what value of null

for Java programmers, NULL is a headache. It is often harassed by a null pointer exception (NPE). Even the inventor of Java admits it was a big mistake. Why should Java keep null? Null has been around for some time, and I think the Java inventor

PHP determines whether an array of key values exists, using empty or isset or array_key_existsPurpose of this articleThe first few days of work, it is necessary to frequently judge the existence of the key values in the array, initially using the

SQL2000 sa password cannot be changed by resolution, the following error occurred in changing the sa password: Copy Code code as follows: Error 21776: [SQL-DMO] The name ' dbo ' is not found in the Users collection. If The name is a

4.7.1 BACKUP The backup command is used to back up database content or its transaction log to storage media (floppy disk, hard disk, tape). The dump command is used to perform this function when the previous version of SQL Server 7.0 is in use, and

1. isset function: Determine whether the variable is initialized Description: It does not determine whether the variable is empty and can be used to determine whether the elements in the array have been definedNote: When using isset to determine

1. IssetFunction: To determine whether a variable is initializedDescription: It does not determine whether the variable is empty and can be used to determine whether the elements in the array have been definedNote: When using isset to determine

There are two special values in JavaScript: undefined and null, which need to be noticed when comparing them. The undefined value is obtained when reading an unassigned variable or attempting to read an attribute that is not in an object.

We often encounter null in the use or maintenance of SQL Server, so what is NULL? The following is a short description from MSDN (see "Null Values"): A value of NULL indicates that's the value is unknown. A value of NULL is different A empty

1. isset function: Determine whether the variable is initialized Description: It does not determine whether the variable is empty and can be used to determine whether the elements in the array have been defined Note: When using isset to determine

Inquire1. MongoDB uses find to query. The first parameter of find determines which documents to return, and this parameter is a document that specifies the query criteria. An empty query matches the entire contents of the collection. If you do not

Count1) with Count (*) Comparison: If your data table does not have a primary key, COUNT (1) than COUNT (*Fast If there is a primary key, then the primary key (the Federated primary key) as the count condition is also more than count (*) to be quick

In the data we often encounter ntext large text type, this type if and nvarchar type addition can have problems, so there is a way to solve this problem.SQL functions used:TEXTPTR: Returns the value of the text pointer to the textual, ntext, or

In SQL we often use null values,So what is NULL? The following is a brief description of MSDN (see "Null Values"):? A value of NULL indicates the value is unknown. A value of NULL is different from an empty or zero value. No null values are equal.

MySQL series (i)---basic knowledge summary Preface: This article is mainly for MySQL basic knowledge Summary, the basic knowledge of MySQL many, here for a simple general introduction, specific details or need to search by themselves. Of

Use the Find or FindOne function to execute a query in MongoDBFind functionDb.c.find ()--query collection C allDb.c.find ({"Name": "Zhangsan"})Note: The value of a query condition must be a constant, that is, you cannot use a variableDb.c.find

first, the basic conceptThe purpose of the Java Container Class library is to "hold objects" and divide them into two different concepts:1) Collection: A sequence of independent elements that obey one or more rules. the list must save the element in

Oracle NULL represents uncertainty. Will there be a definite data type for uncertain items? Alternatively, let's change the default data type of NULL in Oracle. Here we will discuss this issue. First, I will announce the answer. The default type of

Js data types: Number, String, Boolean, Undefined, Null, and Object. The data we create in js must be one of the above types. 1. the typeof operator js provides the typeof operator to detect the data type. However, the result of the typeof operation

The Java Collection framework is the foundation of the Java programming language and is an important knowledge point in the Java interview. Here, I've listed some important questions and answers about the Java collection.In addition, the Code of the

OverviewSometimes the user device needs to be identified, so you want to get a stable and reliable and unique identification code. Although such device identifiers are available in the Android system, stability and uniqueness are not ideal due to