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SQL Cartesian Product Tips

The Cartesian product, also referred to as a cross-join, returns all the rows in all the tables listed in the query.  Each row in the first table is paired with all the rows in the second table.  This happens when there is no relationship defined between the two tables.  Both the AUTHOR and STORE tables have ten rows.  If we use a Cartesian join in these two tables, we will get back 100 rows.   

SQL> select
  2    author_key,
  3    store_key
  4  from
  5    author, store; 
AUTHOR_KEY  STOR
----------- ----
A101        S101
A101        S102
A101        S103
A101        S104
A101        S105
A101        S106
A101        S107
A101        S108
A101        S109
A101        S110
A102        S101
A102        S102
Ö
A110        S105
A110        S106
A110        S107
A110        S108
A110        S109
A110        S110 
100 rows selected.

Most of the time, we do not want a Cartesian join, and we end up with one because we failed to provide a filter on the join.  If we actually want a Cartesian join, then we should use the ANSI cross join to tell others reading the script that we actually wanted a Cartesian join.

select
  author_key,
  store_key
from
  author cross join store;

One reason to use a Cartesian join is to generate a large amount of rows to use for testing.  I can take a large table and cross join it to another large table and produce a very large results set.  If I cross join dba_objects and dba_views, I can produce the results set below.

SQL> select count(*)
  2  from
  3    dba_objects cross join dba_views; 
  COUNT(*)
----------
 164623840 

Thatís a lot of rows!

 
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