The IS DISTINCT FROM operator is used in SQL to compare two values for inequality, taking into account the possibility of NULL values. This operator is particularly useful because it treats NULL values as distinct from each other, which is different from the standard equality operator (=) or inequality operator (<>).
Syntax
value1 IS DISTINCT FROM value2
Behavior
- It returns TRUE if:
- value1 and value2 are different non-null values.
- One of them is NULL and the other is not.
- It returns FALSE if:
- Both values are identical (and not NULL).
- It returns NULL if both values are NULL, but IS DISTINCT FROM treats this case as if they are considered distinct.
Examples
1. Basic Example:
SELECT 'abc' IS DISTINCT FROM 'abc'; -- Returns FALSE
SELECT 'abc' IS DISTINCT FROM 'def'; -- Returns TRUE
2. Handling NULL Values:
SELECT NULL IS DISTINCT FROM NULL; -- Returns FALSE (but you want this to be "same")
SELECT NULL IS DISTINCT FROM 'abc'; -- Returns TRUE (NULL is distinct from 'abc')
SELECT 'abc' IS DISTINCT FROM NULL; -- Returns TRUE (the opposite)
3. Using in a WHERE Clause:
You might use IS DISTINCT FROM in a WHERE clause during a SELECT statement:
SELECT *
FROM my_table
WHERE column_a IS DISTINCT FROM column_b;
This query will return all rows where column_a and column_b are different, including cases where one of them is NULL.
Conclusion
The IS DISTINCT FROM operator is a valuable tool in SQL for comparing values while effectively handling NULL values. It simplifies the logic when working with nullable data by treating NULL comparisons in a consistent manner.