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Understanding Relational Algebra

Python.P

I am trying to teach myself relational algebra. I came across this one and want to understand exactly what it means.

𝜎(𝑂𝑟𝑑𝑒𝑟𝑠.𝑜𝑑𝑎𝑡𝑒= 𝑆β„Ž𝑖𝑝𝑚𝑒𝑛𝑡.𝑆β„Ž𝑖𝑝𝑑𝑎𝑡𝑎) (𝑂𝑟𝑑𝑒𝑟𝑠×𝑆β„Ž𝑖𝑝𝑚𝑒𝑛𝑡×𝑂𝑟𝑑𝑒𝑟_𝐼𝑡𝑒𝑚)

∧(𝑂𝑟𝑑𝑒𝑟𝑠.𝑜𝑖𝑑= 𝑆β„Ž𝑖𝑝𝑚𝑒𝑛𝑡.𝑂𝑖𝑑)

β‹€(𝑂𝑟𝑑𝑒𝑟𝑠.𝑂𝑖𝑑=𝑂𝑟𝑑𝑒𝑟𝐼𝑡𝑒𝑚.𝑂𝑖𝑑)

∧(𝑂𝑟𝑑𝑒𝑟_𝐼𝑡𝑒𝑚.𝑄𝑡𝑦>30)


Where this part from the first line is shown as superscript:

Reference - Scaler, Quora

(𝑂𝑟𝑑𝑒𝑟𝑠×𝑆β„Ž𝑖𝑝𝑚𝑒𝑛𝑡×𝑂𝑟𝑑𝑒𝑟_𝐼𝑡𝑒𝑚)

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  • I don't use Relational algebra  but It seems to be a cartesian product of the three tables Orders, Shipment and Order_Item.

    In SQL that is a CROSS JOIN

    select * from  Orders cross join Shipment cross join Order_Item

    If orders has 2 rows, Shipment has 3 rows and Order_Item has 4 rows the result-set will have 2x3x4 = 24 rows 

    The cross join will produce rows which combine each row from the Orders table with each row from the table created cross joining the Shipment table with the Order_Item table.

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  • I don't use Relational algebra  but It seems to be a cartesian product of the three tables Orders, Shipment and Order_Item.

    In SQL that is a CROSS JOIN

    select * from  Orders cross join Shipment cross join Order_Item

    If orders has 2 rows, Shipment has 3 rows and Order_Item has 4 rows the result-set will have 2x3x4 = 24 rows 

    The cross join will produce rows which combine each row from the Orders table with each row from the table created cross joining the Shipment table with the Order_Item table.

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