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 @ = passed the first predicate

Let * = passed the second predicate

Table1 CROSS JOIN Table2

a b a c

=========================

1 w 1 r @

1 w 2 s

1 w 3 t *

2 x 1 r

2 x 2 s @

2 x 3 t *

3 y 1 r

3 y 2 s

3 y 3 t @* <== 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;