Will ANSI JOIN vs. non-ANSI JOIN queries perform differently?

Question

I have my business-logic in ~7000 lines of T-SQL stored procedures, and most of them has next JOIN syntax:

SELECT A.A, B.B, C.C
FROM aaa AS A, bbb AS B, ccc          


        

Answer 1

The two queries are the same, except the second is ANSI-92 SQL syntax and the first is the older SQL syntax which didn't incorporate the join clause. They should produce exactly the same internal query plan, although you may like to check.

You should use the ANSI-92 syntax for several of reasons

• The use of the JOIN clause separates the relationship logic from the filter logic (the WHERE) and is thus cleaner and easier to understand.
• It doesn't matter with this particular query, but there are a few circumstances where the older outer join syntax (using + ) is ambiguous and the query results are hence implementation dependent - or the query cannot be resolved at all. These do not occur with ANSI-92
• It's good practice as most developers and dba's will use ANSI-92 nowadays and you should follow the standard. Certainly all modern query tools will generate ANSI-92.
• As pointed out by @gbn, it does tend to avoid accidental cross joins.

Myself I resisted ANSI-92 for some time as there is a slight conceptual advantage to the old syntax as it's a easier to envisage the SQL as a mass Cartesian join of all tables used followed by a filtering operation - a mental technique that can be useful for grasping what a SQL query is doing. However I decided a few years ago that I needed to move with the times and after a relatively short adjustment period I now strongly prefer it - predominantly because of the first reason given above. The only place that one should depart from the ANSI-92 syntax, or rather not use the option, is with natural joins which are implicitly dangerous.