sql loop

Galaxy Glossary

How can I iterate through rows in a SQL table?

SQL doesn't have explicit looping constructs like `for` or `while` found in programming languages. Instead, it achieves iterative processing through techniques like cursors and procedural languages (like PL/SQL). These methods allow you to process data row by row, but they are less common than set-based operations.

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Description

SQL, at its core, is a set-based language. It excels at manipulating entire datasets at once. Unlike procedural languages, it doesn't have built-in looping mechanisms. While you can't directly loop through rows in a standard SQL query, you can achieve similar results using cursors or procedural extensions like PL/SQL. Cursors are essentially pointers to rows in a result set, allowing you to fetch and process them one at a time. Procedural languages provide more control and flexibility, enabling complex logic and data manipulation within the database environment. This approach is often used when you need to perform actions that aren't easily expressed in a set-based query, such as updating multiple rows based on conditions or performing complex calculations on each row.

Why sql loop is important

While SQL's set-based approach is generally preferred, understanding cursors and procedural languages is crucial for handling complex data manipulation tasks that require row-by-row processing. This knowledge is valuable for tasks like complex data transformations, custom reporting, and integration with other applications.

Example Usage

```sql -- Using a cursor to update records DECLARE CURSOR emp_cursor IS SELECT employee_id, salary FROM employees WHERE department_id = 10; v_emp_id employees.employee_id%TYPE; v_salary employees.salary%TYPE; BEGIN OPEN emp_cursor; LOOP FETCH emp_cursor INTO v_emp_id, v_salary; EXIT WHEN emp_cursor%NOTFOUND; UPDATE employees SET salary = v_salary * 1.1 WHERE employee_id = v_emp_id; END LOOP; CLOSE emp_cursor; END; / ```

Common Mistakes

Trying to use programming language loops directly within SQL queries.
Not understanding the limitations of cursors, such as performance issues with large datasets.
Incorrectly using cursor variables or failing to handle exceptions during cursor processing.

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