements returning different results? According to Microsoft documentation , for function: ISNULL(check_expression, replacement_value) replacement_value must be of a type that is implicitly convertible to the type of check_expression . Note that type for 'xy'+NULL is VARCHAR(3) . Because of this your string 'ABCDEFGHIJ' is cast to VARCHAR(3) and thus trimmed. It sounds strange why it is not VARCHAR(2) , but this is the way it is -

I have a query inside a stored procedure that sums some values inside a table: SELECT SUM(columnA) FROM my_table WHERE columnB = 1 INTO res; After this select I subtract res value with an integer retrieved by another query and return the result. If WHERE clause is verified, all works fine. But if it's not, all my function returns is an empty column (maybe because I try to subtract a integer with an empty value). How can I make my query return zero if the WHERE clause is not satisfied? Erwin Brandstetter You could: SELECT COALESCE(SUM(columnA), 0) FROM my_table WHERE columnB = 1 INTO res; This

目录 一.spark 分区 partition的理解 二.coalesce 与 repartition的区别（我们下面说的coalesce都默认shuffle参数为false的情况） 三.实例 1. coalesce 2. repartition 四.总结 一.spark 分区 partition的理解 spark中是以vcore级别调度task的。 如果读取的是hdfs，那么有多少个block，就有多少个partition 举例来说：sparksql 要读表T, 如果表T有1w个小文件，那么就有1w个partition 这时候读取效率会较低。假设设置资源为 --executor-memory 2g --executor-cores 2 --num-executors 5。 步骤是拿出1-10号10个小文件（也就是10个partition） 分别给5个executor读取（spark调度会以vcore为单位，实际就是5个executor，10个task读10个partition） 如果5个executor执行速度相同，再拿11-20号文件 依次给这5个executor读取 而实际执行速度不会完全相同，那就是哪个task先执行完，哪个task领取下一个partition读取执行，以此类推。这样往往读取文件的调度时间大于读取文件本身，而且会频繁打开关闭文件句柄，浪费较为宝贵的io资源

I have the following query SELECT DISTINCT pt.incentive_marketing, pt.incentive_channel, pt.incentive_advertising FROM test.pricing pt WHERE pt.contract_id = 90000 group by 1,2,3 order by pt.incentive_marketing; The above query returns the o/p as shown in the attached image However I want to replace all null values by 0 using COALESCE Please let me know how this can be achieved in above SELECT query Now I further modified the query using coalesce as below SELECT COALESCE( pt.incentive_marketing, '0' ), COALESCE(pt.incentive_channel,'0'), COALESCE( pt.incentive_advertising,'0') FROM test

COALESCE (expression_1, expression_2, ...,expression_n) 依次参考各参数表达式，遇到非null值即停止并返回该值。 如果所有的表达式都是空值，最终将返回一个空值。 使用COALESCE在于大部分包含空值的表达式最终将返回空值。 SELECT coalesce(collect_result,0) as collect_result FROM collect 数据库中如果查询的字段collect_result为空那么赋值0给collect_result。 作者：Devid 链接：https://www.jianshu.com/p/b0f6c574274b 来源：简书 简书著作权归作者所有，任何形式的转载都请联系作者获得授权并注明出处。 来源： https://www.cnblogs.com/telwanggs/p/11386743.html