In SQL, NULL represents the absence of a value or an unknown value. It is a special value that can be assigned to a column when there is no actual data available or when the value is unknown. NULL is different from an empty string ('') or a zero (0), as it specifically denotes the absence of a value.

Here are some key points to understand about NULL values in SQL:

1. Assigning NULL: To assign a NULL value to a column when inserting or updating a row, you can simply omit the value or use the keyword NULL explicitly. For example:

sql
INSERT INTO employees (name, age, department, salary)
VALUES ('John Doe', NULL, 'Sales', 5000);

2. Comparing NULL: Comparisons involving NULL values typically result in NULL rather than true or false. To compare a column with a NULL value, you need to use the IS NULL or IS NOT NULL operators. For example:

sql
SELECT *
FROM employees
WHERE age IS NULL;

3. Handling NULL: NULL values can have implications when performing calculations or aggregations. Most SQL functions and operators return NULL if any of the input values are NULL. To handle NULL values, you can use the ISNULL or COALESCE functions to substitute NULL with a specific value or a default value.

4. NULL in Constraints: When defining table columns, you can specify whether a column allows NULL values or not using the NULL or NOT NULL constraint. If a column has the NOT NULL constraint, it means that NULL values are not allowed in that column.

5. NULL in Joins: When joining tables, NULL values can impact the results. If a column used for joining has NULL values, it may cause rows to be excluded from the join result. Understanding how NULL values behave in joins is important for writing accurate queries.

It's essential to handle NULL values appropriately in SQL queries to ensure accurate results and avoid unexpected behavior. Understanding the behavior of NULL values and using NULL-related operators and functions can help you effectively work with these special values in your database operations.