-- 基本用法：拼接字段和字符串
SELECT CONCAT(字段1, 字段2, '固定字符串') AS 拼接结果 FROM 表名;

-- 示例：拼接用户姓名和邮箱（格式：姓名<邮箱>）
SELECT CONCAT(name, '<', email, '>') AS user_info FROM users;

-- 示例：拼接多个字段
SELECT CONCAT(id, '-', username, '-', status) AS user_code FROM users;

-- 语法：CONCAT_WS(分隔符, 字段1, 字段2, ...)
SELECT CONCAT_WS(分隔符, 字段1, 字段2) AS 拼接结果 FROM 表名;

-- 示例：用逗号分隔姓名、年龄、城市
SELECT CONCAT_WS(',', name, age, city) AS user_profile FROM users;

-- 示例：忽略NULL值（如果age为NULL，只会显示"姓名,城市"）
SELECT CONCAT_WS(',', name, age, city) AS user_profile FROM users;

-- 基本语法
SELECT 分组字段, GROUP_CONCAT(拼接字段 [SEPARATOR '分隔符']) AS 拼接结果 
FROM 表名 
GROUP BY 分组字段;

-- 示例：按班级分组，拼接学生姓名（用逗号分隔）
SELECT class, GROUP_CONCAT(name SEPARATOR ',') AS students 
FROM students 
GROUP BY class;

-- 示例：拼接时排序并去重
SELECT class, GROUP_CONCAT(DISTINCT name ORDER BY name DESC SEPARATOR ';') AS students 
FROM students 
GROUP BY class;

-- 将NULL转换为空字符串
SELECT CONCAT(IFNULL(name, ''), '-', IFNULL(phone, '未填写')) AS contact FROM users;

-- 给手机号添加前缀"Tel:"，后缀";"
SELECT CONCAT('Tel:', phone, ';') AS contact_phone FROM users;