How to use the IFNULL function to replace NULL values ​​in MySQL

How to use the IFNULL function to replace NULL values ​​in MySQL

When using a MySQL database, you often encounter situations where you need to handle NULL values. If left unhandled, NULL values ​​may cause errors during calculations or queries. To solve this problem, MySQL provides the IFNULL function, which can be used to replace NULL values. This article will introduce how to use the IFNULL function and give corresponding code examples.

The syntax of the IFNULL function is as follows:
IFNULL(expr1, expr2)

Among them, expr1 is the expression to check whether it is NULL, expr2 is the value to replace when expr1 is NULL .

The following are some examples of using the IFNULL function:

Example 1: Using constants to replace NULL values
Suppose there is a student score table, in which a NULL value exists in a certain field score, we want to To replace these NULL values ​​with 0. You can use the following SQL statement:

SELECT IFNULL(score, 0) AS new_score
FROM student_scores;

In the above statement, if the value of the score field is NULL, use 0 instead, and the result is saved in the new_score column.

Example 2: Replace NULL values ​​with values ​​from other fields
Sometimes, we want to replace NULL values ​​with values ​​from other fields. For example, you have a student table that contains the student's name and age fields, where the age field may be empty. We want to replace the age field of these records with a null age with 0. You can use the following SQL statement:

SELECT name, IFNULL(age, 0) AS new_age
FROM students;

In the above statement, if the value of the age field is NULL, use 0 instead, and the result is stored in the new_age column.

Example 3: Use expressions to replace NULL values
In addition to using constants or values ​​from other fields, we can also use expressions to replace NULL values. For example, you have an orders table that contains order amount and discount fields, where the discount field may be empty. We want to replace the discount field of these records with an empty discount with 10% of the order amount. You can use the following SQL statement:

SELECT amount, IFNULL(discount, amount * 0.1) AS new_discount
FROM orders;

In the above statement, if the value of the discount field is NULL, use 10% of the order amount instead, and the result is stored in the new_discount column.

Summary:
The IFNULL function is a useful tool for handling NULL values ​​in the MySQL database. By using the IFNULL function, we can replace NULL values ​​in query results, making data processing more accurate and convenient. Whether you use constants, values ​​from other fields, or expressions, you can choose the appropriate replacement value based on the specific situation. I hope the examples in this article will help you use the IFNULL function to replace NULL values ​​in MySQL.

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