Question

I am using java on sql server and I have a DB problem of duplicate customers
(many customers are duplicated in the following way).
My tables are :

CUSTOMER(person_id,customer_id)
ORDER (order_id,person_id )

and a duplicated customer looks like this :

CUSTOMER(333,040535743)
CUSTOMER(334,040535743)
CUSTOMER(335,040535743)

ORDER (1,333 )
ORDER (2,333 )

ORDER (4,334 )
ORDER (5,334 )
ORDER (6,334 )

ORDER (7,335 )
ORDER (8,335 )
ORDER (9,335 )


CUSTOMER(336,009530650)
CUSTOMER(337,009530650)

ORDER (10,336 )
ORDER (11,336 )
ORDER (12,336 )

ORDER (13,337 )
ORDER (14,337 )
ORDER (15,337 )

333,334,335 are three rows for the same customer because they have the same customer_id, I want to keep only the last customer ( 335), and make all the orders of 334 and 333 refer to 333.

my final data should be :

CUSTOMER(335,040535743)
ORDER (1,335)
ORDER (2,335)

ORDER (4,335)
ORDER (5,335)
ORDER (6,335)

ORDER (7,335 )
ORDER (8,335 )
ORDER (9,335 )



CUSTOMER(337,009530650)

ORDER (10,337)
ORDER (11,337)
ORDER (12,337)

ORDER (13,337 )
ORDER (14,337 )
ORDER (15,337 )

how do I do a query that does that for all my duplicated customers? (I have a query returning the list of the customer_id’s that have duplications)

Answer 1

Update the Order table:

UPDATE o
SET o.person_id = cc.max_person_id
FROM
    [Order] AS o
  JOIN
    Customer AS c
        ON c.person_id = o.person_id
  JOIN
    ( SELECT customer_id
           , MAX(person_id) AS max_person_id
      FROM Customer
      GROUP BY customer_id
    ) AS cc
        ON cc.customer_id = c.customer_id ;

Then, update the Customer table:

UPDATE c
SET c.person_id = cc.max_person_id
FROM
    Customer AS c
  JOIN
    ( SELECT customer_id
           , MAX(person_id) AS max_person_id
      FROM Customer
      GROUP BY customer_id
    ) AS cc
        ON cc.customer_id = c.customer_id ;

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After that, it would be good to have Customer(person_id) defined as PRIMARY KEY or with a UNIQUE constraint.

And a FOREIGN KEY constraint from Order(person_id) to Customer(person_id)