SQL经典查询

1. 查询不在表里的数据，一张学生表，一张学生的选课表，要求查出没有选课的学生？

select students.student_name from students left join course_selection on students.student_id=course_selection.student_id where course_selection.student_id is null

2. 查找第N高的数据，查找课程编号为”01“的成绩第三高的学生，如果不存在则返回null

select IFNULL((select scores.score from scores order by scores.score desc limit 1 offset 2) ,null) as "第三高的成绩"

3. 分组排序，按成绩从大到小排序如80，80，76，70，50 对应的排序为1，1，3，4，5

select  *,RANK() over(ORDER BY scores.score DESC) as "排名" from scores

4. 连续出现N次问题，学生连续3个学号相邻的学生出现年龄相同的年龄

select distinct a.age from students a,students b,students c where a.student_id=b.student_id+1 and b.student_id=c.student_id+1 and a.age=b.age and b.age=c.age 

常见知识点：

1. 多层嵌套子查询 + 聚合函数

题目：查询订单总额高于该客户平均订单金额的所有订单

SELECT order_id, customer_id, total_amount
FROM orders o1
WHERE total_amount > (
    SELECT AVG(total_amount)
    FROM orders o2
    WHERE o2.customer_id = o1.customer_id
);

考点：关联子查询、聚合函数、比较运算符

---

2. 多表JOIN + 分组统计

题目：查询每个部门的最高薪员工信息

SELECT d.dept_name, e.emp_name, e.salary
FROM departments d
JOIN employees e ON d.dept_id = e.dept_id
WHERE e.salary = (
    SELECT MAX(salary)
    FROM employees
    WHERE dept_id = d.dept_id
);

考点：内连接、相关子查询、分组极值

---

3. 窗口函数应用

题目：查询每个部门薪资排名前三的员工

SELECT dept_id, emp_name, salary 
FROM (
    SELECT *,
           DENSE_RANK() OVER (PARTITION BY dept_id ORDER BY salary DESC) as rk
    FROM employees
) t
WHERE rk <= 3;

考点：窗口函数、排名函数、子查询

---

4. 递归查询层级数据

题目：查询某员工的所有下级（包含N级）

WITH RECURSIVE emp_tree AS (
    SELECT emp_id, emp_name, manager_id
    FROM employees
    WHERE emp_id = 1001  -- 指定上级ID
    UNION ALL
    SELECT e.emp_id, e.emp_name, e.manager_id
    FROM employees e
    JOIN emp_tree et ON e.manager_id = et.emp_id
)
SELECT * FROM emp_tree;

考点：CTE递归查询、树形结构处理

---

5. 行转列动态查询

题目：动态生成各月销售额的列式报表

SET @sql = NULL;
SELECT
  GROUP_CONCAT(DISTINCT
    CONCAT(
      'SUM(CASE WHEN MONTH(order_date) = ',
      month,
      ' THEN amount ELSE 0 END) AS `',
      month_name, '`'
    )
  ) INTO @sql
FROM (
    SELECT MONTH(order_date) month, 
           DATE_FORMAT(order_date, '%b') month_name
    FROM orders
    GROUP BY 1,2
) m;

SET @sql = CONCAT('SELECT product_id, ', @sql, 
                 ' FROM orders GROUP BY product_id');

PREPARE stmt FROM @sql;
EXECUTE stmt;

考点：动态SQL、PIVOT转换、GROUP_CONCAT函数

---

6. 复杂日期处理

题目：查询连续3天登录的用户

SELECT DISTINCT a.user_id
FROM login_log a
JOIN login_log b ON a.user_id = b.user_id 
    AND b.login_date = DATE_ADD(a.login_date, INTERVAL 1 DAY)
JOIN login_log c ON a.user_id = c.user_id 
    AND c.login_date = DATE_ADD(a.login_date, INTERVAL 2 DAY);

考点：日期函数、自连接、连续性问题

---

7. 存在性检查

题目：查询购买了所有品类商品的客户

SELECT customer_id
FROM orders
GROUP BY customer_id
HAVING COUNT(DISTINCT category_id) = (
    SELECT COUNT(DISTINCT category_id) FROM products
);

考点：HAVING子句、集合运算、全量存在判断

---

8. 分页性能优化

题目：高效实现千万级数据分页

SELECT id, name, create_time
FROM large_table
WHERE create_time > '2023-01-01'
ORDER BY create_time DESC, id DESC
LIMIT 10 OFFSET 100000;

考点：分页优化、索引设计、排序字段选择

---

9. 多重条件聚合

题目：统计各商品不同价格区间的销量

SELECT product_id,
       SUM(CASE WHEN price < 100 THEN 1 ELSE 0 END) AS low_price,
       SUM(CASE WHEN price BETWEEN 100 AND 500 THEN 1 ELSE 0 END) AS mid_price,
       SUM(CASE WHEN price > 500 THEN 1 ELSE 0 END) AS high_price
FROM orders
GROUP BY product_id;

考点：条件聚合、CASE表达式

---

10. 数据去重保留最新

题目：删除重复订单（保留最新记录）

DELETE FROM orders
WHERE order_id NOT IN (
    SELECT MAX(order_id)
    FROM orders
    GROUP BY customer_id, product_id, order_date
);

考点：数据去重、保留极值、子查询删除

---

11. 多结果集合并

题目：合并新老系统用户表（去重）

SELECT user_id, user_name FROM old_users
UNION 
SELECT user_id, user_name FROM new_users;

考点：集合操作、UNION去重

---

12. 事务并发控制

题目：实现库存安全扣减

START TRANSACTION;
SELECT stock FROM products WHERE id=1001 FOR UPDATE;
UPDATE products SET stock = stock - 1 WHERE id=1001;
COMMIT;

考点：事务隔离、悲观锁、并发控制

---