MySQL之使用UNION和UNION ALL合并两个或多个SELECT语句的结果集

SELECT column1, column2, ...
FROM table1
WHERE condition
UNION
SELECT column1, column2, ...
FROM table2
WHERE condition;

-- 创建 employees1 表
CREATE TABLE employees1 (
    employee_id INT,
    first_name VARCHAR(50),
    last_name VARCHAR(50)
);

-- 插入数据到 employees1 表
INSERT INTO employees1 VALUES
(1, 'John', 'Doe'),
(2, 'Jane', 'Smith'),
(3, 'Bob', 'Johnson'),
(4, 'Alice', 'Doe');

-- 创建 employees2 表
CREATE TABLE employees2 (
    employee_id INT,
    first_name VARCHAR(50),
    last_name VARCHAR(50)
);

-- 插入数据到 employees2 表，包含一些与 employees1 表重复的数据
INSERT INTO employees2 VALUES
(3, 'Bob', 'Johnson'),
(4, 'Alice', 'Doe'),
(5, 'Eva', 'Williams');

-- 使用 UNION 合并，去除重复行
SELECT employee_id, first_name, last_name
FROM employees1
UNION
SELECT employee_id, first_name, last_name
FROM employees2;

+-------------+------------+-----------+
| employee_id | first_name | last_name |
+-------------+------------+-----------+
| 1           | John       | Doe       |
| 2           | Jane       | Smith     |
| 3           | Bob        | Johnson   |
| 4           | Alice      | Doe       |
| 5           | Eva        | Williams  |
+-------------+------------+-----------+

-- 使用 UNION ALL 合并，包含重复行
SELECT employee_id, first_name, last_name
FROM employees1
UNION ALL
SELECT employee_id, first_name, last_name
FROM employees2;

+-------------+------------+-----------+
| employee_id | first_name | last_name |
+-------------+------------+-----------+
| 1           | John       | Doe       |
| 2           | Jane       | Smith     |
| 3           | Bob        | Johnson   |
| 4           | Alice      | Doe       |
| 3           | Bob        | Johnson   |
| 4           | Alice      | Doe       |
| 5           | Eva        | Williams  |
+-------------+------------+-----------+