CREATE OR REPLACE FUNCTION add_column_if_not_exists(table_name text, column_name text, column_attr_string text) RETURN VARCHAR(2)
AS
BEGIN
IF NOT EXISTS (
SELECT 1
FROM ALL_TAB_COLUMNS
WHERE TABLE_NAME = table_name
AND COLUMN_NAME = column_name
)
THEN
EXECUTE IMMEDIATE ‘ALTER TABLE "’ || table_name || '" ADD ’ || column_name || ’ '|| column_attr_string;
END IF;
RETURN ‘’;
END;

其中

SELECT 1
FROM ALL_TAB_COLUMNS
WHERE TABLE_NAME = table_name
AND COLUMN_NAME = column_name

返回值是什么？为什么无论参数是什么都是存在？