SQL joining through referential integrity
A REFERENTIAL INTEGRITY is a database concept that is used to build and maintain logical relationships between tables to avoid logical corruption of data. It is a very useful and important part in RDBMS.
Usually, referential integrity is made up of the combination of a primary key and a foreign key.
The main concept of REFERENTIAL INTEGRITY is that it does not allow to add any record in a table that contains the foreign key unless the reference table containing a corresponding primary key.
If any record in referenced table (i.e. the table who contain primary key) is deleted, all the corresponding records in the referencing table will be deleted for the referential integrity.
Sample table: agents
Sample table: orders
To get 'agent_code' and 'agent_name' from 'agents' table and 'ord_num' and 'advance_amount' form 'orders' after a joining with following conditions -
1. 'agent_code' is primary key in 'agents' table
2. 'agent_code' is a foreign key in orders table which is referring to the primary key of 'agents' table.
3. 'agent_code' of 'agents' and 'orders' must be same.
the following SQL statement can be used:
SELECT agents.agent_code,agents.agent_name, orders.ord_num,orders.advance_amount FROM agents,orders WHERE agents.agent_code=orders.agent_code;
AGENT_CODE AGENT_NAME ORD_NUM ADVANCE_AMOUNT ---------- ---------------------------------------- ---------- -------------- A008 Alford 200114 2000 A004 Ivan 200122 400 A006 McDen 200118 100 A010 Santakumar 200119 700 A004 Ivan 200121 600 A011 Ravi Kumar 200130 400 A005 Anderson 200134 1800 A004 Ivan 200108 600 A005 Anderson 200103 700 A011 Ravi Kumar 200105 500 A010 Santakumar 200109 800 A008 Alford 200101 1000 A008 Alford 200111 300 A004 Ivan 200104 500 A002 Mukesh 200106 700 A005 Anderson 200125 600 A001 Subbarao 200117 200 A002 Mukesh 200123 100 A002 Mukesh 200120 100 ................. .................
Here is a new document which is a collection of questions with short and simple answers, useful for learning SQL as well as for interviews.
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