今天和大家聊聊SQL中常見的5個(gè)坑，比如子查詢誤用、窗口函數(shù)誤用等，希望我們少踩坑，寫出的代碼一次就運(yùn)行成功且結(jié)果正確，哈哈！
錯(cuò)誤示例：在子查詢中未考慮多行結(jié)果返回的情況，導(dǎo)致錯(cuò)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(yùn)算符處理多行結(jié)果。
SELECT product_name FROM products WHERE price > ANY (SELECT AVG(price) FROM products GROUP BY category);
錯(cuò)誤示例：在窗口函數(shù)中忽略PARTITION BY或ORDER BY，導(dǎo)致結(jié)果不準(zhǔn)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;
3. 復(fù)雜的聯(lián)結(jié)邏輯錯(cuò)誤
錯(cuò)誤示例：在復(fù)雜的聯(lián)結(jié)操作中未正確處理表關(guān)系，導(dǎo)致錯(cuò)誤的結(jié)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(guān)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(cuò)誤示例：嵌套查詢過多，導(dǎo)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(jìn)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(cuò)誤示例：未設(shè)置事務(wù)隔離級別，可能導(dǎo)致并發(fā)問題。
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;BEGIN TRANSACTION;-- 執(zhí)行一系列操作COMMIT;
正確示例：使用適當(dāng)?shù)氖聞?wù)隔離級別。
SET TRANSACTION ISOLATION LEVEL READ COMMITTED;BEGIN TRANSACTION;-- 執(zhí)行一系列操作COMMIT;
該文章在 2024/1/31 12:33:56 編輯過
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