Linking MySQL Tables with JOIN Keys
Relational tables are linked through key columns so product, customer and transaction data can stay in separate tables without repeating the same details in every row. JOIN clauses then reconstruct the report you need.
Linking of table is a very common requirement in SQL. Different types of data can be stored in different tables and based on the requirement the tables can be linked to each other and the records can be displayed in a very interactive way.
We can link more than one table to get the records in different combinations as per requirement. Keeping data of one area in one table and linking them each other with key field is better way of designing tables than creating single table with more number of fields. For example in a student database you can keep student contact details in one table and its performance report in another table. You can link these two tables by using one unique student identification number ( ID ).
Let us take one example of linking of tables by considering product and customer relationship. We have a product table where all the records of products are stored. Same way we will have customer table where records of customers are stored. The daily sales keep the record of all the sales. This sales table will keep record of which product who has purchased. So linking is to be done from Sales table to product table and customer table.
This is our customer table
| customer_id | name | address | |
| 1 | Robort John | 123 Avenue, WS | robort@sitename.com |
| 2 | Elena Hick | 567 avenue | elana@sitename.com |
| 3 | Greek Tor | 987 street | greek@sitename.com |
| 4 | Marr Batson | 456 Mary road | marry@sitename.com |
| 5 | Don Rafel | 456 Rafel | don@sitename.com |
| product_id | product_name | product_dtl |
| 1 | CPU | CPU unit details here. |
| 2 | Keyboard | Keyboard Details available here |
| 3 | Mouse | Mouse details available here |
| 4 | Monitor | Monitor details here |
| 5 | CD Drive | CD Drive details here |
| sales_id | product_id | customer_id | date_of_sale |
| 1 | 1 | 2 | 2004-11-12 00:00:00 |
| 2 | 2 | 1 | 2004-01-11 00:00:00 |
| 3 | 3 | 1 | 2004-05-06 00:00:00 |
ON conditions. Here
is the command to link three tables.
SELECT p.product_name,
c.name AS customer_name,
s.date_of_sale
FROM sales AS s
INNER JOIN product AS p ON p.product_id = s.product_id
INNER JOIN customer AS c ON c.customer_id = s.customer_id
ORDER BY s.date_of_sale;| product_name | name | date_of_sale |
| CPU | Elena Hick | 2004-11-12 00:00:00 |
| Keyboard | Robort John | 2004-01-11 00:00:00 |
| Mouse | Robort John | 2004-05-06 00:00:00 |
You can download SQL Dump of these three tables here.
USING LEFT , RIGHT, INNER join of tables
We may be interested to know which are the products not sold or who are the customers who have not purchased.We can prepare such reports by using Left Join , RIGHT Join or INNER JOIN.
Joining tables Using SQL LEFT Join, RIGHT Join and Inner Join
LEFT JOIN ( Basic Query) RIGHT JOIN INNER Join CROSS Join
SQL Union
LEFT JOIN using Multiple Tables Exercise on LEFT JOIN using Product & Sales Tables
Exercise : Sales - Agent using table JOIN and Date functions
Related table-linking tutorials
JOIN types · LEFT JOIN · Self join · Multiple-table JOINs · UNION
Subhendu Mohapatra
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