Count rows where value in row is also in previous row

ppetree :

I want to get a count where the contents of a value in one row is also in the previous row.

   Row | Item1 | Item2 | Item 3 |
   1   | Dog   | Cat   | Rat
   2   | Bird  | Cat   | Horse
   3   | Horse | Dog   | Rat
   4   | Bird  | Cat   | Horse
   5   | Horse | Bird  | Cat

Row 2 would increase the count of Cat because Cat is in row 1 and 2

Row 3 would increase the count of Horse because Horse is also in Row 2

Row 4 would increase the count of Horse because Horse is also in Row 3

Row 5 would increase the count of Horse AND Cat because both of those appear in row 4.

There can be a max of 100 items or SKU's and I can index on any or all fields. At any given time there's probably between 1000 and 2000 rows.

I can't even wrap my head around where to begin with this query other than "SELECT * FROM table WHERE"

frost-nzcr4 :

First, create table with all available unique values of SKU:

CREATE TABLE results(
  id    VARCHAR(255) NOT NULL PRIMARY KEY
);

-- All fields should be listed here one-by-one.
INSERT IGNORE INTO results (select Item1 from example);
INSERT IGNORE INTO results (select Item2 from example);
INSERT IGNORE INTO results (select Item3 from example);

Previous row could be obtained by left join primary table again with itself, i.e. LEFT JOIN example AS previous ON previous.id + 1 = example.id.

After that we've to check that each unique result exists in example table within current row and in previous row and finally get this:

SELECT
  r.*,
  SUM(
    CASE WHEN r.id IN (
      prv.Item1, prv.Item2, prv.Item3  -- All fields should be listed here.
    ) THEN 1 ELSE 0 END
  ) AS total
FROM
  results AS r
LEFT JOIN
  example AS cur ON r.id IN (
    cur.Item1, cur.Item2, cur.Item3    -- All fields should be listed here.
  )
LEFT JOIN
  example AS prv ON prv.id + 1 = cur.id
GROUP BY
  r.id
ORDER BY
  cur.id
;

See working example http://www.sqlfiddle.com/#!9/7ebd85/1/0