SQL Union函数

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

SELECT name, salary FROM employees
UNION ALL
SELECT name, salary FROM contractors;

-- 错误示例
SELECT name, salary FROM employees
UNION
SELECT name FROM contractors; -- 列数不一致

-- 正确示例
SELECT name, salary FROM employees
UNION
SELECT name, 0 AS salary FROM contractors; -- 添加默认值保持列数一致

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

-- 正确示例
SELECT name, salary FROM employees
UNION
SELECT name, CAST('unknown' AS DECIMAL(10, 2)) AS salary FROM contractors; -- 转换数据类型

-- 优化示例
SELECT name, salary FROM employees WHERE department = 'IT'
UNION ALL
SELECT name, salary FROM contractors WHERE department = 'IT';