What is the difference between the Primary and Foreign key?
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Answer Posted / slokh
Primary key does not contain NULL or duplicate values.
whereas Foreign key accepts duplicate values but not null
values.
Primary key states the Entity integrity constraint, which
says that every row in a table is unique.
Foreign key states the referential integrity constraint,
that says that you can only insert values that is present
in the primary of the parent table.
| Is This Answer Correct ? | 8 Yes | 2 No |
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Hi, I am new in oracle(SQL), could anyone help me in writing a correct SQL. Below is the table structure. Table: Subsc Fields: 1. Sub_no (this field will hold values of subscriber nos, for e.g. S111111, S222222, S333333, S444444, etc.) 2. s_status (this field will hold values for different status of subscriber, for e.g. 'A', 'S', 'C', etc.) 3. cus_id (this field will hold values of bill nos for e.g. 11111111, 22222222, 33333333, 44444444, etc.) Table: Bill Fields: 1. Bill_no this field will hold values of bill nos for e.g. 11111111, 22222222, 33333333, 44444444, etc.) 2. b_status = (this field will hold values for different status of bill for e.g. 'O', 'C', 'S', etc.) Note: 1. The Sub_no is a Primary key of Subsc table. 2. The cus_id is a foreign in Subsc table (referred from Bill_no field of Bill table) 3. The Bill_no field is the Primary key of Bill table. Query A --> I wrote a query to select cus_id/Bill_no which is in status open (b_status = 'O') and having more than two active subscriber (i.e. S_status = 'A') in it ( i.e. more the two subscribers in same bill). select s.cus_id from subsc s where exists (select 1 from bill where bill_no = s.cus_id and b_status = 'O') and s_status = 'A' group by s.cus_id having count(sub_no) = 2 Problem : The above query will give the cus_id (or rather bill_no) which are in open status (b_status ='O) and which are having TWO ACTIVE Subscribers (s_status ='A') in it. However, this query will also lists the cus_id/bill_no which are having more than TWO subscribers in it (but only two subscriber will be in Active status (s_status = 'A') and the others will be in s_status = 'C' or s_status = 'S'. Help needed: I want to write a query which will fetch ONLY the cus_id/bill_no which are in open status (b_status ='O') and which are having ONLY TWO ACTIVE subscribers (s_status ='A') in it. B--> If I include the sub_no in the above query then NO row are returned. select s.cus_id, s.sub_no from subsc s where exists (select 1 from bill where bill_no = s.cus_id and b_status = 'O') and s_status = 'A' group by s.cus_id, s.sub_no having count(sub_no) = 2 Help needed: I want to modify the above query which will fetch ONLY the cus_id/bill_no which are in open status (b_status ='O') and which are having ONLY TWO ACTIVE subscribers (s_status ='A') in it ALONG with the sub_no. Thanks a lot in advance. Regards, Nitin
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