Rule Definition
Improperly written Triangular Joins are worse than even Cursors or While Loops and can bring a CPU and Disk System right to it's knees. their processing algorithm is close to quadratic complexity. For very small counts, the number of internal rows spawned are nearly trivial. But, it doesn't take very many rows to spawn nightmarish numbers of internal rows that can bring a server to its knees. There's danger in thinking that just avoiding the loop leads to set based code. Triangular Joins are a form of "Hidden row by row agonizing row" that can and will crush performance in the face of even the smallest scalability expectations.

Remediation
restrict the conditions using other critersion

Violation Code Sample

SELECT x.OrderID, x.Freight, 
        (SELECT SUM(y.Freight) FROM dbo.Orders y
          WHERE y.OrderID <= x.OrderID) AS RunningTotal,
        (SELECT COUNT(y.Freight)
           FROM dbo.Orders y
          WHERE y.OrderID <= x.OrderID) AS RunningCount
  FROM dbo.Orders X
 ORDER BY x.OrderID