Usage of ifnull in sql

IFNULL function is used to check whether the expression is NULL, if so, it returns the specified default value, otherwise it returns the value of the expression. It prevents null values ​​from causing errors, allows manipulation of null values, and improves the readability of queries. Usage includes: replacing null values ​​with default values, excluding null values ​​from calculations, and nested usage to handle multiple null value situations.

Usage of IFNULL in SQL

The IFNULL function is used to check whether the expression is NULL. If it is NULL, Returns the specified default value; if not NULL, returns the value of the expression.

Syntax:

<code class="sql">IFNULL(expression, default_value)</code>

Where:

• expression: The expression to be checked.
• default_value: The value to be returned if expression is NULL.

Usage:

The IFNULL function is mainly used to replace null values ​​with specified default values. For example, the following query uses IFNULL to replace null values ​​in the name column with the string "Unnamed":

<code class="sql">SELECT IFNULL(name, '无名') AS name FROM table_name;</code>

Advantages:

• Prevent errors or unexpected results due to null values.
• Allow calculations or operations to be performed on null values.
• Improve the readability and maintainability of queries.

Note:

• default_value can be any data type.
• If the data types of expression and default_value do not match, the SQL engine will automatically perform an implicit conversion.
• IFNULL function can also be used nested to handle multiple null value situations.

Example:

• Replace null values ​​in the age column with 0:

  <code class="sql">SELECT IFNULL(age, 0) AS age FROM table_name;</code>

• Exclude null values ​​in the COUNT() function:

  <code class="sql">SELECT COUNT(IFNULL(name, '')) AS num_names FROM table_name;</code>

• Use the nested IFNULL function to handle multiple null values:

  <code class="sql">SELECT IFNULL(name, IFNULL(nickname, '无名')) AS name FROM table_name;</code>

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