SQL join two tables related by a composite columns primary key or foreign key

Introduction

This page delves into the concept of joining two tables in SQL based on composite columns as primary key and foreign key pairs. Unlike typical joins that rely on direct relationships between tables, this example demonstrates a scenario where the join condition is based on matching values in a specific column (ord_amount) across two unrelated tables: despatch and orders.

This type of join can be useful in situations where tables do not share explicit foreign key relationships but have overlapping data points in certain columns.

Example:

DES_NUM    DES_DATE  DES_AMOUNT    ORD_NUM ORD_DATE  ORD_AMOUNT AGENT_CODE
---------- --------- ---------- ---------- --------- ---------- ----------
D002       10-JUN-08       2000     200112 30-MAY-08       2000 A007
D005       19-OCT-08       4000     200119 16-SEP-08       4000 A010
D001       12-JAN-08       3800     200113 10-JUN-08       4000 A002
D003       25-OCT-08        900     200117 20-OCT-08        800 A001
D004       20-AUG-08        450     200120 20-JUL-08        500 A002
D006       24-JUL-08       4500     200128 20-JUL-08       3500 A002

Sample table: orders

   ORD_NUM ORD_AMOUNT ADVANCE_AMOUNT ORD_DATE  CUST_CODE       AGENT_CODE      ORD_DESCRIPTION
---------- ---------- -------------- --------- --------------- --------------- -----------------
    200114       3500           2000 15-AUG-08 C00002          A008
    200122       2500            400 16-SEP-08 C00003          A004
    200118        500            100 20-JUL-08 C00023          A006
    200119       4000            700 16-SEP-08 C00007          A010
    200121       1500            600 23-SEP-08 C00008          A004
    200130       2500            400 30-JUL-08 C00025          A011
    200134       4200           1800 25-SEP-08 C00004          A005
    200108       4000            600 15-FEB-08 C00008          A004
    200103       1500            700 15-MAY-08 C00021          A005
    200105       2500            500 18-JUL-08 C00025          A011
...........

View the table

To get 'des_num' and 'des_date' columns from the table 'despatch' and sum of 'ord_amount' column from the table 'orders' together after a joining, with following conditions -

1. 'a', and 'b' are the aliases of 'despatch' and 'orders',

2. 'ord_amount' of 'despatch' and 'orders' must be same,

3. the same combination of 'des_num' and 'des_date' of 'despatch' should be grouped,

the following SQL statement can be used:

SQL Code:

-- Selecting specific columns: 'des_num' and 'des_date' from the 'despatch' table, and the sum of 'ord_amount' from the 'orders' table
SELECT a.des_num, a.des_date, SUM(b.ord_amount)
-- Performing a Cartesian product (cross join) between the 'despatch' and 'orders' tables (implicit join)
FROM despatch a, orders b
-- Defining the join condition in the WHERE clause where 'a.ord_amount' equals 'b.ord_amount'
WHERE a.ord_amount = b.ord_amount
-- Grouping the result set by 'des_num' and 'des_date'
GROUP BY a.des_num, a.des_date;

Explanation:

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


• It selects specific columns from the 'despatch' table: 'des_num' and 'des_date', along with the sum of 'ord_amount' from the 'orders' table.


• The query performs a Cartesian product (cross join) between the 'despatch' and 'orders' tables, implicitly joining every row from the 'despatch' table with every row from the 'orders' table.


• The join condition is specified in the WHERE clause, where 'a.ord_amount' (from 'despatch') must equal 'b.ord_amount' (from 'orders'). This join condition seems incorrect as it's comparing 'ord_amount' from two different tables, which may not result in meaningful data.


• The result set is then grouped by 'des_num' and 'des_date' using the GROUP BY clause. This ensures that the sum of order amounts is calculated for each unique combination of dispatch number and date.


• Due to the join condition being incorrect, this query may not produce the desired results. It seems to be attempting to sum order amounts based on some common value in the 'ord_amount' column, which is likely incorrect.


• It's important to review and correct the join condition to ensure that the query accurately retrieves the desired data.


• This query is intended to calculate the total order amount for each dispatch number and date combination, but it requires correction to function properly.

Output:

DES_NUM  DES_DATE  SUM(B.ORD_AMOUNT)
-------- --------- -----------------
D004     20-AUG-08              3000
D002     10-JUN-08             10000
D005     19-OCT-08             16000
D001     12-JAN-08             16000
D003     25-OCT-08               800
D006     24-JUL-08             10500

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