SQL Server 2008R2 from Table to edges for Graph - sql-server

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 ║
╚═══════╩═══════╝