What does null mean in sql

The NULL value in SQL indicates unknown or non-existent, unlike other values ​​such as the empty string or 0. It is not equal to any value, including itself. Methods to identify NULL values ​​include: IS NULL operator and COALESCE() function. You need to be careful when handling NULL values ​​in queries because they may lead to unexpected results. It is recommended to use IS NULL or COALESCE() function to identify NULL values ​​and use the CASE statement. Provide default values ​​and use foreign key constraints to ensure data integrity.

NULL in SQL

NULL in SQL represents an unknown or non-existent value. It is different from other values ​​such as an empty string or 0, which indicates that the value is undefined or missing.

Characteristics of NULL

• #The NULL value is not a number, string, or Boolean value.
• It is a unique value that is different from any other value, including the empty string or 0.
• The NULL value is not equal to any other value, including itself.

How to identify NULL values

In SQL, NULL values ​​are usually represented as NULL or null. You can also check for NULL values ​​using the following method:

• IS NULL Operator: SELECT * FROM table_name WHERE column_name IS NULL;
• COALESCE() Function: SELECT COALESCE(column_name, 'default_value') FROM table_name;

NULL value processing

You need to be careful when handling NULL values ​​in SQL queries. This is because NULL values ​​can lead to unexpected results, such as:

• In mathematical operations, NULL values ​​produce NULL values.
• In comparison operations, a NULL value is not equal to any other value, including itself.
• In logical operations, NULL values ​​can produce undefined results.

To correctly handle NULL values, the following techniques can be used:

• Use the IS NULL or COALESCE() function identification NULL value.
• Use the CASE statement to provide a default value for NULL values.
• Use foreign key constraints to ensure data integrity and reduce NULL values.

Other notes

• The NULL value is a special value in SQL and is different from other values.
• It represents an unknown or non-existent value, not other values ​​such as empty string or 0.
• Extreme care is required when handling NULL values ​​to avoid unexpected results.

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