Common Table Expressions (CTEs)

A Common Table Expression (CTE) is a temporary named result set that exists only during the execution of a single SQL statement. CTEs improve query readability, allow for recursive queries, and can simplify complex queries that would otherwise require nested subqueries or derived tables.

Basic Syntax

WITH cte_name AS (
    SELECT column1, column2
    FROM table_name
    WHERE condition
)
SELECT *
FROM cte_name
WHERE another_condition;

Explanation:

• WITH introduces the CTE.
• cte_name is the temporary name for the result set.
• The CTE can then be referenced in the main SELECT query as if it were a table.

Example 1: Simple CTE

Tables:

employees

employee_id	name	department_id	salary
1	Alice	101	70000
2	Bob	102	80000
3	Charlie	101	60000

Goal: Find employees earning more than the average salary.

WITH avg_salary AS (
    SELECT AVG(salary) AS avg_sal
    FROM employees
)
SELECT name, salary
FROM employees, avg_salary
WHERE salary > avg_salary.avg_sal;

Output:

name	salary
Bob	80000
Alice	70000

Explanation:

• The CTE avg_salary calculates a single value.
• The main query references it for filtering, making the query clearer than a nested subquery.

Example 2: Multiple CTEs

You can define multiple CTEs in a single query.

WITH dept_avg AS (
    SELECT department_id, AVG(salary) AS avg_sal
    FROM employees
    GROUP BY department_id
),
high_earners AS (
    SELECT e.name, e.salary, e.department_id
    FROM employees e
    JOIN dept_avg d
    ON e.department_id = d.department_id
    WHERE e.salary > d.avg_sal
)
SELECT *
FROM high_earners;

Output:

name	salary	department_id
Alice	70000	101
Bob	80000	102

Explanation:

• First CTE calculates department average salary.
• Second CTE filters employees above department average.
• Main query selects from the second CTE.
• Readability and modularity are greatly improved.

Example 3: Recursive CTE

Definition: Recursive CTEs reference themselves. Useful for hierarchical data, e.g., organizational charts.

Table: employees (manager hierarchy)

employee_id	name	manager_id
1	Alice	NULL
2	Bob	1
3	Charlie	2
4	David	2

Goal: Get all employees under manager Alice.

WITH RECURSIVE subordinates AS (
    SELECT employee_id, name, manager_id
    FROM employees
    WHERE manager_id IS NULL  -- Alice

    UNION ALL

    SELECT e.employee_id, e.name, e.manager_id
    FROM employees e
    INNER JOIN subordinates s
    ON e.manager_id = s.employee_id
)
SELECT *
FROM subordinates;

Output:

employee_id	name	manager_id
1	Alice	NULL
2	Bob	1
3	Charlie	2
4	David	2

Explanation:

• Recursive CTE starts with the anchor member (Alice).
• Recursively joins to find subordinates at all levels.

Advantages of CTEs

1. Improved Readability: Break complex queries into named steps.
2. Reusable within Query: The CTE name can be referenced multiple times.
3. Supports Recursion: Useful for hierarchical data.
4. Better than nested subqueries: Simplifies complex filtering, aggregation, or joins.

When to Use Joins, Subqueries, and CTEs

Operation	When to Use	Notes
JOIN	Combining columns from multiple tables in a single query.	Efficient for large datasets; supported in all databases.
Subquery	Filtering based on aggregated or conditional values; existence checks; single-step computations.	Can be correlated (row-by-row) or nested.
CTE	Breaking complex queries into readable steps; recursive hierarchies; multiple references to same intermediate result.	Optimized by the query planner; makes query maintenance easier.

Database Optimization Notes

Feature	PostgreSQL	MySQL	SQL Server	Oracle
JOIN optimization	Uses hash join, merge join, nested loop; well-optimized	Similar; uses nested loops or hash joins	Uses cost-based optimizer; multiple join strategies	Similar; cost-based optimizer
Subquery optimization	Can convert to JOIN internally for efficiency	Correlated subqueries may be slow; sometimes optimized to JOIN	Correlated subqueries optimized to JOIN	Similar; optimizer can flatten subqueries
CTE optimization	Non-recursive CTE may be inlined	Non-recursive CTE inlined (MySQL 8+)	Non-recursive CTE inlined	Non-recursive CTE inlined; recursive executed iteratively
Recursive CTE	Well-supported; optimized	Supported from 8.0; can be slower on large data	Supported; optimized	Supported; can handle large hierarchies

Note:

• Non-recursive CTEs are often just syntactic sugar for derived tables; performance is similar to subqueries.
• Recursive CTEs are unique; they are optimized differently than joins or subqueries.

Key Takeaways

1. CTEs are temporary result sets that improve query clarity and structure.
2. Recursive CTEs are ideal for hierarchical data.
3. Subqueries are better for filtered or aggregated computations.
4. Joins are most efficient for combining tables.
5. Use CTEs when queries are complex, reused multiple times, or require recursion.
6. Databases optimize these differently: CTEs are usually inlined, subqueries may be flattened into joins, and recursive CTEs are iteratively evaluated.