Derive daily delta from total daily counts? Tsql
If this is for SQL Server 2012 or later version, you could use the LAG analytic function to produce the required result:

SELECT
  ID,
  tableName,
  rowCountDelta = totalRowCount - LAG(totalRowCount, 1, 0) OVER (PARTITION BY tableName
                                                                     ORDER BY Date),
  Date
FROM
  dbo.yourTable
;

The above query assumes that the first entry in a partition is indeed the very first addition of rows. (LAG(totalRowCount, 1, 0) means the absence of the previous totalRowCount will be treated as a zero.) If that cannot be assumed safely in every case and you would prefer to return NULL for first entries, you could use just LAG(totalRowCount) OVER ... instead:

SELECT
  ID,
  tableName,
  rowCountDelta = totalRowCount - LAG(totalRowCount) OVER (PARTITION BY tableName
                                                               ORDER BY Date),
  Date
FROM
  dbo.yourTable
;

If you insist on storing the results in a table, you can just add an INTO clause to the query. A possibly better idea, though, might be to save the query as a view.