Let us see how to write a query to count the number of invoices with practical examples.

Invoice ID is a unique ID or a table column that exists in the Products table.

It is associated with the product information.

One invoice may have multiple products.

See the example below I have used the Current_Products table.

Select * from Current_Products

Invoice_ID ProductID            ProductName                   Sales
1                      A                          Mobile                          10000
2                      B                          Desktop                        20000
3                      C                          Printer                         15000
4                      D                          Headset                       2500
5                      E                          Speakers                      1300

Using this table I want to count the number of invoices.

For that, I can write a query like this

Select Count(Invoice_ID) from Current_Products

Results would be

Count(Invoice_ID)
5

Remember one thing here Invoice will be generated only when the product is sold.

Let us take the sales invoice detail table here.

Table: Sales_Invoice_Detail

Invoice_Date_ID Invoice_ID  Product_ID     Quantity    Unit_Discuount
1                           1                             A                     1              0
2                           1                             A                     1              0
3                           2                             B                     1              0

Let us say I want to find out the list of all products that have not sold.

For that, I can write this SQL Query. 

Select * from Current_Products Where
Product_ID NOT IN (Select distinct Product_ID from Sales_Invoice_Details)

Here output would be

Invoice_ID                      ProductID                 ProductName              Sales
4                                       D                                Headset                      2500
5                                       E                                 Speakers                    1300

I hope it is useful.