How to get result from parent child table

Question

Work on SQL-Server. My table structure is below

CREATE TABLE [dbo].[AgentInfo](
    [AgentID] [int] NOT NULL,
    [ParentID] [int] NULL,
 CONST         


        

Answer 1

Try something like this:

WITH Merged (AgentId, ParentId) AS (
     SELECT AgentId, ParentId FROM AgentInfo WHERE ParentId = -1
     UNION ALL
     SELECT AgentInfo.AgentId, AgentInfo.ParentId FROM AgentInfo INNER JOIN Merged ON AgentInfo.AgentId = Merged.ParentId
)
SELECT * FROM Merged

Answer 2

You could use a recursive SELECT, see the examples in the documentation for WITH, starting with example D.

The general idea within the recursive WITH is: You have a first select that is the starting point, and then a UNION ALL and a second SELECT which describes the step from on level to the next, where the previous level can either be the result of the first select or the result of the previous run of the second SELECT.

Answer 3

You can try this, to get a tree of the elements:

WITH CTE_AgentInfo(AgentID, ParentID, BaseAgent)
AS(
  SELECT 
    AgentID,
    ParentID,
    AgentID AS BaseAgent
   FROM AgentInfo
   WHERE ParentID = -1
  UNION ALL
     SELECT 
    a.AgentID,
    a.ParentID,
    a.AgentID AS BaseAgent
   FROM AgentInfo a
   INNER JOIN CTE_AgentInfo c ON
    c.AgentID = a.ParentID
  )
SELECT * FROM CTE_AgentInfo

And here is an SQLFiddle demo to see it.

Answer 4

You can use a Common Table Expression to do this.

The sql statement will then look like this:

WITH [Parents]([AgentID], [ParentID], [BaseAgent])
AS
(
    SELECT 
      [AgentID],
      [ParentID],
      [AgentID] AS [BaseAgent]
    FROM [AgentInfo]
    WHERE [ParentID] = -1
    UNION ALL
    SELECT 
      [ai].[AgentID],
      [ai].[ParentID],
      [p].[BaseAgent]
    FROM [AgentInfo] [ai]
    INNER JOIN [Parents] [p] 
      ON [ai].[ParentID] = [p].[AgentID]
)

SELECT *
FROM [Parents]
ORDER BY 
  [BaseAgent] ASC,
  [AgentID] ASC

But, the results are different from your desired output, since every Agent is only listed once. The output is:

AGENTID     PARENTID    BASEAGENT
1           -1          1
3            1          1
2           -1          2
4            2          2

The Fiddle is over here.

And here is a nice post on working with hierarchies: What are the options for storing hierarchical data in a relational database?