ctala / SQL-Queries-Introduction

Testing SQL Queries With SQLite

SQL Queries Introduction

We are going to use SQLite as a Database Engine to try the SQL Queries.

• Version : 1.0.0
• Created : 10/01/2017
• Updated : 11/11/2017

Files attached :

• Northwind.sqlite : The actual database that can be open with any SQLite editor.
• Northwind.Sqlite3.sql : Includes the SQL code to create the Database.

Docs

• Functions and Operators

Examples

• Northwind Database examples

How to run the database:

The easiest way to run the database is to download the code of this repository and upload the Northwind.sqlite to an online SQLite manager like https://sqliteonline.com/.

Queries

Showing Available Tables

Query

SELECT name FROM sqlite_master ;

Showing the content of one table

Query to get all the content

SELECT * FROM Categories;

In this case "*" represents "ALL" fields. Everytime that we just need to get all the columns or fields for all the results or rows we will use asterisk.

Query to get specific fields

SELECT CategoryName,Description FROM Categories;

When we just need to get specific fields from the table we must specified which fields do we need. In this case the query will return all the rows but only the columns called Categoryname and Descrition.

Limiting the quantity of results

SELECT CategoryName,Description FROM Categories limit 1;

With the last query we will run the same example as before where we got only CategoryName and Description columns, but in this case we will get only one result. We can use any number that we want !

Conditioning the results

SELECT * FROM Customers WHERE ContactName = "Maria Anders";

We will use the "Where" clause when we want to condition the output of the results. In this case we will show all the results from the table Customers in which the ContactName is equals to "Maria Anders".

Conditioning the results with like.

SELECT * FROM Customers WHERE ContactName Like "Maria%";
SELECT * FROM Customers WHERE ContactName Like "%Maria%";
SELECT * FROM Customers WHERE ContactName Like "%Maria";

With the last examples we can have 3 possible outputs.

• All the customers which name starts with Maria.
• All the customers which name contains Maria.
• All the customers which name finishes with Maria.

Multiple Conditioning.

SELECT *  FROM Customers
WHERE ContactName like "A%"
AND
ContactTitle = "Owner";

In this case we are going to obtain all the results of customers on which their name starts with an "A" and their Contact Title is owner. We will introduce as many conditions as we want separated by an "AND".

Showing the content from multiple tables.

SELECT * FROM Orders, Customers
WHERE
Orders.CustomerID = Customers.CustomerID

In this case we are getting the results from two different tables, Orders and Customers, and we want to get all the orders and the customer information from those orders. In this case we need to link together both tables. In the orders table we have a field called CustomerID that correspond to the unique ID of the customer, there is only one CustomerID per customer making it unique.

If we don't link together the tables I will have as a result all the possible combinations of customers with all the orders.

Math Operations on Queries.

Counting the results.

SELECT count(*) FROM Customers;

Calculate the sum on the results.

SELECT SUM(Quantity) as "Total Products" FROM "Order Details";

In this case we will get the sum of all products in all the orders, so the total quantity of products sold.

Calculate the total cost per product in an order

SELECT OrderID, ProductID, (UnitPrice*Quantity)-(UnitPrice*Quantity*Discount) as ProductTotal FROM "Order Details";

In this case we are getting the order id, the product id and the total cost including the discount of the product.

Everything together

SELECT OD.OrderID,ProductName, (OD.UnitPrice*OD.Quantity)-(OD.UnitPrice*OD.Quantity*OD.Discount) as ProductTotal
FROM "Order Details" as OD, Products
WHERE
OD.ProductID = Products.ProductID

In this case we will get the Total Price quantity of every product per OrderID from The Order Details table and the name of the product from the Products table.