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Oracle中null问题

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1、Oracle中null和空字符串''是一回事。 2、null的判断只能是is null 和is not null 3、null不能和其他类型的数据运算，比如算数运算符+、-、*、/，关系运算符=、<>等等，参与运算的结果依然是null --普通的算术运算和null一起运算，得到的总是null select null+2+3+4+5+6 from dual;--结果为null --解决这一隐患的方法，使用nvl函数转换 select nvl(null,0)+2+3+4+5+6 from dual;--结果为20 4、一些函数会自动过滤掉数值为null的字段，比如聚合函数，sum、avg、max、min等 5、使用 is null 和is not null作为判断条件时，改字段将不会使用到索引；暂时总结这么多，以后接触到新的坑再总结。来源： CSDN 作者：就是个名字链接： https://blog.csdn.net/zk052300/article/details/103569911

Identifying and removing null characters in UNIX

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问题 I have a text file containing unwanted null characters (ASCII NUL, \0 ). When I try to view it in vi I see ^@ symbols, interleaved in normal text. How can I: Identify which lines in the file contain null characters? I have tried grepping for \0 and \x0 , but this did not work. Remove the null characters? Running strings on the file cleaned it up, but I'm just wondering if this is the best way? 回答1: I’d use tr : tr < file-with-nulls -d '\000' > file-without-nulls If you are wondering if input

Why static fields are not initialized in time?

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问题 The following code prints null once. class MyClass { private static MyClass myClass = new MyClass(); private static final Object obj = new Object(); public MyClass() { System.out.println(obj); } public static void main(String[] args) {} } Why are the static objects not initialized before the constructor runs? Update I'd just copied this example program without attention, I thought we were talking about 2 Object fields, now I saw that the first is a MyClass field.. :/ 回答1: Because statics are

7-51 两个有序链表序列的合并 (20分)

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7-51 两个有序链表序列的合并 (20分) AC代码 # include <iostream> # include <cstdio> # include <vector> # include <algorithm> # include <cstdlib> using namespace std ; typedef struct node { int data ; struct node * nxt ; } node , * linklist ; linklist Read ( ) { linklist L , op ; L = ( linklist ) malloc ( sizeof ( node ) ) ; L -> data = - 1 ; L -> nxt = NULL ; op = L ; int data ; scanf ( "%d" , & data ) ; while ( data != - 1 ) { linklist tmp = ( linklist ) malloc ( sizeof ( node ) ) ; tmp -> nxt = NULL ; tmp -> data = data ; op -> nxt = tmp ; op = op -> nxt ; scanf ( "%d" , & data ) ; } return L ; } linklist

Do nullable columns occupy additional space in PostgreSQL?

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问题 I have a table with 7 columns and 5 of them will be null. I will have a null columns on int , text , date , boolean , and money data types. This table will contain millions of rows with many many nulls. I am afraid that the null values will occupy space. Also, do you know if Postgres indexes null values? I would like to prevent it from indexing nulls. 回答1: Basically, NULL values occupy 1 bit in the NULL bitmap. But it is not as simple as that. The null bitmap (per row) is only there if at

Is there any reason to check for a NULL pointer before deleting?

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问题 I often see legacy code checking for NULL before deleting a pointer, similar to, if (NULL != pSomeObject) { delete pSomeObject; pSomeObject = NULL; } Is there any reason to checking for a NULL pointer before deleting it? What is the reason for setting the pointer to NULL afterwards? 回答1: It's perfectly "safe" to delete a null pointer; it effectively amounts to a no-op. The reason you might want to check for null before you delete is that trying to delete a null pointer could indicate a bug in

How much size “Null” value takes in SQL Server

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问题 I have a large table with say 10 columns. 4 of them remains null most of the times. I have a query that does null value takes any size or no size in bytes. I read few articles some of them are saying : http://www.sql-server-citation.com/2009/12/common-mistakes-in-sql-server-part-4.html There is a misconception that if we have the NULL values in a table it doesn't occupy storage space. The fact is, a NULL value occupies space – 2 bytes SQL: Using NULL values vs. default values A NULL value in

What is the point of the class Option[T]?

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问题 I am not able to understand the point of Option[T] class in Scala. I mean, I am not able to see any advanages of None over null . For example, consider the code: object Main{ class Person(name: String, var age: int){ def display = println(name+" "+age) } def getPerson1: Person = { // returns a Person instance or null } def getPerson2: Option[Person] = { // returns either Some[Person] or None } def main(argv: Array[String]): Unit = { val p = getPerson1 if (p!=null) p.display getPerson2 match{

Why can't primitive data types be “null” in Java?

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问题 When declaring any primitive type data like int or double they get initialized to 0 or 0.0 . Why can we not set them to null ? 回答1: A primitive type is just data. What we call objects, on the other hand, are just pointers to where the data is stored. For example: Integer object = new Integer(3); int number = 3; In this case, object is just a pointer to an Integer object whose value happens to be 3. That is, at the memory position where the variable object is stored, all you have is a

Possible causes of Java VM EXCEPTION_ACCESS_VIOLATION?

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问题 When a Java VM crashes with an EXCEPTION_ACCESS_VIOLATION and produces an hs_err_pidXXX.log file, what does that indicate? The error itself is basically a null pointer exception. Is it always caused by a bug in the JVM, or are there other causes like malfunctioning hardware or software conflicts? Edit: there is a native component, this is an SWT application on win32. 回答1: Most of the times this is a bug in the VM. But it can be caused by any native code (e.g. JNI calls). The hs_err_pidXXX.log