MySQL CTE (Common Table Expressions) 详解

CTE (Common Table Expression，公共表表达式) 是 MySQL 8.0 引入的重要特性，它允许在查询中创建临时命名结果集，提高复杂查询的可读性和可维护性。

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基本语法

WITH cte_name AS (SELECT ...  -- CTE查询定义
)
SELECT * FROM cte_name;  -- 主查询

CTE 主要特点

1. 临时结果集：只在查询执行期间存在

2. 可引用性：可以在主查询中多次引用

3. 可读性强：比嵌套子查询更易理解

4. 递归支持：支持递归查询（MySQL 8.0+）

非递归 CTE

简单 CTE 示例

WITH department_stats AS (SELECT department_id, COUNT(*) as employee_count,AVG(salary) as avg_salaryFROM employeesGROUP BY department_id
)
SELECT d.department_name,ds.employee_count,ds.avg_salary
FROM departments d
JOIN department_stats ds ON d.department_id = ds.department_id;

多 CTE 示例

WITH 
high_earners AS (SELECT * FROM employees WHERE salary > 100000
),
it_employees AS (SELECT * FROM employees WHERE department_id = 10
)
SELECT h.employee_id,h.name,'High Earner' as category
FROM high_earners h
UNION ALL
SELECT i.employee_id,i.name,'IT Employee' as category
FROM it_employees i;

递归 CTE

递归 CTE 可以处理层次结构数据，如组织结构、评论树等。

基本递归 CTE 结构

WITH RECURSIVE cte_name AS (-- 基础部分（种子查询）SELECT ... WHERE ...UNION [ALL]-- 递归部分SELECT ... FROM cte_name JOIN ...WHERE ...
)
SELECT * FROM cte_name;

递归 CTE 示例：组织结构查询

WITH RECURSIVE org_hierarchy AS (-- 基础部分：查找顶级管理者SELECT employee_id,name,manager_id,1 as levelFROM employeesWHERE manager_id IS NULLUNION ALL-- 递归部分：查找下属员工SELECT e.employee_id,e.name,e.manager_id,oh.level + 1FROM employees eJOIN org_hierarchy oh ON e.manager_id = oh.employee_id
)
SELECT * FROM org_hierarchy ORDER BY level, employee_id;

递归 CTE 示例：生成序列

WITH RECURSIVE number_sequence AS (SELECT 1 AS nUNION ALLSELECT n + 1 FROM number_sequence WHERE n < 10
)
SELECT * FROM number_sequence;

CTE 的优势

1. 提高可读性：将复杂查询分解为逻辑块

2. 避免重复：可以多次引用同一个CTE

3. 替代视图：不需要创建永久视图

4. 递归能力：处理层次结构数据

5. 更好的优化：MySQL优化器能更好处理CTE

CTE 与派生表的比较

特性	CTE	派生表
可读性	高	低
可重用性	可在查询中多次引用	每次使用都需要重新定义
递归支持	支持	不支持
性能	通常更好	可能较差
语法清晰度	更清晰	嵌套较深时难以理解

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实际应用场景

1. 数据报表：构建复杂报表的多步数据处理

   WITH 
   monthly_sales AS (SELECT DATE_FORMAT(order_date, '%Y-%m') as month,SUM(amount) as total_salesFROM ordersGROUP BY month
   ),
   growth_rate AS (SELECT month,total_sales,LAG(total_sales) OVER (ORDER BY month) as prev_sales,(total_sales - LAG(total_sales) OVER (ORDER BY month)) / LAG(total_sales) OVER (ORDER BY month) * 100 as growth_pctFROM monthly_sales
   )
   SELECT * FROM growth_rate;

2. 数据清洗：多步数据转换

   WITH 
   raw_data AS (SELECT * FROM source_table WHERE quality_check = 1
   ),
   cleaned_data AS (SELECT id,TRIM(name) as name,CASE WHEN age < 0 THEN NULL ELSE age END as ageFROM raw_data
   )
   SELECT * FROM cleaned_data;

3. 路径查找：图数据查询

   WITH RECURSIVE path_finder AS (SELECT start_node as path,start_node,end_node,1 as lengthFROM graphWHERE start_node = 'A'UNION ALLSELECT CONCAT(pf.path, '->', g.end_node),g.start_node,g.end_node,pf.length + 1FROM graph gJOIN path_finder pf ON g.start_node = pf.end_nodeWHERE FIND_IN_SET(g.end_node, REPLACE(pf.path, '->', ',')) = 0
   )
   SELECT * FROM path_finder;

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性能考虑

1. 物化：MySQL可能会物化CTE结果

2. 递归深度：默认递归深度限制为1000，可通过cte_max_recursion_depth参数调整

   SET SESSION cte_max_recursion_depth = 2000;

3. 优化器提示：可以使用提示影响CTE处理

   WITH cte_name AS (SELECT /*+ MERGE() */ * FROM table_name
   )
   SELECT * FROM cte_name;

限制

1. MySQL 8.0 之前版本不支持CTE

2. 某些复杂递归查询可能有性能问题

3. 在存储过程和函数中使用有限制

CTE是MySQL中处理复杂查询的强大工具，合理使用可以显著提高SQL代码的可读性和维护性。