SQL's FOR EACH statement isn't a standard SQL command found in major database systems like MySQL, PostgreSQL, or SQL Server. The functionality often associated with a FOR EACH loop—iterating over rows and performing actions—is typically achieved using other constructs. This article explores common scenarios where developers might think they need a FOR EACH and shows how to achieve the desired results using standard SQL techniques, drawing upon insights from Stack Overflow.
The Myth of FOR EACH in Standard SQL
Many Stack Overflow questions reveal developers coming from procedural programming backgrounds, seeking a direct FOR EACH equivalent in SQL. For example, a question might ask: "How to loop through each row in a SQL table and update a value based on a condition?" While there's no direct FOR EACH statement, the solution usually lies in set-based operations.
Set-Based Operations: The SQL Way
Instead of row-by-row processing (which is generally inefficient in SQL), we use SQL's power to operate on entire sets of data simultaneously. This is where UPDATE statements with WHERE clauses, CASE statements, and sometimes even common table expressions (CTEs) come into play.
Let's illustrate with an example inspired by Stack Overflow discussions regarding updating rows based on conditions. Imagine a table products with columns product_id, price, and discount. We want to apply a 10% discount to products where the price is above $100.
Inefficient Row-by-Row Approach (Conceptual - Avoid This!):
A naive approach might attempt to emulate FOR EACH with procedural code within a stored procedure (this is generally discouraged for performance reasons).
Efficient Set-Based Approach:
UPDATE products
SET discount = CASE
WHEN price > 100 THEN price * 0.10
ELSE 0
END
WHERE price > 100;
This single SQL statement efficiently updates all qualifying rows simultaneously. The CASE statement handles the conditional logic, avoiding the need for any explicit iteration.
Another Example: Row-by-Row Processing in PL/SQL (Oracle)
While standard SQL lacks a FOR EACH statement, procedural extensions within specific database systems do provide looping constructs. In Oracle's PL/SQL, for instance, we might use a cursor to iterate.
DECLARE
CURSOR product_cursor IS
SELECT product_id, price
FROM products
WHERE price > 100;
product_record product_cursor%ROWTYPE;
BEGIN
OPEN product_cursor;
LOOP
FETCH product_cursor INTO product_record;
EXIT WHEN product_cursor%NOTFOUND;
-- Perform actions on each row here, e.g., update discount
UPDATE products
SET discount = product_record.price * 0.10
WHERE product_id = product_record.product_id;
END LOOP;
CLOSE product_cursor;
END;
/
This PL/SQL block achieves row-by-row processing, but its performance is generally less efficient than the set-based approach in pure SQL. The set-based method is typically preferred unless there's a compelling reason to process each row individually (e.g., complex calculations involving data from multiple tables that are difficult to express in a single SQL statement).
Conclusion
While a dedicated FOR EACH statement doesn't exist in standard SQL, its functionality can be efficiently replicated using set-based operations. Understanding this core difference between procedural and declarative programming is crucial for writing high-performing SQL queries. Embrace the power of SQL's set-based operations to avoid unnecessary iteration and improve database performance. Remember to consult your database system's documentation for specific syntax and capabilities, particularly when dealing with procedural extensions like stored procedures and cursors. Remember to always benchmark your solutions to ensure they meet your performance needs.