Question

I have a table with data like so:

[ID, Name]
1, Bob
1, Joe
1, Joe
1, Bob

I want to retrieve a list of records showing the relationship between the records with the same ID.
For instance, I want the following result set from my query:

Bob, Joe
Joe, Bob
Bob, Bob
Joe, Joe

This shows me the “from” and “to” for every item in the table.

I can get this result by using the following query:

SELECT DISTINCT [NAME] 
FROM TABLE A
INNER JOIN TABLE B ON A.ID = B.ID

Is there anyway for me to achieve the same result set without the use of the “distinct” in the select statement? If I don’t include the distinct, I get back 16 records, not 4.

Answer 1

The reason you get duplicate rows without DISTINCT is because every row of ID = x will be joined with every other row with ID = x. Since the original table has (1, “Bob”) twice, both of those will be joined to every row in the other table with ID = 1.

Removing duplicates before doing a join will do two things: decrease the time to run the query, and prevent duplicate rows from showing up in the result.

Something like (using MySQL version of SQL):

SELECT L.NAME, R.NAME
FROM (SELECT DISTINCT ID, NAME FROM A) AS L
INNER JOIN (SELECT DISTINCT ID, NAME FROM B) AS R
ON L.ID = R.ID

Edit: is B an alias for table A?