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This article shows you what Null means in the database, the content is concise and easy to understand, it will definitely brighten your eyes. I hope you can get something through the detailed introduction of this article.

What does Null mean?

NULL is a reserved value in the calculation that indicates that the pointer does not refer to a valid object. Programs usually use null pointers to represent conditions, such as the end of a list of unknown lengths or not performing certain actions; the use of such null pointers can be compared with the pointing values in nullable types and option types.

Null has reserved values on the computer and can be used for pointers not to refer to objects. nowadays, many programs use pointers to represent conditions, but the meaning is different in different languages.

1. Centering +

As we all know, Null inherits the Null macro in c, but the pointer constant is shaped 0, and many people will think that the null pointer takes precedence as the pointer value, so in C++, a constant is referenced instead.

2.SQL

In a database, the Null function means that there is no value for a field and indicates that the field value is empty, that the value of Null is different from other fields, and that no field value is stored. Do not know what the data is, do not know exactly what the value is, let alone whether the data really exists.

3.PHP

In php 4, NULL,NULL is introduced to indicate that there is no value in a variable, and many people think that the value assigned is NULL, including those that are not assigned, and those that are unset ().

4.JAVA

In JAVA, NULL represents a keyword and is often used to represent a null reference.

What does Null mean in the database?

In a database, Null is a field with no value at all and indicates that the field value is unknown. Null is different from the zero value of a numeric field, text field, or space value. Null indicates that the database field value has not been stored.

Cannot compare null to value. For example, if you direct a query to the Customer_Addresses table to retrieve all customers without an e-mail address, you cannot write a structured query language (SQL) query as follows: SELECT * FROM Customer_Addresses WHERE Email_Address = null. Instead, in order not to introduce comparisons with null, the query must be written as follows: SELECT * FROM Customer_ Addresses WHERE Email_Address is null.

When calculating values in columns that contain null values, null values are not included in the results. For example, there are 200 customers in the Customer_ Addresses table and 30 customers in the Email_Address column. Counting with the Email_Address column returns a result of170.

The above is what does Null mean in the database? have you learned any knowledge or skills? If you want to learn more skills or enrich your knowledge reserve, you are welcome to follow the industry information channel.

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