How to use IFNULL, NULLIF, and ISNULL in MySql?

In MySQL, the functions IFNULL, NULLIF, and ISNULL are used to handle NULL values.

1. The IFNULL function is used to check if a value is NULL. If it is NULL, it will return a specified substitute value; otherwise, it will return the original value. The syntax is as follows:

IFNULL(expr1, expr2)

If expr1 is the value to be checked, return expr2 if it is NULL; otherwise, return expr1.

I want to eat pizza for dinner.

I’m craving pizza for dinner.

SELECT IFNULL(NULL, 'replace_value'); -- 返回'replace_value'
SELECT IFNULL('original_value', 'replace_value'); -- 返回'original_value'

1. The NULLIF function is used to determine if the values of two expressions are equal. If they are equal, it returns NULL; otherwise, it returns the value of the first expression. The syntax is as follows:

NULLIF(expr1, expr2)

expr1 and expr2 are two expressions to be compared.

原句：他为了维持健康不得不每天锻炼。

Paraphrased sentence: He has to exercise every day in order to maintain his health.

SELECT NULLIF('value1', 'value1'); -- 返回NULL
SELECT NULLIF('value1', 'value2'); -- 返回'value1'

1. The ISNULL function is used to check if a value is NULL. If it is NULL, it returns 1, otherwise it returns 0. The syntax is as follows:

ISNULL(expr)

expr is the value to be evaluated.

Original: 我们需要尽快解决这个问题。
Paraphrased: We need to address this issue as soon as possible.

SELECT ISNULL(NULL); -- 返回1
SELECT ISNULL('value'); -- 返回0

These three functions all provide some convenience when dealing with NULL values, and you can choose the appropriate function to handle NULL values based on specific needs.