41 lines
1.5 KiB
Markdown
41 lines
1.5 KiB
Markdown
---
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title: SQL高阶用法
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date: 2025-12-21
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---
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# 一、WITH
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## 1.1 定义
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  SQL 中的WITH子句也被称为**公用表表达式**（CTE，Common Table Expression），它的作用是**在执行主查询之前，先定义一个临时的结果集，这个结果集可以在后续的查询中被多次引用**，就像一个临时表一样。它能让复杂的 SQL 查询变得更清晰、更易读，还能简化嵌套查询的逻辑。
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## 1.2 使用
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```sql
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-- 定义第一个CTE：数学高分学生
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WITH math_high_score AS (
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SELECT name, score FROM student_score WHERE subject = '数学' AND score > 85
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),
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-- 定义第二个CTE：语文高分学生
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chinese_high_score AS (
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SELECT name, score FROM student_score WHERE subject = '语文' AND score > 85
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)
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-- 主查询：查询既在数学高分又在语文高分的学生
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SELECT m.name
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FROM math_high_score m
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JOIN chinese_high_score c ON m.name = c.name;
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```
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  递归 CTE：
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```sql
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WITH recursive dept_hierarchy AS (
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-- 锚点成员：查询顶级部门（parent_id为NULL）
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SELECT dept_id, dept_name, parent_id, 1 AS level
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FROM department
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WHERE parent_id IS NULL
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UNION ALL
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-- 递归成员：查询子部门，关联自身的dept_id和parent_id
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SELECT d.dept_id, d.dept_name, d.parent_id, dh.level + 1 AS level
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FROM department d
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JOIN dept_hierarchy dh ON d.parent_id = dh.dept_id
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)
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-- 主查询：获取所有部门的层级
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SELECT * FROM dept_hierarchy;
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``` |