Doubt in Joins - 1Z0-051 Preparation

Question

Hi,
In Chapter 7, p.no 317 of 'OCA Oracle Databse 11g, SQL Fundametals I Exam Guide' regarding the joins, there is below lines.
If no joins are N-1 joins are specified in the WHERE clause conditions, where N refers to the number of tables in the query, then a cartesian or cross join is performed.
If an adequate number of joins conditions are specified, then the first optional conditional clause specifies a natural join, while the second two optional clauses specify the syntax for right and left outer joins.I didnt quite understand this well. For ex.
select regions.region_name , countries.countries_name from  regions , countries where  regions.region_id = countries.region_id.
Countries table has coutry_id , country_name, region_id.
Regions table has region_id, region_name.In the above case, it is a natural join, But as per the lines from book, it has N-1 (2-1) joins in the where clause, so this is a cartesian or cross join.
I might be wrong in uderstanding it clearly.
Can anyone help me in understanding those 2 lines regarding the joins.
Thanks in advance. :-)

Answer 1

agathya wrote:
If no joins are N-1 joins are specified in the WHERE clause conditions, where N refers to the number of tables in the query, then a cartesian or cross join is performed.Hmm... Might be a printing mistake. If there are less than n-1 join conditions or if there are no joins, then the result will be a Cartesian Product or what you consider as CROSS JOIN.
The number of rows in a Cartesian product is equal to the product of number of rows in the first table and number of rows in the second table and the number of rows in the third table. In the following example it will be 5*2*3 = 30 rows.
SQL> set feedback on;
SQL> SELECT * FROM test_tab_1
  2  /
     COL_1
         1
         2
         3
         4
         5
5 rows selected.
SQL> SELECT * FROM test_tab_2
  2  /
     COL_2
         1
         2
         3
3 rows selected.
SQL> SELECT * FROM test_tab_3
  2  /
     COL_3
         1
         2
2 rows selected.
SQL> SELECT a1.*, a2.*, a3.*
  2    FROM test_tab_1 a1, test_tab_2 a2, test_tab_3 a3
  3  /
     COL_1      COL_2      COL_3
         1          1          1
         2          1          1
         3          1          1
         4          1          1
         5          1          1
         1          2          1
         2          2          1
         3          2          1
         4          2          1
         5          2          1
         1          3          1
     COL_1      COL_2      COL_3
         2          3          1
         3          3          1
         4          3          1
         5          3          1
         1          1          2
         2          1          2
         3          1          2
         4          1          2
         5          1          2
         1          2          2
         2          2          2
     COL_1      COL_2      COL_3
         3          2          2
         4          2          2
         5          2          2
         1          3          2
         2          3          2
         3          3          2
         4          3          2
         5          3          2
30 rows selected.The following code gives a demonstration where you will have less than N-1 Joins (In this case you should have 2 Joins to avoid Cartesian Product). In this case it will be the resultant of the join conditions between test_tab_1 and test_tab_3 multiplied by the number of rows in the unjoined table (test_tab_3)
SQL> SELECT a1.*, a2.*, a3.*
  2    FROM test_tab_1 a1, test_tab_2 a2, test_tab_3 a3
  3   WHERE a1.col_1 = a2.col_2
  4  /
     COL_1      COL_2      COL_3
         1          1          2
         1          1          1
         2          2          2
         2          2          1
         3          3          2
         3          3          1
6 rows selected.
SQL>Hope this helps,
The example mentioned above assumes that there is 1 to 1 relationship between the three tables.
Regards,
Jo