SQL函数遍历表

Question

我是tSQL函数的新手，正在尝试执行以下操作：

我有一个像这样的表叫做Projects：

+----------+--------------+
| Code     | Previous_Code|
+----------+--------------+
| 001      | NULL         |
| 002      | 001          |
| 003      | 002          |
| 004      | NULL         |
| 005      | NULL         |
+----------+--------------+

而且我想创建一个SQL函数以产生以下查询结果：

+----------+--------------+
| Code     | Original_Code|
+----------+--------------+
| 001      | 001          |
| 002      | 001          |
| 003      | 001          |
| 004      | 004          |
| 005      | 005          |
+----------+--------------+

我想出了以下功能：

CREATE FUNCTION OriginalProjectCode (@Code VARCHAR(3))
RETURNS VARCHAR(3)
AS
BEGIN
DECLARE @Code2 AS VARCHAR(3);
WHILE @Code IS NOT NULL
    BEGIN
        SET @Code2 = @Code;
        SET @Code = ('SELECT [Previous_Code] FROM 
        Projects WHERE [Code] = ' + @Code);
        SET @Code2 = @Code;
    END;
RETURN @Code2;
END;

但是它运行时似乎处于无限循环中。 有人对实现相同结果有任何想法或更好的方法吗？ 提前致谢。

Answer 1

似乎简单的case表达式可以工作：

select t.*, (case when Previous_Code is not null 
                  then coalesce(lag(Previous_Code, 1) over (order by code), Previous_Code) 
                  else code
             end) as Original_Code
from table t;

Answer 2

希望这可以帮助！！

    CREATE TABLE #Projects(Code VARCHAR(10),Previous_Code VARCHAR(10))

    INSERT INTO #Projects
    SELECT '001',NULL  Union ALL
    SELECT '002','001' Union ALL
    SELECT '003','002' Union ALL
    SELECT '004',NULL  Union ALL
    SELECT '005',NULL 

    DECLARE @id varchar(10)='003'

    ;with cte
    AS
    (
         select code,code as Original_Code,ISNULL(Previous_Code,code)Previous_Code
           from #Projects
           union all
          select c.Code,p.code as Original_Code, p.Previous_Code
           from #Projects p
             join cte c on c.Previous_Code = p.code  

    )

    SELECT code,Original_Code
    from cte 
    WHERE
        1= ( CASE WHEN Previous_Code IS NULL AND code !=Original_Code then 1
            WHEN code=Original_Code and Original_Code=Previous_Code then 1
            ELSE 0 END)
    order by code,Previous_Code


    DROP TABLE #Projects