SQL Inner Join. ON condition vs WHERE clause

Question

I am busy converting a query using the old style syntax to the new join syntax. The essence of my query is as follows :

Original Query

SELECT i.*  
FROM 
  InterestRunDailySum i, 
  InterestRunDetail ird, 
  InterestPayments p
WHERE 
   p.IntrPayCode = 187
   AND i.IntRunCode = p.IntRunCode AND i.ClientCode = p.ClientCode
   AND ird.IntRunCode = p.IntRunCode AND ird.ClientCode = p.ClientCode

New Query

SELECT  i.*
  FROM InterestPayments p
    INNER JOIN InterestRunDailySum i 
      ON (i.IntRunCode = p.IntRunCode AND i.ClientCode = p.ClientCode)
    INNER JOIN InterestRunDetail ird 
      ON (ird.IntRunCode = p.IntRunCode AND ird.IntRunCode = p.IntRunCode)
  WHERE 
    p.IntrPayCode = 187

In this example, "Original Query" returns 46 rows, where "New Query" returns over 800

Can someone explain the difference to me? I would have assumed that these queries are identical.

Answer 1

The problem is with your join to InterestRunDetail. You are joining on IntRunCode twice.

The correct query should be:

SELECT  i.*
  FROM InterestPayments p
    INNER JOIN InterestRunDailySum i 
      ON (i.IntRunCode = p.IntRunCode AND i.ClientCode = p.ClientCode)
    INNER JOIN InterestRunDetail ird 
      ON (ird.IntRunCode = p.IntRunCode AND ird.ClientCode = p.ClientCode)
  WHERE 
    p.IntrPayCode = 187

Answer 2

The "new query" is the one compatible with the current ANSI SQL standard for JOINs.

Also, I find query #2 much cleaner:

• you are almost forced to think about and specify the join condition(s) between two tables - you will not accidentally have cartesian products in your query. If you happen to list ten tables, but only six join conditions in your WHERE clause - you'll get a lot more data back than expected!

• your WHERE clause isn't cluttered with join conditions and thus it's cleaner, less messy, easier to read and understand

• the type of your JOIN (whether INNER JOIN, LEFT OUTER JOIN, CROSS JOIN) is typically a lot easier to see - since you spell it out. With the "old-style" syntax, the difference between those join types is rather hard to see, buried somewhere in your lots of WHERE criteria.....

Functionally, the two are identical - #1 might be deprecated sooner or later by some query engines.

Also see Aaron Bertrand's excellent Bad Habits to Kick - using old-style JOIN syntax blog post for more info - and while you're at it - read all "bad habits to kick" posts - all very much worth it!