What does null mean in sql

NULL in SQL represents a missing or unknown value, which is different from 0 or an empty string. It is usually represented by the keyword NULL, and the comparison result is always unknown or NULL. Uses of NULL include representing missing values, distinguishing missing values ​​from zero values, enforcing data integrity, and using NULL comparisons. When working with NULL values, you can use functions or methods such as COALESCE, IFNULL, or CASE statements.

The meaning of NULL in SQL

NULL represents a missing or unknown value in SQL, which indicates to the database No value is stored in a specific column or field. NULL differs from 0, the empty string, or a space in that it indicates that the value does not actually exist or has not yet been determined.

How to identify NULL values

In SQL, NULL values ​​are usually represented by the keyword NULL. It can appear in any data type column, and comparisons with other values ​​always result in unknown or NULL. For example:

<code>SELECT * FROM table_name WHERE column_name IS NULL;</code>

Uses of NULL

The NULL value has several uses in SQL:

• means missing or unknown Value: Use NULL when the data is missing, has not been collected, or cannot be determined.
• Distinguish between missing values ​​and zero values: NULL can help distinguish between non-existent values ​​and values ​​with zero value.
• Enforce data integrity: You can enforce that a column always contains a value by defining it as NOT NULL.
• Using NULL comparison: You can use the IS NULL and IS NOT NULL operators to compare NULL values.

Handling NULL Values

It is important to be careful when handling NULL values, as they can lead to unexpected results. Some common ways to handle NULL values ​​include:

• Using the COALESCE function:The COALESCE function can be used to return the first non-NULL value of a non-NULL expression.
• Using the IFNULL function: The IFNULL function can be used to return a value if an expression is NULL, otherwise it returns the expression.
• Using the CASE statement: The CASE statement can be used to return different values ​​based on the presence of a NULL value.

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