Answer

In most SQL implementations (e.g., T-SQL, PL/pgSQL, PL/SQL), control structures are available: conditions (IF/CASE), loops (WHILE/FOR/LOOP), error handling constructs. They are convenient for automating business logic on the server—such as creating complex reports, validating business rules, orchestrating ETL processes, and automating event responses.

However, excessive use of loops and complex conditions within SQL procedures can lead to decreased readability and performance (turning SQL into "procedural hell").

It is recommended to use these constructs only for operations that cannot be implemented using set-based processing: for example, processing one record at a time with dynamic logic.

Example of a well-written procedure (T-SQL)

CREATE PROCEDURE ProcessOrders
AS
BEGIN
    DECLARE @OrderID INT, @Total FLOAT
    DECLARE OrderCursor CURSOR FOR
        SELECT OrderID FROM Orders WHERE Status = 'NEW'
    OPEN OrderCursor
    FETCH NEXT FROM OrderCursor INTO @OrderID
    WHILE @@FETCH_STATUS = 0
    BEGIN
        SELECT @Total = SUM(Price) FROM OrderItems WHERE OrderID = @OrderID
        IF @Total > 10000
            UPDATE Orders SET Status='MANUAL_CHECK' WHERE OrderID=@OrderID
        ELSE
            UPDATE Orders SET Status='APPROVED' WHERE OrderID=@OrderID
        FETCH NEXT FROM OrderCursor INTO @OrderID
    END
    CLOSE OrderCursor
    DEALLOCATE OrderCursor
END

Trick question

Does CASE in SELECT replace nested IF conditions in procedures, and can they be interchangeable?

Answer: No. CASE can only be applied in SELECT/UPDATE/INSERT expressions—for computed fields. IF is only in control blocks of procedures/functions/triggers and is not allowed in "pure" SELECT.

Example:

SELECT CASE WHEN score > 80 THEN 'High' ELSE 'Low' END AS Level FROM students
-- IF is not possible here

Examples of real errors

---

Story

The team tried to rewrite the entire business process from code into SQL procedures with a large amount of nested IF/ELSE and loops. The result was tangled, untestable code that was almost impossible to maintain. They reverted to moving logic back into the application, leaving only critical procedures.

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Story

In PL/pgSQL, due to improper use of LOOP without a proper exit (EXIT) in the stored procedure, a "infinite" loop formed, blocking the connection to the database. This was resolved by implementing a loop iteration limit and proper exit conditions.

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Story

An error in the logic of a nested IF in the order processing procedure: there was no ELSE construct. Some orders remained in an incorrect status (NULL), lacking subsequent validation. A specific ELSE with explicit default behavior was added.