如何在Amazon Redshift中创建可从表取值替换字符串的UDF？

Redshift不支持在自定义SQL函数中直接引用表，下面提供两种可行的替代方案：

方案1：递归CTE迭代替换 #

利用Redshift支持的递归CTE，遍历映射表中的每一条替换规则，逐步完成字符串替换。假设你的映射表StringReplace包含find（待替换字符串）和sub（目标替换值）两个字段，示例代码如下：

WITH RECURSIVE replace_steps AS (
    -- 初始化：传入原始字符串，标记初始步骤
    SELECT 
        '需要替换的输入字符串' AS current_str,
        1 AS step,
        (SELECT COUNT(*) FROM StringReplace) AS total_steps
    UNION ALL
    -- 递归应用每一条替换规则
    SELECT 
        REPLACE(rs.current_str, sr.find, sr.sub) AS current_str,
        rs.step + 1 AS step,
        rs.total_steps
    FROM replace_steps rs
    JOIN (
        -- 给替换规则编号，确保执行顺序可控（按需调整ORDER BY）
        SELECT 
            find, 
            sub,
            ROW_NUMBER() OVER (ORDER BY find) AS rule_num
        FROM StringReplace
    ) sr ON rs.step = sr.rule_num
    WHERE rs.step <= rs.total_steps
)
-- 提取最终替换结果
SELECT current_str AS final_result
FROM replace_steps
WHERE step = total_steps + 1;

如果需要复用逻辑，可以封装为存储过程：

CREATE OR REPLACE PROCEDURE replace_string_from_table(IN input_str VARCHAR, OUT output_str VARCHAR)
AS $$
BEGIN
    WITH RECURSIVE replace_steps AS (
        SELECT 
            input_str AS current_str,
            1 AS step,
            (SELECT COUNT(*) FROM StringReplace) AS total_steps
        UNION ALL
        SELECT 
            REPLACE(rs.current_str, sr.find, sr.sub) AS current_str,
            rs.step + 1 AS step,
            rs.total_steps
        FROM replace_steps rs
        JOIN (
            SELECT 
                find, 
                sub,
                ROW_NUMBER() OVER (ORDER BY find) AS rule_num
            FROM StringReplace
        ) sr ON rs.step = sr.rule_num
        WHERE rs.step <= rs.total_steps
    )
    SELECT current_str INTO output_str
    FROM replace_steps
    WHERE step = total_steps + 1;
END;
$$ LANGUAGE plpgsql;

-- 调用示例
CALL replace_string_from_table('测试输入字符串', @result);
SELECT @result;

方案2：动态生成嵌套REPLACE语句 #

如果映射表中的替换规则数量固定且较少，可以通过字符串聚合生成嵌套的REPLACE调用语句，直接执行即可：

-- 生成替换SQL语句
SELECT 'SELECT ' || LISTAGG(
    'REPLACE(' || LAG(expr, 1, '''需要替换的输入字符串''') OVER (ORDER BY find) || ', ''' || find || ''', ''' || sub || ''')', 
    ''
) WITHIN GROUP (ORDER BY find) || ' AS final_result;'
FROM StringReplace;

-- 执行生成的语句得到结果

关键注意点 #

• 若替换规则有顺序依赖（比如先替换"A"为"B"，再替换"B"为"C"），必须在ROW_NUMBER()或LISTAGG的ORDER BY中指定正确的优先级顺序。
• 如果find字段包含单引号等特殊字符，需要提前做转义（比如用REPLACE(find, '''', '''''')处理），避免SQL语法错误。

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