SQL NULL Values: When to Use '= NULL' vs. 'IS NULL'?

Distinguishing between "= null" and "IS NULL" in SQL

In SQL, the distinction between "= null" and "IS NULL" is crucial for accurate database operations. While both expressions handle null values, they serve different purposes and are used in distinct contexts.

Use of "= null":

The expression "= null" is used for assigning null values to columns or variables. It sets a column to an unknown or undefined state. Null values signify that no meaningful value can be associated with the column. For example, you might use "= null" to clear the value of a column during an update query:

UPDATE TableX SET Column=NULL WHERE>

Use of "IS NULL":

In contrast, "IS NULL" is used in WHERE clauses to check whether a column contains null values. It tests for the absence of any valid data in the column. This operator is particularly useful when you need to exclude null values from your query results. For instance:

SELECT * FROM TableX WHERE Column IS NULL;

Key Difference:

The fundamental difference between "= null" and "IS NULL" lies in their purpose. "= null" assigns null values, while "IS NULL" checks for their presence. In a WHERE clause, "= null" is never true because null values cannot be equated to any specific value, including null itself. Therefore, it is incorrect to use "= null" in such contexts. Instead, you must use "IS NULL" or "IS NOT NULL" to test for the existence or absence of null values.

Additional Resources:

• Wikipedia: Null (SQL)
• w3schools: SQL NULL Values
• SQL Tutorial: IS NULL Operator

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