GROUP BY

引入或更新于：v1.2.32

GROUP BY 子句用于根据相同的分组表达式对行进行分组，并对每个结果组应用聚合函数。分组表达式可包含列名或别名、SELECT 列表中位置的数值引用、通用表达式，或 SELECT 列表中的所有非聚合项。

Databend 的 GROUP BY 子句提供以下扩展功能，支持更全面的数据分组和灵活分析：

• GROUP BY CUBE
• GROUP BY GROUPING SETS
• GROUP BY ROLLUP

语法

SELECT ...
    FROM ...
    [ ... ]
GROUP BY [ ALL | groupItem [ , groupItem [ , ... ] ] ]
    [ ... ]

参数说明：

• ALL：使用该关键字时，Databend 会按 SELECT 列表中的所有非聚合项分组
• groupItem：分组项可为以下任意类型：
  • SELECT 列表中定义的列名或别名
  • SELECT 列表中列位置的数值引用
  • 涉及当前查询表列的任意表达式

示例

本节示例基于以下数据准备：

-- 创建示例表 "employees"
CREATE TABLE employees (
    id INT,
    first_name VARCHAR(50),
    last_name VARCHAR(50),
    department_id INT,
    job_id INT,
    hire_date DATE
);

-- 向 "employees" 表插入数据
INSERT INTO employees (id, first_name, last_name, department_id, job_id, hire_date)
VALUES (1, 'John', 'Doe', 1, 101, '2021-01-15'),
       (2, 'Jane', 'Smith', 1, 101, '2021-02-20'),
       (3, 'Alice', 'Johnson', 1, 102, '2021-03-10'),
       (4, 'Bob', 'Brown', 2, 201, '2021-03-15'),
       (5, 'Charlie', 'Miller', 2, 202, '2021-04-10'),
       (6, 'Eve', 'Davis', 2, 202, '2021-04-15');

单列分组

按 department_id 分组统计各部门员工数：

SELECT department_id, COUNT(*) AS num_employees
FROM employees
GROUP BY department_id;

输出：

+---------------+---------------+
| department_id | num_employees |
+---------------+---------------+
|             1 |             3 |
|             2 |             3 |
+---------------+---------------+

多列分组

按 department_id 和 job_id 分组统计各组合员工数：

SELECT department_id, job_id, COUNT(*) AS num_employees
FROM employees
GROUP BY department_id, job_id;

输出：

+---------------+--------+---------------+
| department_id | job_id | num_employees |
+---------------+--------+---------------+
|             1 |    101 |             2 |
|             1 |    102 |             1 |
|             2 |    201 |             1 |
|             2 |    202 |             2 |
+---------------+--------+---------------+

ALL 分组

使用 GROUP BY ALL 按 SELECT 列表所有非聚合列分组：

SELECT department_id, job_id, COUNT(*) AS num_employees
FROM employees
GROUP BY ALL;

输出：

+---------------+--------+---------------+
| department_id | job_id | num_employees |
+---------------+--------+---------------+
|             1 |    101 |             2 |
|             1 |    102 |             1 |
|             2 |    201 |             1 |
|             2 |    202 |             2 |
+---------------+--------+---------------+

位置分组

按 SELECT 列表第 1 位（department_id）分组：

SELECT department_id, COUNT(*) AS num_employees
FROM employees
GROUP BY 1;

输出：

+---------------+---------------+
| department_id | num_employees |
+---------------+---------------+
|             1 |             3 |
|             2 |             3 |
+---------------+---------------+

表达式分组

按入职年份分组统计每年招聘人数：

SELECT EXTRACT(YEAR FROM hire_date) AS hire_year, COUNT(*) AS num_hires
FROM employees
GROUP BY EXTRACT(YEAR FROM hire_date);

输出：

+-----------+-----------+
| hire_year | num_hires |
+-----------+-----------+
|      2021 |         6 |
+-----------+-----------+