Usage of coalesce in sql

The COALESCE function returns the specified non-null value, or the specified default value if all values ​​are NULL. Usage: 1. Get the first non-null value; 2. Provide a default value; 3. Check multiple values ​​in cascade. Note: Only the first non-null value is returned to ensure that the default value type is consistent. This function avoids NULL values ​​in the query, ensuring that meaningful values ​​are returned.

Usage of COALESCE function in SQL

COALESCE function is used to return the specified non- NULL, returns the specified default value if all values ​​are NULL. The syntax is as follows:

<code class="sql">COALESCE(value1, value2, ..., valueN, default_value)</code>

Among them:

• value1, value2,..., valueN is Multiple values ​​to check.
• default_value is the default value to be returned if all values ​​are NULL.

Usage:

The COALESCE function is used to avoid NULL values ​​in queries, especially in scenarios where it is necessary to ensure that non-null values ​​are returned. Here are some usage examples:

• Get the first non-null value:

<code class="sql">SELECT COALESCE(name, email) FROM users;</code>

The above query will return the user's name if there is no name , the email is returned.

• Provide a default value:

<code class="sql">SELECT COALESCE(address, 'Unknown') FROM customers;</code>

The above query will return the customer's address, or if there is no address, the default value "Unknown" will be returned.

• Cascade check for multiple values:

<code class="sql">SELECT COALESCE(first_name, last_name, username) FROM users;</code>

The above query will return the user's last name, first name, or username, checking these values ​​in order , until a non-null value is found.

Note:

• The COALESCE function will return the first non-null value, not the combination of all non-null values.
• Make sure default_value is of the same data type as the value being checked.
• The COALESCE function is useful for handling missing data and ensuring that meaningful values ​​are returned.

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