Multi-Row Subqueries

In Oracle SQL, Multi-Row Subqueries are subqueries that return multiple rows and possibly multiple columns. These types of subqueries are used when you need to compare a value against a set of values or a list, or when you want to check for existence or a condition that applies to multiple rows in a subquery.

Key Points about Multi-Row Subqueries

1.     Definition: A Multi-Row Subquery is a subquery that returns multiple rows of results. This can be a list of values, a result set from a complex query, or aggregated data.

2.     Usage Contexts: Multi-row subqueries are commonly used in conjunction with operators that allow for comparisons between a single value from the outer query and a set of values from the inner query.

• In the WHERE Clause: For comparisons like IN, ANY, ALL, EXISTS.
• In the SELECT Clause: Sometimes used to return multiple values or a list of values.
• In the HAVING Clause: To filter based on aggregated results from multiple rows.

Operators for Multi-Row Subqueries:

Multi-row subqueries are typically used with the following operators:

• IN: Checks if a value exists within a set of returned values.
• ANY: Compares a value to the result of a subquery with a condition applied to any of the rows returned.
• ALL: Compares a value to the result of a subquery where the condition is satisfied for all rows returned.
• EXISTS: Checks for the existence of rows in the subquery.
• NOT IN: Checks if a value is not in a set of values returned by a subquery.

Examples of Multi-Row Subqueries:

1. Using IN with a Multi-Row Subquery:

The IN operator allows you to check if a value from the outer query matches any value returned by the subquery.

SELECT employee_id, name

FROM employees

WHERE department_id IN (SELECT department_id FROM departments WHERE location_id = 1000);

• The subquery (SELECT department_id FROM departments WHERE location_id = 1000) returns multiple department_id values.
• The outer query filters employees whose department_id is in the list of department IDs returned by the subquery.

2. Using ANY with a Multi-Row Subquery:

The ANY operator allows you to compare a value to the result of a multi-row subquery. It will return TRUE if the condition matches any row returned by the subquery.

SELECT employee_id, name

FROM employees

WHERE salary > ANY (SELECT salary FROM employees WHERE department_id = 10);

• This query will return employees whose salary is greater than any salary in department 10.
• The subquery returns all salaries from department 10, and the outer query checks if an employee's salary is greater than any of them.

3. Using ALL with a Multi-Row Subquery:

The ALL operator compares a value to the result of a multi-row subquery. It returns TRUE if the condition is true for all rows returned by the subquery.

SELECT employee_id, name

FROM employees

WHERE salary > ALL (SELECT salary FROM employees WHERE department_id = 10);

• This query will return employees whose salary is greater than all salaries in department 10.
• The subquery returns all salaries from department 10, and the outer query checks if the employee's salary is greater than all of them.

4. Using EXISTS with a Multi-Row Subquery:

The EXISTS operator checks whether the subquery returns any rows. It is not concerned with the actual values returned by the subquery, only with whether any rows exist.

SELECT employee_id, name

FROM employees e

WHERE EXISTS (SELECT 1 FROM departments d WHERE d.department_id = e.department_id AND d.location_id = 1000);

• This query checks if there is any department with the same department_id as the employee and where the location_id is 1000.
• The EXISTS subquery doesn't care about the specific values returned, just that at least one row matches the condition.

5. Using NOT IN with a Multi-Row Subquery:

The NOT IN operator checks if a value from the outer query does not match any of the values returned by the subquery.

SELECT employee_id, name

FROM employees

WHERE department_id NOT IN (SELECT department_id FROM departments WHERE location_id = 1000);

• This query returns employees whose department_id is not in the list of department IDs located at 1000.

6. Using ANY and ALL with Aggregate Functions:

Multi-row subqueries are often used with aggregate functions to compare values to an entire set.

Example with ANY:

SELECT employee_id, salary

FROM employees

WHERE salary < ANY (SELECT salary FROM employees WHERE department_id = 10);

This returns employees whose salary is less than any of the salaries in department 10.

Example with ALL:

SELECT employee_id, salary

FROM employees

WHERE salary > ALL (SELECT salary FROM employees WHERE department_id = 10);

This returns employees whose salary is greater than all of the salaries in department 10.

Performance Considerations for Multi-Row Subqueries:

·        Efficiency: Multi-row subqueries, particularly those with operators like IN and EXISTS, can sometimes be less efficient compared to other joins, especially if they return large result sets.

• When using IN, Oracle often optimizes the subquery to use a hash join or merge join, depending on the size of the result set.
• EXISTS can sometimes be more efficient than IN because it stops searching as soon as a match is found.

·        Indexing: Ensure relevant columns in the subquery (like those used in the WHERE clause) are indexed to improve performance.

·        Avoiding Errors: Ensure that your subquery returns the expected number of rows. For example, if using IN, ensure that the number of rows returned by the subquery is manageable. If the subquery returns a very large result set, it could impact performance.

Example with Large Data Sets:

Suppose you want to find employees whose salary is greater than any salary in a specific department:

SELECT employee_id, name

FROM employees

WHERE salary > ANY (SELECT salary FROM employees WHERE department_id = 10);

• If the subquery returns a large number of salaries, the ANY operator can be inefficient, as it needs to check the employee's salary against every value returned. You can optimize the query by limiting the rows returned by the subquery using ROWNUM or a FETCH FIRST clause.

Handling Multiple Rows in a Subquery:

• IN: Used when you want to check if a value is in a set of returned values.
• ANY/ALL: Used when you need to compare a value against one or more values in the result set.
• EXISTS: Used when you just need to know whether any rows exist that match a certain condition.