SQL中常见的5个坑，你一定踩过！
|
admin
2024年1月30日 16:18
本文热度 503
|
今天和大家聊聊SQL中常见的5个坑，比如子查询误用、窗口函数误用等，希望我们少踩坑，写出的代码一次就运行成功且结果正确，哈哈！
错误示例：在子查询中未考虑多行结果返回的情况，导致错误或性能问题。
SELECT product_nameFROM productsWHERE price > (SELECT AVG(price) FROM products);
正确示例：使用比较运算符处理多行结果。
SELECT product_name FROM products WHERE price > ANY (SELECT AVG(price) FROM products GROUP BY category);
错误示例：在窗口函数中忽略PARTITION BY或ORDER BY，导致结果不准确。
SELECT product_name, SUM(price) OVER () AS total_priceFROM products;
正确示例：使用正确的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;
错误示例：在复杂的联结操作中未正确处理表关系，导致错误的结果。
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;
正确示例：正确处理表关系，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;
错误示例：嵌套查询过多，导致查询性能下降。
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'));
正确示例：使用JOIN进行更简洁的查询。
SELECT product_nameFROM productsJOIN categories ON products.category_id = categories.category_idJOIN departments ON categories.department_id = departments.department_idWHERE departments.department_name = 'Electronics';
错误示例：未设置事务隔离级别，可能导致并发问题。
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;BEGIN TRANSACTION;-- 执行一系列操作COMMIT;
正确示例：使用适当的事务隔离级别。
SET TRANSACTION ISOLATION LEVEL READ COMMITTED;BEGIN TRANSACTION;-- 执行一系列操作COMMIT;
该文章在 2024/1/31 12:33:56 编辑过
400 186 1886
