UNION ALL FAQS

 1. What is the difference between UNION and UNION ALL?

• UNION: Combines the result sets of two or more SELECT statements and removes duplicate rows.
• UNION ALL: Combines the result sets of two or more SELECT statements without removing duplicates. It returns all rows, including duplicates.

2. What happens if there are duplicate rows in the result set when using UNION ALL?

• UNION ALL includes all duplicate rows in the final result set. If the same row appears in both queries, it will be shown twice (or more, depending on how many times it appears in the queries).

3. Is UNION ALL faster than UNION?

Yes, UNION ALL is faster than UNION because it does not have to perform the extra processing step of removing duplicate rows. Since UNION sorts and checks for duplicates, it requires more resources and time, especially with large datasets.

4. Do the SELECT statements need to have the same number of columns when using UNION ALL?

Yes, both SELECT statements must return the same number of columns. Also, the corresponding columns must have compatible data types (e.g., a VARCHAR column can be combined with another VARCHAR column).

5. Can UNION ALL be used with different data types?

The columns being combined with UNION ALL must have compatible data types. If the data types are not the same, Oracle will try to implicitly convert them if possible. If the types are not compatible, an error will occur.

6. Can I use ORDER BY with UNION ALL?

Yes, you can use ORDER BY with UNION ALL. However, ORDER BY should be placed at the end of the entire query and will apply to the combined result set:

SELECT column1 FROM table1

UNION ALL

SELECT column1 FROM table2

ORDER BY column1;

7. What happens if the SELECT queries in UNION ALL have different column names?

The column names in the result set will be determined by the first SELECT statement in the UNION ALL. If the column names differ, you can alias the columns for consistency across queries.

8. Can UNION ALL be used with subqueries?

Yes, UNION ALL can be used with subqueries. Each subquery must return the same number of columns and compatible data types.

Example:

SELECT column1 FROM (SELECT column1 FROM table1 WHERE condition1)

UNION ALL

SELECT column1 FROM (SELECT column1 FROM table2 WHERE condition2);

9. How can I avoid including duplicate rows in UNION ALL?

If you need to eliminate duplicates, you should use UNION instead of UNION ALL, as UNION removes duplicates by default.

10. What is the advantage of using UNION ALL?

• Faster performance: Since it does not remove duplicates, UNION ALL is generally faster and requires fewer resources compared to UNION.
• Keeps all data: If duplicates are allowed or expected, UNION ALL will give you all the rows from each query without removing any.

11. Can I use UNION ALL to combine data from different tables or databases?

Yes, you can use UNION ALL to combine data from multiple tables. These tables can reside in the same schema, different schemas, or even different databases (with the appropriate database links). The only requirement is that the SELECT statements must return the same number of columns with compatible data types.

12. Does UNION ALL guarantee the order of the rows?

No, UNION ALL does not guarantee any specific order for the rows. If you want the rows to be sorted, you must use an ORDER BY clause at the end of the entire query.

13. Can I use GROUP BY or HAVING with UNION ALL?

Yes, you can use GROUP BY, HAVING, or any other clause (like WHERE) with individual SELECT statements inside the UNION ALL. For example:

SELECT department, COUNT(*) FROM employees GROUP BY department

UNION ALL

SELECT department, COUNT(*) FROM contractors GROUP BY department;

14. Can I use UNION ALL with different WHERE conditions for each query?

Yes, each SELECT query within the UNION ALL can have its own WHERE condition. For example:

SELECT employee_id, department FROM employees WHERE department = 'HR'

UNION ALL

SELECT contractor_id, department FROM contractors WHERE department = 'HR';

15. What happens if I try to UNION ALL queries with incompatible data types?

If the corresponding columns in the SELECT statements have incompatible data types, Oracle will throw an error. For example, you cannot UNION ALL a NUMBER column with a VARCHAR column unless they can be implicitly converted to a compatible data type.