COALESCE FAQS

 1. What is the difference between COALESCE and NVL?

• COALESCE: Returns the first non-NULL value from a list of expressions. It can handle more than two expressions.
• Example: COALESCE(expression_1, expression_2, ..., expression_n)
• NVL: Only handles two arguments and replaces NULL with a specified value.
• Example: NVL(column, replacement_value)

2. Can I use COALESCE with more than two arguments?

Yes, COALESCE can take multiple arguments. It evaluates each argument in order and returns the first non-NULL value it encounters.

SELECT COALESCE(col1, col2, col3, 'default') FROM table_name;

In this case, the function will return the first non-NULL value from col1, col2, or col3. If all are NULL, it will return 'default'.

3. Does COALESCE work with NULL values?

Yes, the primary purpose of COALESCE is to handle NULL values. It returns the first non-NULL value it encounters from the list of expressions provided.

4. What is the return type of the COALESCE function?

The return type of the COALESCE function is the data type of the first non-NULL value in the list of expressions. If all expressions are NULL, the return type is NULL.

5. How is COALESCE different from CASE statements?

• COALESCE: Simpler and more concise for checking multiple expressions for non-NULL values. It only returns the first non-NULL value.
• CASE: More versatile and allows for complex conditional logic. It can handle more complex expressions and conditions, but it is less efficient than COALESCE for simple NULL checks.

Example of CASE:

SELECT CASE

           WHEN col1 IS NOT NULL THEN col1

           WHEN col2 IS NOT NULL THEN col2

           ELSE 'default_value'

       END AS result

FROM table_name;

6. Can COALESCE handle multiple columns with different data types?

Yes, COALESCE can handle expressions of different data types, and it will return the value with the data type of the first non-NULL expression. Oracle implicitly converts the data types to a common type based on its internal rules.

7. Can COALESCE be used with aggregate functions?

Yes, COALESCE is frequently used with aggregate functions to handle NULL values. For example, you can replace NULL with 0 in a sum operation:

SELECT department_id, COALESCE(SUM(salary), 0) AS total_salary

FROM employees

GROUP BY department_id;

If all salaries are NULL, the function will return 0 instead of NULL.

8. Can COALESCE be used in WHERE or HAVING clauses?

Yes, COALESCE can be used in WHERE or HAVING clauses to replace NULL values in conditions:

SELECT employee_id, COALESCE(phone_number, 'Not Provided') AS contact

FROM employees

WHERE COALESCE(phone_number, 'Not Provided') != 'Not Provided';

In this query, the COALESCE function replaces NULL values with 'Not Provided' and then compares it with that string in the WHERE clause.

9. Does COALESCE work with empty strings?

No, COALESCE treats empty strings as valid values, not as NULL. It will only return the first non-NULL value, but if an empty string is the first value, it will be returned as is.

SELECT COALESCE('', 'Not Available') FROM dual;

-- Output: ''

10. Can COALESCE be used for string concatenation?

Yes, COALESCE can be useful in string concatenation to ensure that NULL values are replaced with a default value.

SELECT employee_id, COALESCE(first_name, '') || ' ' || COALESCE(last_name, 'No Last Name') AS full_name

FROM employees;

This ensures that NULL values for first_name or last_name are replaced with default values before concatenating.