SQL LINKING two & more TABLES in MySQL using select query

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.

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 email

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 table is below

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 table is here below.

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

From these three tables let us find out the information on sales by linking these tables. We will look into sales table and link it to the customer table by the customer id field and in same way we will link product table by product ID field. We will use WHERE sql command to link different tables. Here is the command to do link three tables.

SELECT product_name, customer.name, date_of_sale  FROM sales, product, customer WHERE  product.product_id = sales.product_id and customer.customer_id >= sales.customer_id LIMIT 0, 30

The above SQL command links three tables and display the required result. The tables are linked by their ID fields. The output is here

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

This way we can link three tables and get a meaningful report.
We may be interested to know which are the products not sold or who are the customers who have not purchased today. We can prepare such reports by using Left Join.

SQL LINKING TABLE Command

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