Apache Arrow DataFusion SQL查询语法详解

Apache Arrow DataFusion SQL查询语法详解

arrow-datafusion Apache Arrow DataFusion SQL Query Engine 项目地址: https://gitcode.com/gh_mirrors/arr/arrow-datafusion

概述

Apache Arrow DataFusion是一个高性能的查询引擎，它提供了丰富的SQL查询功能。本文将深入解析DataFusion中的SELECT语句语法，帮助开发者掌握其核心查询能力。

基础语法结构

DataFusion的SELECT查询遵循标准SQL语法，但有一些特定的实现细节需要注意：

[WITH with_query [, ...]]
SELECT [ALL | DISTINCT] select_expr [, ...]
[FROM from_item [, ...]]
[JOIN join_item [, ...]]
[WHERE condition]
[GROUP BY grouping_element [, ...]]
[HAVING condition]
[UNION [ALL | select]]
[ORDER BY expression [ASC | DESC] [, ...]]
[LIMIT count]
[EXCLUDE | EXCEPT]

关键子句详解

WITH子句（公共表表达式）

WITH子句允许为子查询命名，并在后续查询中引用这些名称：

WITH department_stats AS (
    SELECT department, AVG(salary) AS avg_salary 
    FROM employees 
    GROUP BY department
)
SELECT * FROM department_stats WHERE avg_salary > 5000;

SELECT子句

SELECT子句指定要返回的列，支持DISTINCT去重：

-- 返回所有行
SELECT employee_id, name, salary FROM employees;

-- 返回去重后的结果
SELECT DISTINCT department FROM employees;

FROM子句

FROM子句指定数据源，支持表别名：

SELECT e.name, d.department_name 
FROM employees AS e 
JOIN departments AS d ON e.department_id = d.id;

WHERE子句

WHERE子句用于过滤行：

SELECT * FROM products WHERE price > 100 AND stock > 0;

JOIN操作

DataFusion支持多种JOIN类型：

内连接(INNER JOIN)

SELECT o.order_id, c.customer_name
FROM orders o
INNER JOIN customers c ON o.customer_id = c.id;

左外连接(LEFT OUTER JOIN)

SELECT e.name, d.department_name
FROM employees e
LEFT JOIN departments d ON e.department_id = d.id;

右外连接(RIGHT OUTER JOIN)

SELECT e.name, d.department_name
FROM employees e
RIGHT JOIN departments d ON e.department_id = d.id;

全外连接(FULL OUTER JOIN)

SELECT e.name, d.department_name
FROM employees e
FULL OUTER JOIN departments d ON e.department_id = d.id;

自然连接(NATURAL JOIN)

SELECT * FROM employees NATURAL JOIN departments;

交叉连接(CROSS JOIN)

SELECT * FROM colors CROSS JOIN sizes;

GROUP BY子句

GROUP BY用于分组聚合，支持带排序的聚合函数：

-- 基本分组
SELECT department, COUNT(*) AS employee_count 
FROM employees 
GROUP BY department;

-- 带排序的聚合
SELECT department, ARRAY_AGG(name ORDER BY hire_date) AS employees
FROM employees
GROUP BY department;

HAVING子句

HAVING用于过滤分组结果：

SELECT department, AVG(salary) AS avg_salary
FROM employees
GROUP BY department
HAVING AVG(salary) > 5000;

UNION操作

合并多个查询结果：

SELECT product_id FROM current_products
UNION ALL
SELECT product_id FROM discontinued_products;

ORDER BY子句

排序结果集：

-- 升序排序
SELECT name, salary FROM employees ORDER BY salary;

-- 降序排序
SELECT name, salary FROM employees ORDER BY salary DESC;

-- 多列排序
SELECT name, department, salary 
FROM employees 
ORDER BY department ASC, salary DESC;

LIMIT子句

限制返回行数：

SELECT * FROM large_table LIMIT 100;

EXCLUDE/EXCEPT子句

排除特定列：

-- 排除敏感信息
SELECT * EXCLUDE(salary, ssn) FROM employees;

-- 等同于
SELECT * EXCEPT(salary, ssn) FROM employees;

注意事项

1. 大小写敏感性：DataFusion在查询时会自动将列名转为小写，但如果模式中定义了大小写混合的列名，需要使用双引号引用。

2. 性能考虑：复杂的JOIN操作和聚合函数可能影响查询性能，建议在大型数据集上谨慎使用。

3. 语法兼容性：虽然DataFusion支持标准SQL语法，但某些数据库特有的语法可能不被支持。

通过掌握这些SQL查询语法，开发者可以充分利用DataFusion的强大功能进行高效的数据分析和处理。

arrow-datafusion Apache Arrow DataFusion SQL Query Engine 项目地址: https://gitcode.com/gh_mirrors/arr/arrow-datafusion