Question

Suppose I have a table of customers:

CREATE TABLE customers (
    customer_number  INTEGER,
    customer_name    VARCHAR(...),
    customer_address VARCHAR(...)
)

This table does not have a primary key. However, customer_name and customer_address should be unique for any given customer_number.

It is not uncommon for this table to contain many duplicate customers. To get around this duplication, the following query is used to isolate only the unique customers:

SELECT
  DISTINCT customer_number, customer_name, customer_address
FROM customers

Fortunately, the table has traditionally contained accurate data. That is, there has never been a conflicting customer_name or customer_address for any customer_number. However, suppose conflicting data did make it into the table. I wish to write a query that will fail, rather than returning multiple rows for the customer_number in question.

For example, I tried this query with no success:

SELECT
  customer_number, DISTINCT(customer_name, customer_address)
FROM customers
GROUP BY customer_number

Is there a way to write such a query using standard SQL? If not, is there a solution in Oracle-specific SQL?

EDIT: The rationale behind the bizarre query:

Truth be told, this customers table does not actually exist (thank goodness). I created it hoping that it would be clear enough to demonstrate the needs of the query. However, people are (fortunately) catching on that the need for such a query is the least of my worries, based on that example. Therefore, I must now peel away some of the abstraction and hopefully restore my reputation for suggesting such an abomination of a table…

I receive a flat file containing invoices (one per line) from an external system. I read this file, line-by-line, inserting its fields into this table:

CREATE TABLE unprocessed_invoices (
    invoice_number   INTEGER,
    invoice_date     DATE,
    ...
    // other invoice columns
    ...
    customer_number  INTEGER,
    customer_name    VARCHAR(...),
    customer_address VARCHAR(...)
)

As you can see, the data arriving from the external system is denormalized. That is, the external system includes both the invoice data and its associated customer data on the same line. It is possible that multiple invoices will share the same customer, therefore it is possible to have duplicate customer data.

The system cannot begin processing the invoices until all customers are guaranteed to be registered with the system. Therefore, the system must identify the unique customers and register them as necessary. This is why I wanted the query: because I was working with denormalized data I had no control over.

SELECT
  customer_number, DISTINCT(customer_name, customer_address)
FROM unprocessed_invoices
GROUP BY customer_number

Hopefully this helps clarify the original intent of the question.

EDIT: Examples of good/bad data

To clarify: customer_name and customer_address only have to be unique for a particular customer_number.

 customer_number | customer_name | customer_address
----------------------------------------------------
 1               | 'Bob'         | '123 Street'
 1               | 'Bob'         | '123 Street'
 2               | 'Bob'         | '123 Street'
 2               | 'Bob'         | '123 Street'
 3               | 'Fred'        | '456 Avenue'
 3               | 'Fred'        | '789 Crescent'

The first two rows are fine because it is the same customer_name and customer_address for customer_number 1.

The middle two rows are fine because it is the same customer_name and customer_address for customer_number 2 (even though another customer_number has the same customer_name and customer_address).

The last two rows are not okay because there are two different customer_addresses for customer_number 3.

The query I am looking for would fail if run against all six of these rows. However, if only the first four rows actually existed, the view should return:

 customer_number | customer_name | customer_address
----------------------------------------------------
 1               | 'Bob'         | '123 Street'
 2               | 'Bob'         | '123 Street'

I hope this clarifies what I meant by “conflicting customer_name and customer_address“. They have to be unique per customer_number.

I appreciate those that are explaining how to properly import data from external systems. In fact, I am already doing most of that already. I purposely hid all the details of what I’m doing so that it would be easier to focus on the question at hand. This query is not meant to be the only form of validation. I just thought it would make a nice finishing touch (a last defense, so to speak). This question was simply designed to investigate just what was possible with SQL. 🙂

Answer 1

A scalar sub-query must only return one row (per result set row…) so you could do something like:

select distinct
       customer_number,
       (
       select distinct
              customer_address
         from customers c2
        where c2.customer_number = c.customer_number
       ) as customer_address
  from customers c