mysql如何输出拼接字符串

-- 基本用法
SELECT CONCAT('Hello', ' ', 'World');
-- 输出: Hello World

-- 连接列
SELECT CONCAT(first_name, ' ', last_name) AS full_name FROM users;
-- 假设users表中有first_name和last_name两列，输出将是拼接后的全名

-- 连接多个列和常量
SELECT CONCAT('User: ', first_name, ' ', last_name, ' - Email: ', email) AS user_info FROM users;
-- 输出: User: John Doe - Email: john.doe@example.com

SELECT CONCAT('Age: ', CAST(age AS CHAR)) AS age_info FROM users;
-- 假设users表中有age列，输出将是Age: 25

SELECT CONCAT('User: ', COALESCE(first_name, 'Unknown'), ' ', COALESCE(last_name, 'Unknown')) AS full_name FROM users;
-- 如果first_name或last_name为空，输出将是User: Unknown Unknown