Page 182 - Data Science class 10
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In this join, none of the associated tables have primary key column. It means that all the columns of primary key
        fields are associated with all the columns of related tables. It occurs when each record in Table A may have many
        linked records in Table B and vice-versa.

        For example, there are many people associated with each project, and every person can associate with more than
        one project.




                                 Many to many
                                                                             Project



                                                         Works on/
                                                        Team includes




                                        Employee




        Let us take an another example to understand the types of joins.
        Consider a database of shop in which there are ‘Customer’ table, ‘Item’ table, ‘Order’ table, ‘Address’ table and
        ‘Ordered Items’ table with fields as: ‘CustomerID’, ‘OrderID’, ‘ItemID’, etc.
           • There is one to one join between the CustomerID in the Customers table and the CustomerID in the Address
          table. This join indicates that each customer may be associated with an address.
           • There is one to many join between the OrderID in the Orders table and the OrderID in the Ordered Items table.
          This relationship indicates that each order may contain multiple items, but each Ordered Item record may only
          correspond to a single order.
           • There is many to many join between the ItemID in the Items table and the ItemID in the Ordered Items table.
          This relationship indicates that each item of stock may be associated with many orders.


        4.3. WHAT IS Z-SCORE?

        A z-score defines the position of a point in terms of its distance from the mean when it is calculated in standard
        deviation units. If the value is above the mean, the z-score is positive; if it is below the mean, it is negative.
        Z-score is also termed as standard score as it permits comparison of scores on different types of variables by
        standardising the distribution. A standard normal distribution is a normally shaped distribution with a mean of 0
        and a standard deviation of 1.


        4.3.1. How to Calculate a Z-score?
        The formula for calculating a z-score is:

           z =  (x – μ)
                 σ
        Where
           x = Raw score

           μ = Population mean
           σ = Population Standard Deviation



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