SQL 字符串函数如何去掉重复字符？

WITH Characters AS (
    SELECT 
        SUBSTRING('aabbcc', 1, 1) AS ch,
        1 AS pos
    UNION ALL
    SELECT 
        SUBSTRING('aabbcc', pos + 1, 1),
        pos + 1
    FROM Characters
    WHERE pos < LEN('aabbcc')
)
SELECT STRING_AGG(ch, '') AS unique_string
FROM (
    SELECT DISTINCT ch
    FROM Characters
) t;

SELECT string_agg(DISTINCT letter, '' ORDER BY letter)
FROM (
    SELECT unnest(regexp_split_to_array('aabbcc', '')) AS letter
) t
WHERE letter != '';

CREATE FUNCTION dbo.RemoveDuplicateChars(@input NVARCHAR(MAX))
RETURNS NVARCHAR(MAX)
AS
BEGIN
    DECLARE @result NVARCHAR(MAX) = '';
    DECLARE @i INT = 1;
    DECLARE @ch NCHAR(1);
<pre class='brush:php;toolbar:false;'>WHILE @i <= LEN(@input)
BEGIN
    SET @ch = SUBSTRING(@input, @i, 1);
    IF CHARINDEX(@ch, @result) = 0
        SET @result = @result + @ch;
    SET @i += 1;
END

RETURN @result;

SELECT dbo.RemoveDuplicateChars('aabbcc'); -- 返回 abc