mysql 创建表union

SELECT name, age FROM students
UNION
SELECT name, age FROM teachers;

-- 错误示例
SELECT name, age FROM students
UNION
SELECT name FROM teachers; -- 列数不匹配

-- 正确示例
SELECT name, age FROM students
UNION
SELECT name, age FROM teachers; -- 列数匹配

-- 错误示例
SELECT name, age FROM students
UNION
SELECT name, salary FROM teachers; -- 数据类型不兼容

-- 正确示例
SELECT name, age FROM students
UNION
SELECT name, CAST(salary AS UNSIGNED) AS age FROM teachers; -- 数据类型兼容

SELECT name, age FROM students
UNION ALL
SELECT name, age FROM teachers;

-- 创建示例表
CREATE TABLE students (
    id INT PRIMARY KEY,
    name VARCHAR(50),
    age INT
);

CREATE TABLE teachers (
    id INT PRIMARY KEY,
    name VARCHAR(50),
    age INT
);

-- 插入示例数据
INSERT INTO students (id, name, age) VALUES (1, 'Alice', 20);
INSERT INTO students (id, name, age) VALUES (2, 'Bob', 22);
INSERT INTO teachers (id, name, age) VALUES (1, 'Charlie', 40);
INSERT INTO teachers (id, name, age) VALUES (2, 'David', 45);

-- 使用 UNION
SELECT name, age FROM students
UNION
SELECT name, age FROM teachers;

-- 使用 UNION ALL
SELECT name, age FROM students
UNION ALL
SELECT name, age FROM teachers;