DISTINCT FAQS

1. What does the DISTINCT keyword do in Oracle SQL?

• The DISTINCT keyword is used to remove duplicate rows from the result set, ensuring that only unique values are returned.

2. How do I use DISTINCT in a SELECT statement?

• You simply place DISTINCT right after SELECT to return only unique records.

SELECT DISTINCT column_name

FROM table_name;

3. Can I use DISTINCT with multiple columns?

• Yes, you can apply DISTINCT to multiple columns. The query will return unique combinations of the values from all specified columns.

SELECT DISTINCT column1, column2

FROM table_name;

4. Does DISTINCT affect all columns in the SELECT clause?

• Yes, when DISTINCT is applied, it affects all columns in the SELECT clause. It will remove duplicate rows based on the combination of all selected columns.

5. How does DISTINCT handle NULL values?

• DISTINCT treats NULL values as equal. So if there are multiple rows with NULL in the same column, only one NULL will appear in the result set.

6. Can I use DISTINCT with aggregate functions like COUNT()?

• Yes, you can use DISTINCT with aggregate functions to count unique values in a column.

SELECT COUNT(DISTINCT column_name)

FROM table_name;

7. What is the difference between DISTINCT and GROUP BY?

• DISTINCT removes duplicate rows from the result set, while GROUP BY groups rows by specified columns and is typically used in conjunction with aggregate functions. You don't need DISTINCT when using GROUP BY.

SELECT department_id

FROM employees

GROUP BY department_id;  -- This already removes duplicates

8. Is DISTINCT useful in JOIN operations?

• Yes, DISTINCT can be used in JOIN queries to return unique results when joining multiple tables.

SELECT DISTINCT column_name

FROM table1

JOIN table2 ON table1.id = table2.id;

9. Does DISTINCT affect performance?

• Yes, using DISTINCT can impact query performance, especially on large datasets, because Oracle needs to process and remove duplicate rows.

10. Can I use DISTINCT with ORDER BY?

• Yes, DISTINCT can be combined with ORDER BY to return unique values in a specific order.

SELECT DISTINCT column_name

FROM table_name

ORDER BY column_name;

11. Can DISTINCT be used with INSERT or UPDATE statements?

• DISTINCT can only be used in SELECT statements. It cannot be directly used in INSERT or UPDATE statements. However, you can use DISTINCT in a SELECT query to get unique data before inserting or updating.

12. Is it possible to use DISTINCT with NULL values?

• Yes, DISTINCT considers all NULL values as identical, meaning only one NULL will appear in the result set, even if multiple rows have NULL.

13. Can DISTINCT be used on DATE values?

• Yes, DISTINCT can be used on DATE values. It will remove duplicate date records, treating dates with the same value as identical.

14. What happens if I use DISTINCT with a large number of columns?

• Using DISTINCT with a large number of columns might have performance implications, especially if the dataset is large. Oracle has to compare all the columns in the result set to ensure uniqueness.

15. Is DISTINCT the same as DISTINCT ON in other SQL dialects?

• No, Oracle doesn't support the DISTINCT ON syntax that is available in some other SQL dialects (like PostgreSQL). In Oracle, you use DISTINCT directly without the ON clause.