学ぶ Executing the Plan: Row Processing | Execution and Result Delivery

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When a SQL query is executed, the engine follows the execution plan to process data row by row. The engine begins by accessing the relevant tables and reading each row in turn. For each row, it evaluates any conditions specified in the WHERE clause. Only rows that meet these conditions are passed to the next stage of processing. This filtering step is essential for reducing the amount of data that moves through the rest of the query.


              1234
            
-- Select employees with a salary greater than 60000
SELECT first_name, last_name, salary
FROM employees
WHERE salary > 60000;

As the engine processes each row, it checks whether the row satisfies the condition in the WHERE clause. If the condition is met, the row is included in the result set; if not, the row is skipped. This row-by-row evaluation continues until all rows in the source table have been examined. The result set at this stage contains only those rows that passed the filter.


              1234
            
-- Select employees and sort them by salary in descending order
SELECT first_name, last_name, salary
FROM employees
ORDER BY salary DESC;

1. What happens to each row during the execution phase of a SQL query?

2. Which clause determines the order of rows in the final result set?

3. Fill in the blank: The execution engine applies ________ to determine which rows are returned.

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