今天和大家聊聊SQL中常見的5個坑，比如子查詢誤用、窗口函數誤用等，希望我們少踩坑，寫出的代碼一次就運行成功且結果正確，哈哈！
錯誤示例：在子查詢中未考慮多行結果返回的情況，導致錯誤或性能問題。
SELECT product_nameFROM productsWHERE price > (SELECT AVG(price) FROM products);
正確示例：使用比較運算符處理多行結果。
SELECT product_name FROM products WHERE price > ANY (SELECT AVG(price) FROM products GROUP BY category);
錯誤示例：在窗口函數中忽略PARTITION BY或ORDER BY，導致結果不準確。
SELECT product_name, SUM(price) OVER () AS total_priceFROM products;
正確示例：使用正確的PARTITION BY和ORDER BY子句。
SELECT product_name, SUM(price) OVER (PARTITION BY category ORDER BY product_name) AS category_total_priceFROM products;
錯誤示例：在復雜的聯結操作中未正確處理表關系，導致錯誤的結果。
SELECT customers.customer_name, orders.order_idFROM customersJOIN order_items ON customers.customer_id = order_items.order_idJOIN orders ON order_items.order_id = orders.order_id;
正確示例：正確處理表關系，使用JOIN ON子句。
SELECT customers.customer_name, orders.order_idFROM customersJOIN orders ON customers.customer_id = orders.customer_idJOIN order_items ON orders.order_id = order_items.order_id;
錯誤示例：嵌套查詢過多，導致查詢性能下降。
SELECT product_nameFROM productsWHERE category_id IN (SELECT category_id FROM categories WHERE department_id IN (SELECT department_id FROM departments WHERE department_name = 'Electronics'));
正確示例：使用JOIN進行更簡潔的查詢。
SELECT product_nameFROM productsJOIN categories ON products.category_id = categories.category_idJOIN departments ON categories.department_id = departments.department_idWHERE departments.department_name = 'Electronics';
錯誤示例：未設置事務隔離級別，可能導致并發問題。
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;BEGIN TRANSACTION;-- 執行一系列操作COMMIT;
正確示例：使用適當的事務隔離級別。
SET TRANSACTION ISOLATION LEVEL READ COMMITTED;BEGIN TRANSACTION;-- 執行一系列操作COMMIT;
該文章在 2024/1/31 12:33:56 編輯過
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