SQL中如何给函数命名以便后续引用？Sakila数据库实操问题

SQL 语法规则中，同个SELECT子句内无法直接引用当前层级刚定义的字段别名，想要复用拼接结果避免重复写逻辑，可以参考以下兼容不同版本的实现方案，同时也简化了原代码中冗余的嵌套CONCAT写法：

方案1：子查询嵌套（兼容所有MySQL版本） #

该方案适配性最高，低版本MySQL也可以正常运行：

SELECT 
    Actor_Name,
    LENGTH(Actor_Name) AS Name_Length
FROM (
    -- 内层先完成姓名格式化拼接并定义别名
    SELECT
        CONCAT(
            LEFT(first_name, 1), LOWER(SUBSTRING(first_name, 2)),
            ' ',
            LEFT(last_name, 1), LOWER(SUBSTRING(last_name, 2))
        ) AS Actor_Name
    FROM actor
) AS formatted_actor;

方案2：CTE公共表表达式（MySQL 8.0+支持） #

逻辑层级更清晰，适合多步逻辑复用的场景：

WITH formatted_actor AS (
    SELECT
        CONCAT(
            LEFT(first_name, 1), LOWER(SUBSTRING(first_name, 2)),
            ' ',
            LEFT(last_name, 1), LOWER(SUBSTRING(last_name, 2))
        ) AS Actor_Name
    FROM actor
)
SELECT 
    Actor_Name,
    LENGTH(Actor_Name) AS Name_Length
FROM formatted_actor;

内容的提问来源于stack exchange，提问作者Rohit_Pathak21