SQL NULL Comparisons: Why Use 'x IS NULL' Instead of 'x = NULL'?

SQL: Understanding the difference between 'x is null' and 'x = null'

SQL provides two different ways to test a column for NULL values: 'x is null' and 'x = null'. Although these two expressions look similar, their behavior is significantly different.

'x = null' vs 'x is null': key differences

The expression 'x = null' is a comparison operation that checks whether the value of 'x' is equal to the literal NULL value. However, in SQL, any comparison involving NULL values, including 'x = null', will always return 'null'. This is because NULL values ​​in SQL represent unknown values, so it cannot be determined whether they are equal to any other value.

In contrast, 'x is null' is an expression that specifically tests for the presence of a NULL value. Returns 'true' if 'x' is NULL; otherwise returns 'false'. This allows for accurate testing of NULL values ​​without the ambiguity associated with 'x = null'.

Why 'x = null' doesn't work

As mentioned before, 'x = null' will always return 'null' in SQL. This means that when used in a WHERE clause, it will not produce any matching rows because 'null' is considered false in this case.

Example

Consider the following form:

X	Y
1	null
null	1

If you execute the query 'SELECT * FROM t WHERE x = null', you will not get any rows even if there are rows where 'x' is 'null'. This is because 'x = null' always evaluates to 'null', which is false in the WHERE clause.

Use 'x is null' for accurate NULL value testing

To correctly test for NULL values, use 'x is null' instead of 'x = null'. Here is an example that will return the required rows:

SELECT * FROM t WHERE x is null

This query will return:

X	Y
null	1

Conclusion

Understanding the difference between 'x is null' and 'x = null' is critical to effective SQL programming. 'x = null' compares 'x' to a literal NULL value and always returns 'null', while 'x is null' specifically tests for the presence of a NULL value. When testing NULL values, always use 'x is null' for accurate results.

The above is the detailed content of SQL NULL Comparisons: Why Use 'x IS NULL' Instead of 'x = NULL'?. For more information, please follow other related articles on the PHP Chinese website!