Introduction to SQLite NULL Data Type

SQLite is a lightweight relational database management system that is widely used in embedded devices and mobile applications. SQLite supports various data types, including NULL, INTEGER, TEXT, REAL, BLOB, etc. This article will focus on the NULL data type.

Syntax

In SQLite, the NULL data type is used to represent a missing or unknown value. Its syntax is as follows:

NULL

Use Cases

The NULL data type is suitable for representing missing or unknown data. For example, when a field has no value, it can be set to NULL. Compared to other data types, the NULL data type has the following advantages:

• Can represent empty values for any data type: The NULL data type can represent empty values for any data type, including integers, floating-point numbers, text, etc.
• Can improve query accuracy: IS NULL or IS NOT NULL operators can be used in queries to filter out NULL or non-NULL values.

Therefore, using the NULL data type when representing missing or unknown data can improve the flexibility of the database and the accuracy of queries.

Examples

Below are two examples that illustrate how to use the NULL data type.

Example 1: Creating a table with NULL fields

Suppose we want to create a table to store information about students, including their student ID (integer type), name (text type), age (integer type), and birthdate (text type). We can use the following SQL statement to create the table:

CREATE TABLE students (
  id INTEGER PRIMARY KEY,
  name TEXT,
  age INTEGER,
  birthday TEXT
);

The age field in this table can be represented using the NULL data type to indicate unknown or missing age information for students.

Example 2: Inserting and querying NULL data

Now that we have created a table with NULL fields, let’s see how to insert and query data of this type.

First, we can use the following SQL statement to insert a student’s information into the table, where the age is unknown:

INSERT INTO students (id, name, age, birthday)
VALUES (10001, '张三', NULL, '2000-01-01');

This statement inserts a student’s information into the students table, where the age field is represented using the NULL data type to indicate unknown or missing age information for the student.

Next, we can use the following SQL statement to query all students whose age is unknown from the students table:

SELECT id, name FROM students WHERE age IS NULL;

This statement returns the student ID and name for all students in the students table whose age is unknown.

Conclusion

The NULL data type is one of the commonly used data types in SQLite, suitable for representing missing or unknown data. Using the NULL data type can improve the flexibility of the database and the accuracy of queries when creating and manipulating tables with NULL fields.