Re: converting a query with *= into SQL 92

From: --CELKO-- <joe.celko_at_northface.edu>
Date: 5 Feb 2004 16:16:55 -0800
Message-ID: <a264e7ea.0402051616.17b18602_at_posting.google.com>

Here is how OUTER JOINs work in SQL-92. Assume you are given:

Table1       Table2
 a   b        a   c
 ======       ======
 1   w        1   r
 2   x        2   s
 3   y        3   t

 4 z

and the outer join expression:

 Table1
 LEFT OUTER JOIN
 Table2

 ON Table1.a = Table2.a      <== join condition
    AND Table2.c = 't';      <== single table condition

We call Table1 the "preserved table" and Table2 the "unpreserved table" in the query. What I am going to give you is a little different, but equivalent to the ANSI/ISO standards.

1. We build the CROSS JOIN of the two tables. Scan each row in the result set.
2. If the predicate tests TRUE for that row, then you keep it. You also remove all rows derived from it from the CROSS JOIN
3. If the predicate tests FALSE or UNKNOWN for that row, then keep the columns from the preserved table, convert all the columns from the unpreserved table to NULLs and remove the duplicates.

So let us execute this by hand:

 Let _at_ = passed the first predicate
 Let * = passed the second predicate

 Table1 CROSS JOIN Table2
 a b a c

---

 1   w       1   r _at_
 1   w       2   s
 1   w       3   t *
 2   x       1   r
 2   x       2   s _at_
 2   x       3   t *
 3   y       1   r
 3   y       2   s
 3   y       3   t _at_* <== the TRUE set
 4   z       1   r
 4   z       2   s
 4   z       3   t *

 Table1 LEFT OUTER JOIN Table2
 a   b        a   c
 =========================
 3   y     3      t      <= only TRUE row


 -----------------------

 1   w     NULL   NULL   Sets of duplicates
 1   w     NULL   NULL
 1   w     NULL   NULL


 -----------------------

 2   x     NULL   NULL
 2   x     NULL   NULL
 2   x     NULL   NULL
 3   y     NULL   NULL  <== derived from the TRUE set - Remove  
 3   y     NULL   NULL


 -----------------------

 4   z     NULL   NULL
 4   z     NULL   NULL
 4   z     NULL   NULL

the final results:

 Table1 LEFT OUTER JOIN Table2
 a b a c

---

 1   w     NULL   NULL
 2   x     NULL   NULL
 3   y     3      t
 4   z     NULL   NULL

The basic rule is that every row in the preserved table is represented in the results in at least one result row.

There are limitations and very serious problems with the extended equality version of an outer join used in some diseased mutant products. Consider the two Chris Date tables

 Suppliers        SupParts
 supno             supno partno qty
 =========        ==============
 S1               S1   P1    100
 S2               S1   P2    250
 S3               S2   P1    100
                  S2   P2    250

and let's do an extended equality outer join like this:

 SELECT *
  FROM Supplier, SupParts
 WHERE Supplier.supno *= SupParts.supno
   AND qty < 200;

If I do the outer first, I get:

 Suppliers LOJ SupParts
 supno supno partno qty

---

 S1     S1   P1    100
 S1     S1   P2    250
 S2     S2   P1    100
 S2     S2   P2    250

 S3 NULL NULL NULL Then I apply the (qty < 200) predicate and get

 Suppliers LOJ SupParts
 supno supno partno qty

---

 S1 S1 P1 100
 S2 S2 P1 100

Doing it in the opposite order

 Suppliers LOJ SupParts
 supno supno partno qty

---

 S1 S1 P1 100
 S2 S2 P1 100
 S3 NULL NULL NULL Sybase does it one way, Oracle does it the other and Centura (nee Gupta) lets you pick which one -- the worst of both non-standard worlds! In SQL-92, you have a choice and can force the order of execution. Either do the predicates after the join ...

 SELECT *
   FROM Supplier

        LEFT OUTER JOIN
        SupParts
        ON Supplier.supno = SupParts.supno

 WHERE qty < 200;

 ... or do it in the joining:

 SELECT *
  FROM Supplier

       LEFT OUTER JOIN
       SupParts
       ON Supplier.supno = SupParts.supno
          AND qty < 200;

Another problem is that you cannot show the same table as preserved and unpreserved in the extended equality version, but it is easy in SQL-92. For example to find the students who have taken Math 101 and might have taken Math 102:

 SELECT C1.student, C1.math, C2.math
  FROM (SELECT * FROM Courses WHERE math = 101) AS C1

       LEFT OUTER JOIN
       (SELECT * FROM Courses WHERE math = 102) AS C2
       ON C1.student = C2.student;

Received on Fri Feb 06 2004 - 01:16:55 CET