在查询声明的列并定义它时，sql如何返回所有行？

How is this possible in SQL?

这怎么可能在SQL中？

DECLARE @liststr VARCHAR(MAX)
;WITH COMMA (name)
AS
(
    SELECT 'a' 
    UNION
    SELECT 'b'
    UNION
    SELECT 'c'
    UNION
    SELECT 'd'
)
SELECT @liststr = CASE WHEN  @liststr+',' IS NULL THEN '' ELSE @liststr+',' END + name 
FROM comma

SELECT @liststr as Result
DE