Developers often face a common challenge when writing SQL: choosing the right tool for the job. While the world of SQL offers many options, understanding the core differences between subqueries, Common Table Expressions (CTEs), and stored procedures is key to writing clean, efficient code.
Subqueries
A subquery is a query nested inside another query. They are executed first and their result is then used by the outer query. They are simple to use for one-off tasks but can become hard to read and manage in complex scenarios.
Scope: The subquery is only visible to the outer query that contains it. It cannot be referenced by other parts of the script.
Reusability: Very low. A subquery's logic must be rewritten every time it's needed.
Best Use Case: Simple, single-use queries where the result of an inner query is needed for a single value or list in the outer query.
Example; finding all employees whose salary is above the average.
SELECT
first_name,
last_name,
salary
FROM
Employees
WHERE
salary > (SELECT AVG(salary) FROM Employees);
The inner query finds the average salary, and the main query uses that single value to find the right employees. While effective for simple tasks, if you start nesting them deeper, your code can become very difficult to read and manage.
Common Table Expressions (CTEs)
A CTE is a named, temporary result set defined within a single SQL statement. They are introduced by the WITH keyword and help to break down complex queries into logical, readable parts.
Scope: A CTE is only accessible within the query that defines it.
Reusability: A single CTE can be referenced multiple times within the same query, but not by other queries.
Best Use Case: Improving the readability of complex, multi-step queries, or for recursive queries (where a query references itself). CTEs are often used as an alternative to complex subqueries or temporary tables
Example: The same query, rewritten with a CTE.
Here, we define a CTE named AvgSalary to hold the average salary.
WITH AvgSalary AS (
SELECT AVG(salary) AS avg_sal
FROM Employees
)
SELECT
e.first_name,
e.last_name,
e.salary
FROM
Employees e,
AvgSalary a
WHERE
e.salary > a.avg_sal;
When to use it: When you have a complex query that is difficult to read. CTEs are excellent for improving readability and are a go-to for recursive queries.
The downside: CTEs are temporary and are scoped to a single query. You cannot reference them from a different query or script.
Stored Procedures
A stored procedure is a pre-compiled set of one or more SQL statements stored on the database server. They are like functions in a programming language, accepting parameters and executing complex business logic.
Scope: Stored procedures are objects in the database schema and can be called from any application or query that has permission.
Reusability: High. They can be executed multiple times from anywhere in the database or an application, reducing code duplication.
Best Use Case: Encapsulating complex business logic, performing repetitive tasks, or improving performance by reducing network traffic and using pre-compiled execution plans.
Example: Creating a stored procedure to get employee details by department.
First, you create the procedure.
CREATE PROCEDURE GetEmployeesByDepartment
@DepartmentName VARCHAR(50)
AS
BEGIN
SELECT
first_name,
last_name,
salary
FROM
Employees
WHERE
department = @DepartmentName;
END;
Then, you can execute it with a simple command, eliminating the need to write the query every time.
EXEC GetEmployeesByDepartment 'Sales';
When to use it: For any task that needs to be performed over and over again, for business logic that should be centralized, or for improving performance and security.
The upside: Stored procedures are highly reusable, reduce network traffic (only the EXEC command is sent), and can improve performance because the database creates an optimized execution plan the first time it runs.
Conclusion
The best choice depends on the job at hand. For a quick, simple solution within a single query, a subquery is a good choice. If your query is getting complicated and you want to improve its readability, use a CTE to break it down. For any task that needs to be repeated, is critical to your business logic, or requires top performance, a stored procedure is the ideal solution. Mastering all 3 helps in writing better SQL codes.
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