如何在 SQL 服务器中递归获取值？

Question

I have below sample data.

IF OBJECT_ID('tempdb..#tempData1') IS NOT NULL 
     DROP TABLE #tempData1

CREATE TABLE #tempData1
(
     ParentItemId varchar(20),
     ChildItemId varchar(20)
)

INSERT INTO #tempData1 VALUES('2000-194-819','2000-212-595')
INSERT INTO #tempData1 VALUES('2000-212-771','2000-212-704')
INSERT INTO #tempData1 VALUES('2000-212-704','2000-212-705')
INSERT INTO #tempData1 VALUES('2000-212-595','2000-211-801')
INSERT INTO #tempData1 VALUES('2000-212-801','2000-211-578')

I want to find ParentItemId and its children

WHERE ParentItemId = '2000-194-819'

I wrote this query:

SELECT B1.ParentItemID 
FROM #tempData1 B1 
WHERE NOT EXISTS (SELECT B2.ParentItemID 
                  FROM #tempData1 B2 
                  WHERE B2.ParentItemID = B2.ChildItemId)
  AND ParentItemID = '2000-194-819'

Current output:

'2000-194-819'

Expected output:

2000-194-819
2000-212-595
2000-212-801

Please help.

Answer 1

You can use a recursive CTE. For example:

with
n as (
  select ParentItemId, ChildItemId 
  from #tempData1 
  where ParentItemID = '2000-194-819'
 union all
  select d.ParentItemId, d.ChildItemId
  from n 
  join #tempData1 d on d.ParentItemId = n.ChildItemId
)
select * from n

The recursive CTE is made of two parts:

• The first part of the CTE -- the anchor member -- is located before the UNION ALL clause and retrieves the first set of rows; it's the starting point:

   select ParentItemId, ChildItemId from #tempData1 where ParentItemID = '2000-194-819'

• The second part of the CTE -- the recursive member -- is placed after the UNION ALL clause. This part is executed iteratively for the new rows retrieved until it doesn't return any more rows. This way you can walk a graph, as we are doing here.

   select d.ParentItemId, d.ChildItemId from n join #tempData1 d on d.ParentItemId = n.ChildItemId