NOT EXISTS Subquery

The NOT EXISTS operator is used in Oracle SQL to test whether the subquery returns no rows. It is the logical opposite of the EXISTS operator, which checks for the existence of rows in the subquery. NOT EXISTS is useful when you want to retrieve rows from the outer query where there are no matching rows in the subquery.

Key Characteristics of NOT EXISTS:

1.     Opposite of EXISTS:

o   While EXISTS returns TRUE if the subquery returns one or more rows, NOT EXISTS returns TRUE when the subquery returns no rows.

o   If the subquery finds at least one row, NOT EXISTS will evaluate to FALSE and exclude that row from the result set.

2.     Boolean Logic:

o   TRUE: If the subquery returns no rows.

o   FALSE: If the subquery returns one or more rows.

3.     Used in WHERE or HAVING:

o   Typically, NOT EXISTS is used in the WHERE clause to exclude rows from the outer query based on the absence of related rows in the subquery.

4.     Non-Correlated vs Correlated Subqueries:

o   Non-Correlated Subquery: The subquery does not reference any columns from the outer query and can be executed independently.

o   Correlated Subquery: The subquery references columns from the outer query and is executed once for each row in the outer query.

 

Syntax of NOT EXISTS Subquery:

Basic Syntax:

SELECT column_name
FROM outer_table
WHERE NOT EXISTS (
    SELECT 1
    FROM inner_table
    WHERE condition
);

·        SELECT 1: This can be any constant because the subquery only needs to check for the existence of rows and does not care about the actual values.

·        The subquery checks if no rows are returned for the given condition.

 

How NOT EXISTS Works:

1.     Outer Query: For each row in the outer query, the subquery is executed.

2.     Subquery: The subquery checks for the existence of related rows based on the condition. If the subquery finds no matching rows, NOT EXISTS evaluates to TRUE, and the outer query includes the row. If the subquery finds any matching rows, NOT EXISTS evaluates to FALSE, and the outer query excludes that row.

Example:

SELECT employee_name
FROM employees e
WHERE NOT EXISTS (
    SELECT 1
    FROM dependents d
    WHERE d.employee_id = e.employee_id
);

·        In this example, the query returns employees who do not have any dependents. The subquery checks if there are no rows in the dependents table for each employee, and if none exist, the employee is included in the result.

 

Types of NOT EXISTS Subqueries:

1. Non-Correlated NOT EXISTS Subquery:

In a non-correlated subquery, the subquery is independent of the outer query and can be executed separately. It returns TRUE if it finds no rows matching the condition.

Example (non-correlated):

SELECT employee_name
FROM employees
WHERE NOT EXISTS (
    SELECT 1
    FROM departments
    WHERE location_id = 1400
);

·        In this case, the outer query will return employees only if no department has location_id = 1400.

2. Correlated NOT EXISTS Subquery:

In a correlated subquery, the subquery depends on the outer query and references columns from the outer query. The subquery is executed for each row in the outer query.

Example (correlated):

SELECT employee_name
FROM employees e
WHERE NOT EXISTS (
    SELECT 1
    FROM dependents d
    WHERE d.employee_id = e.employee_id
);

·        The subquery checks if there are no dependents for the employee being processed in the outer query. If no dependents exist for that employee, the outer query will return the employee's name.

 

Use Cases for NOT EXISTS:

1.     Find Rows with No Related Data: You can use NOT EXISTS when you need to find rows in the outer query that do not have matching rows in another table.

Example: Find employees who do not work on any project.

SELECT employee_name
FROM employees e
WHERE NOT EXISTS (
    SELECT 1
    FROM project_assignments pa
    WHERE pa.employee_id = e.employee_id
);

2.     Exclude Rows Based on Absence of Related Data: If you want to exclude records from the outer query based on the absence of related records, NOT EXISTS is useful.

Example: Find customers who have not placed any orders:

SELECT customer_name
FROM customers c
WHERE NOT EXISTS (
    SELECT 1
    FROM orders o
    WHERE o.customer_id = c.customer_id
);

3.     Removing Duplicate Rows: When dealing with complex join conditions, NOT EXISTS can help filter out duplicate rows, especially when an entity (like a customer or employee) could appear multiple times due to relationships in other tables.

4.     Anti-Join Equivalent: NOT EXISTS is often used as an equivalent to an anti-join, which finds records in one table that do not have corresponding records in another table.

 

Performance Considerations:

1.     Short-Circuiting:

o   The NOT EXISTS operator performs short-circuiting, meaning the subquery stops execution as soon as it finds a match. If the subquery finds any row, NOT EXISTS evaluates to FALSE, and the outer query will not include that row.

o   This can improve performance, especially for large datasets, because the subquery doesn’t need to process all rows once it finds a match.

2.     Indexing:

o   To optimize performance, ensure that the columns used in the subquery’s WHERE condition are indexed (e.g., employee_id, customer_id, etc.). This speeds up the search for matching rows in the subquery.

3.     Comparison with IN:

o   NOT EXISTS can be more efficient than NOT IN, especially when the subquery might return a large number of rows. NOT IN evaluates the subquery for all possible values, while NOT EXISTS stops as soon as a match is found.

 

Example Queries Using NOT EXISTS:

Example 1: Employees who have no dependents:

SELECT employee_name
FROM employees e
WHERE NOT EXISTS (
    SELECT 1
    FROM dependents d
    WHERE d.employee_id = e.employee_id
);

·        This query retrieves employees who do not have any dependents.

Example 2: Customers who have never placed an order:

SELECT customer_name
FROM customers c
WHERE NOT EXISTS (
    SELECT 1
    FROM orders o
    WHERE o.customer_id = c.customer_id
);

·        This query returns customers who have never placed an order.

Example 3: Products that have never been sold:

SELECT product_name
FROM products p
WHERE NOT EXISTS (
    SELECT 1
    FROM sales s
    WHERE s.product_id = p.product_id
);

·        This query returns products that have not been sold.

Example 4: Employees not assigned to any department:

SELECT employee_name
FROM employees e
WHERE NOT EXISTS (
    SELECT 1
    FROM departments d
    WHERE d.department_id = e.department_id
);

·        This query retrieves employees who are not assigned to any department.

 

Key Points to Remember:

1.     NOT EXISTS checks if the subquery returns no rows and returns TRUE if the subquery is empty, and FALSE if it returns one or more rows.

2.     It is typically used in the WHERE or HAVING clauses to filter results based on the absence of related data.

3.     Correlated subqueries reference the outer query’s columns, while non-correlated subqueries do not.

4.     Performance: NOT EXISTS can be more efficient than alternatives like NOT IN when the subquery is large or when short-circuiting behavior is important.

5.     Indexing is crucial for performance when using NOT EXISTS, particularly on the columns involved in the subquery's WHERE clause.

 

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