What does nn mean in mysql

NN in MySQL stands for "NOT NULL", forcing fields to contain values, ensuring data integrity, avoiding exceptions and simplifying operations.

The meaning of NN in MySQL

In MySQL, NN means "NOT NULL". This means that a field cannot be empty. It is a constraint that specifies that the field must always contain a value, whether it is a default value, an inserted value, or an updated value.

Purpose

By using NN constraints, you can ensure:

• Data integrity and consistency in the table
• Avoid null pointer exceptions and logic errors
• Simplify query and data manipulation tasks

Use cases

NN constraints are important for containing critical information or Fields used to determine record uniqueness are particularly useful. For example:

• Primary key field
• Foreign key field
• Username field
• Email address field
• Other required fields

Usage Example

The following example creates a table named "Personnel" where the "ID" field is specified as NN:

<code class="sql">CREATE TABLE 人员 (
  ID INT NOT NULL AUTO_INCREMENT,
  姓名 VARCHAR(255),
  年龄 INT
);</code>

Important Notes

• NN constraints apply to all insert and update operations.
• MySQL will throw an error when trying to insert or update a null value into an NN field.
• If you need to allow null values, you can use the "NULL" constraint.

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