我要评论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。
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
- 临时结果集：只在查询执行期间存在
- 可引用性：可以在主查询中多次引用
- 可读性强：比嵌套子查询更易理解
- 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_salary
from employees
group 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 level
from employees
where manager_id is null
union all
-- 递归部分：查找下属员工
select
e.employee_id,
e.name,
e.manager_id,
oh.level + 1
from employees e
join 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 n
union all
select n + 1 from number_sequence where n < 10
)
select * from number_sequence;cte 的优势
- 提高可读性：将复杂查询分解为逻辑块
- 避免重复：可以多次引用同一个cte
- 替代视图：不需要创建永久视图
- 递归能力：处理层次结构数据
- 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 |
实际应用场景
- 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_sales
from orders
group 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_pct
from monthly_sales
)
select * from growth_rate;- 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 age
from raw_data
)
select * from cleaned_data;- 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 length
from graph
where start_node = 'a'
union all
select
concat(pf.path, '->', g.end_node),
g.start_node,
g.end_node,
pf.length + 1
from graph g
join path_finder pf on g.start_node = pf.end_node
where find_in_set(g.end_node, replace(pf.path, '->', ',')) = 0
)
select * from path_finder;性能考虑
- 物化：mysql可能会物化cte结果
- 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;
- 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
- mysql 8.0 之前版本不支持cte
- 某些复杂递归查询可能有性能问题
- 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。
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