在SQL中使用不存在和子查询

Question

I have to write an sql query in oracle 11g for the following scenario.

I have a table customers. It has three columns customer_id,cust_cat and cust_deterioration_date. customer_id is the primary key.

customers
(customer_id varchar2(20),
 cust_cat varchar2(1),
 cust_deterioration_date date
);

I have another table Accounts. It has 4 columns loan_account_number,product_code,account_deterioration_date,customer_id. loan_account_number is the primary key.

Accounts
(loan_account_number varchar2(20),
 product_code varchar2(4),
 account_deterioration_date date,
 customer_id  varchar2(20)
 )

I have to select the customers who has loan accounts having product code as 9213 or 9450 in the accounts table and whose cust_cat is 'G'.

If such a customer has multiple loan accounts with product code 9213 or 9450,and Only if the account_deterioration_date for all those loan accounts are null,I should update cust_deterioration_date in customers table as null.

Here is my query to select the reuired customers.

SELECT UNIQUE s1.customer_no
FROM   accounts  a1
      ,customers s1
WHERE  a1.customer_id = s1.customer_no
AND    NVL(s1.cust_cat,
           'G') = 'G'
AND    a1.product_code IN ('9213',
                           '9450')
AND    a1.account_deterioration_date IS NULL
AND    NOT EXISTS (SELECT 1
        FROM   accounts a
        WHERE  a.customer_id = s1.customer_no
        AND    a.account_deterioration_date IS NOT NULL
        AND    a.product_code IN ('9213',
                                  '9450'))

This query is fetching the required result but at the cost of performance. Is there any better way to achieve this functionality?

Thanks in advance

Answer 1

You can aggregate your 9213/9450 account records per customer and see whether there are multiple entries ( count(*) > 1 ) and no non-null account_deterioration_dates ( count(account_deterioration_date) = 0 ). With the customer IDs thus found, you can access the customers table.

update customers
set cust_deterioration_date = null
where nvl(cust_cat,'G') = 'G'
and customer_id in
(
  select customer_id
  from accounts
  where product_code in ('9213', '9450')
  group by customer_id
  having count(*) > 1
  and count(account_deterioration_date) = 0
);