What is a cartesian product why we need it and where we need it

Question

Hi,
One of my interview they asked this question
Can any one please tell detail about it.
Thanks
Kalpana

Answer 1

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what is a cartesian product
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A cartesian join is when you do not specify any join condition between tables.
So for two tables A and B the result is that every row of table B is appended to every row of table A.
If there are 10 rows in Table A and 20 rows in Table b there will be 200 rows in the result set and each row will contain every column from table A and every column from table b unless you specify specific columns.
See Cartesian Products in the SQL Language doc
http://docs.oracle.com/cd/B28359_01/server.111/b28286/queries006.htm
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Cartesian Products
If two tables in a join query have no join condition, then Oracle Database returns their Cartesian product. Oracle combines each row of one table with each row of the other. A Cartesian product always generates many rows and is rarely useful. For example, the Cartesian product of two tables, each with 100 rows, has 10,000 rows. Always include a join condition unless you specifically need a Cartesian product. If a query joins three or more tables and you do not specify a join condition for a specific pair, then the optimizer may choose a join order that avoids producing an intermediate Cartesian product.
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As for when? In earlier versions of Oracle I would use a cartesian join when creating report ready data tables and there needed to be data for every report period. One example is writing a Crystal Report (now business objects) to report sales for 2011 and you want the report to have a section for each month even if there was no data for that month.
Then we would use a cartesian join on a date table that had 12 records (one for each month) with totals of zero. This would make sure that there was at lease one summary record for each month so that every month would show up on the report even if we only had data for March.