What's the Difference Between '=' and 'IS NULL' in SQL Queries?

Understanding the Distinction Between "=" and "IS NULL" in SQL

When constructing SQL queries, it is crucial to comprehend the distinctions between assigning a value of "null" using the "=" operator and comparing a column to "NULL" using the "IS NULL" operator.

Assigning Null Values with "="

The "="" operator can be used to set a column to the null value. For instance, the following query assigns the null value to the "Age" column of the "Persons" table:

UPDATE Persons SET Age = NULL
WHERE Name = 'John Doe';

In this case, the "=" operator is utilized to set the "Age" column for the record where the "Name" column equals "John Doe" to null.

Comparing Columns to NULL with "IS NULL"

In contrast to assignment, the "IS NULL" operator is employed in SQL queries to determine whether a column contains the null value. It returns a boolean value, indicating whether the column is null or not. For example, the following query selects all rows from the "Persons" table where the "Age" column is null:

SELECT *
FROM Persons
WHERE Age IS NULL;

Note that using "Age = NULL" in a WHERE clause is invalid and will always return false because null values cannot be directly compared for equality. Instead, the "IS NULL" operator must be used for such comparisons.

Key Differences

• The "=" operator is used to assign a null value to a column, while "IS NULL" is utilized to compare a column to null.
• "Age = NULL" is not a valid expression in WHERE clauses, whereas "Age IS NULL" is correct.
• "IS NULL" returns a boolean value, indicating whether the compared value is null or not, while "=" is used for assignment.

The above is the detailed content of What's the Difference Between '=' and 'IS NULL' in SQL Queries?. For more information, please follow other related articles on the PHP Chinese website!