SQL Server 2008R2 from Table to edges for Graph

Question

I have stumbled upon an interesting challenge. I have data in a SQL Server table with the following format/content.

Date     | Name
---------+---------
1/1/2010 | John
1/1/2010 | Mark
1/1/2010 | Peter
1/1/2010 | Mia
2/4/2010 | John
2/4/2010 | Billy

I am trying to convert that table into a file containing edges of a graph.

I'll need the edges file to have to columns and all the combinations that the table shows.

John | Mark
John | Peter
John | Mia
Mark | Mia
Mark | Peter
Peter | Mia
John | Billy

I suspect part of this can be achieved with pivot/unpivot but don't know how to proceed with limiting the pivot to only two columns.

Also, I don't know how to make sure I get all the possible combinations of nodes, see that the first four 'nodes' need to become six 'edges.'

Answer 1

You could use ROW_NUMBER and "triangle join":

WITH cte AS
(
  SELECT *, rn = ROW_NUMBER() OVER(PARTITION BY date ORDER BY Name)
  FROM tab
)
SELECT c.Name, c2.Name
FROM cte c
JOIN cte c2
  ON c.Date = c2.Date
  AND c.rn < c2.rn;

LiveDemo

Output:

╔═══════╦═══════╗
║ Name  ║ Name  ║
╠═══════╬═══════╣
║ John  ║ Mark  ║
║ John  ║ Mia   ║
║ John  ║ Peter ║
║ Mark  ║ Mia   ║
║ Mark  ║ Peter ║
║ Mia   ║ Peter ║      -- ORDER BY based on `Name`
║ Billy ║ John  ║      -- same here `B` before `J`
╚═══════╩═══════╝

Note:

To get stable sort you need to add column that will indicate order within group with the same date. I used Name but it swaps the names in last two rows.

---

Version with ID column:

CREATE TABLE tab(ID INT IDENTITY(1,1)
                 ,Date DATE  NOT NULL 
                 ,Name VARCHAR(6) NOT NULL);

INSERT INTO tab(Date,Name) 
VALUES ('1/1/2010','John'), ('1/1/2010','Mark'), ('1/1/2010','Peter')
      ,('1/1/2010','Mia'), ('2/4/2010','John'),('2/4/2010','Billy');

WITH cte AS
(
  SELECT *, rn = ROW_NUMBER() OVER(PARTITION BY date ORDER BY ID)
  FROM tab
)
SELECT c.Name, c2.Name
FROM cte c
JOIN cte c2
  ON c.Date = c2.Date
  AND c.rn < c2.rn;

LiveDemo 2

Output:

╔═══════╦═══════╗
║ Name  ║ Name  ║
╠═══════╬═══════╣
║ John  ║ Mark  ║
║ John  ║ Peter ║
║ John  ║ Mia   ║
║ Mark  ║ Peter ║
║ Mark  ║ Mia   ║
║ Peter ║ Mia   ║
║ John  ║ Billy ║
╚═══════╩═══════╝