mysql 多个表union查询

SELECT name, role FROM employees
UNION
SELECT name, role FROM contractors;

-- 错误示例
SELECT name, role FROM employees
UNION
SELECT name FROM contractors; -- 列数不匹配

-- 正确示例
SELECT name, role FROM employees
UNION
SELECT name, 'contractor' AS role FROM contractors; -- 添加默认值

-- 错误示例
SELECT name, role FROM employees
UNION
SELECT name, salary FROM contractors; -- 数据类型不匹配

-- 正确示例
SELECT name, role FROM employees
UNION
SELECT name, CAST(salary AS VARCHAR(255)) AS role FROM contractors; -- 转换数据类型

-- 示例：添加 WHERE 子句进行过滤
SELECT name, role FROM employees WHERE department = 'Sales'
UNION
SELECT name, role FROM contractors WHERE department = 'Sales';