Question

Here is my table:

Table Name: UserLinks

Link_ID   User_1   User_2
1         234325   100982
2         116727   299011
3         399082   197983
4         664323   272351

Basically, in this table a duplicate value is:

Link_ID   User_1    User_2
1         232       109
2         109       232

I have looked around and found that I should use INSERT IGNORE to prevent duplicate entries, but I am not sure how to write a query that considers that the relationship between User_1 and User_2 is the same as between User_2 and User_1.

Any advice/help is really appreciated.

Answer 1

Thats a bit nasty, a commutative relationship between the 2 fields, but a unique index will not help given the values can be either way around.

If you could alter the code / data to ensure that the lower value of the ids was always placed in the user_1 field, that would at least then let the unique index work – but its a bit nasty.

Alternatively if the insertion is set based (e.g. not a row at a time but a set of rows) you could join to the existing data and anti-join based on both ways round e.g. :

(existing.user_1 = new.user_1 and existing.user_2 = new user_2)
 OR (existing.user_1 = new.user_2 and existing.user_2 = new user_1)

and in the where clause check to ensure no match was made (the anti part of the join)

where existing.link_id is null

That wouldn’t be efficient for row at a time insertion though.