Great! The non-equi JOIN comes in handy especially when you have to join a table with itself. For example you can select all the items from a table and pair them, either in unique or non-unique pairs.

Suppose you have the Items table, and you want to find all possible combinations of all item pairs for a special promotion. You would like to show all possible pairs, excluding only the case when a given item matches itself (that is, the pair consists of two copies of the same item):

SELECT
  Item1.Name,
  Item2.Name
FROM Items Item1
JOIN Items Item2
  ON Item1.Id != Item2.Id

Great! Everything works... However, with this query, we don't get unique pairs - because the result also includes pairs in reverse order. What about unique pairs? Well, in that case, we have to change the inequality operator to the less-than operator (<):

SELECT
  Item1.Name,
  Item2.Name
FROM Items Item1
JOIN Items Item2
  ON Item1.Id < Item2.Id

Why? Suppose we have only three items in the items table. The item with the smallest ID (Id = 1) can be listed as the first element of two pairs: 1 & 2 and 1 & 3. But because of the less-than operator, the item with Id = 2 can only be listed together with item 3. And the item with Id = 3 is never listed as the first member of a pair. With this little trick, we have unique pairs!