null

SELECT null =0, null =12345, null <>12345, null +12345, NULL | | ' abc ', NULL = NULL, NULL <> NULL, NULL and TRUE, NULL and FALSE, NULL or FALSE, NULL or TRUE, not (null); If this is an interview question, I don't know how many programmers or even

Null Pointer exception is the most common and annoying exception. This topic describes how to avoid NULL pointer exceptions. First, let's look at the following example private Boolean isFinished(String status) { if

Believe that a lot of people who have been using MySQL for a long time, the concept of these two field properties is not very clear, the general will have the following questions:1, my field type is not NULL, why I can insert null value2, the

It is best not to look for NULL, isnotnull, isnull, or null values. What is NULL? A: When we do not know the specific data, it is also unknown, It is best not to look for NULL, or is not null, is null, and cannot compare null values. Q: What is NULL?

Null and SQLnull in SQL A summary of null in Oracle SQL is as follows:1.1 null The null value is still null. Null is an unallocated, unknown, or not applicable value. Ø null is neither 0 nor a space.  1.1.1 null Value Calculation The result of the

database table has not null,default,check,unique,primary key,foreign KEY six kinds of constraints.A, NOT NULL----non-null constraintNULL indicates no data and does not represent a specific numeric value, so null in the database is not equal to NULL.

There are two primitive types in javascript: null and undefined. These two types often make JavaScript developers confused, when is null, and when is undefined?The undefined type has only one value, which is undefined. When the declared variable has

In JavaJudge whether it is nullIs nullIf (s = null ){// Do something} Not nullIf (s! = Null ){// Do something} Determines whether null or empty stringIs null or empty string: method 1If (s = null | s. length () = 0 ){// Do something} Is null or

The null summary in Oracle SQL is as follows:1.1 Null? The result of a null value calculation is still null? Null is an unassigned, unknown, or non-applicable value? Null is not 0, nor is it a space1.1.1 Operation of NULL valuesThe result of the

In MySQL, each field definition contains additional constraints or modifiers that can be used to increase the constraints on the input data. Today, let's take a look at MySQL field constraints:NULLAndNot

before we speak NULL, let's look at an exampleThe table data is as follows:3306>select * from T1;+------+-------+| id | name |+------+-------+| 1 | chen || 2 | zhang || 3 | NULL |+------+-------+3 rows in Set (0.00 sec)then we want to find out all

Correct use and space occupation of NULL in SQL Server, servernull We often encounter NULL in the use or maintenance of SQL Server. What is NULL? The following is a brief description provided by MSDN (see "Null Values "): A value of NULL

Several methods for determining null variables in PHP 1. isset function: determines whether a variable is initialized. Note: it does not determine whether the variable is null and can be used to determine whether the elements in the array have

NULL in the computer and programming world represents an unknown, indeterminate. Although the Chinese translation is empty, this empty (null) is not Pethin (empty). Null represents an unknown state. In the future, I don't know how much money I have

Php 0, null, empty, null, false, detailed description of the string relationship. I encountered a strange problem in a project, and it took me a lot of time to solve it. The final debugging found that it was a judgment problem-about 0 and ''(null

In-depth understanding of the relationship between 0, null, empty, null, false, and strings in php // Judge the relationship between 0 and ''and empty null false. start // If ('safdasefasefasf' = 0 ){

Detailed description of unique constraints and NULL in MySQL, and detailed description of null constraints in mysql Preface In the previous requirement, the simplified description is to accept MQ messages from other groups and insert a record in the

In many cases, the default values of the database are null, but many cases occur after program processing. The database value is null rather than null. Note This when creating a unique index. At this time, the database will take null as multiple

NULL in subquery the subquery returns a single row, multiple rows, and null value. It is applicable to the comparison operators of single-row subqueries: =, >,=, SQL> select last_name,department_id from employees where department_id is

Processing the ten NULL values of MySQ learning notes is a special feature of MySQL. In concept, NULL means "no value" or "unknown value", and it is regarded as a slightly different value. To test NULL, you cannot use arithmetic comparison