SQL joining through referential integrity

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.

Example:

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:

SQL Code:

-- Selecting specific columns: 'agent_code' and 'agent_name' from the 'agents' table, and 'ord_num' and 'advance_amount' from the 'orders' table
SELECT agents.agent_code, agents.agent_name, orders.ord_num, orders.advance_amount
-- Performing a Cartesian product (cross join) between the 'agents' and 'orders' tables (implicit join)
FROM agents, orders
-- Defining the join condition in the WHERE clause where 'agents.agent_code' equals 'orders.agent_code'
WHERE agents.agent_code = orders.agent_code;

Explanation:

• This SQL query retrieves data from two tables: 'agents' and 'orders'.


• It selects specific columns from these tables: 'agent_code' and 'agent_name' from the 'agents' table, and 'ord_num' and 'advance_amount' from the 'orders' table.


• Instead of using explicit JOIN syntax, this query uses a Cartesian product (cross join) by listing the tables in the FROM clause separated by a comma. This results in combining every row from the 'agents' table with every row from the 'orders' table.


• The join condition is specified in the WHERE clause, where 'agents.agent_code' must equal 'orders.agent_code'. This filters the result to include only rows where the agent codes match between the two tables.


• While this query produces the desired result, it's important to note that it may be less efficient than using explicit JOIN syntax, especially on large tables. Explicit JOINs provide better readability and maintainability of the query.


• This query is useful for retrieving information about agents and their corresponding orders, linking the two tables based on the agent code.

Output:

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
.................
.................

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