I have two tables:

Users. Where ID is the primary key

++++++++++++++++++
+ID......... Name+
+1 ......... Alex+
+2 ......... Rony+
+3 ......... Fred+
++++++++++++++++++

Order: Where Order_id is the primary key, and User_Id is a foreign key

Order_Id ............. User_ID ............ Day
123456   .............    3    ............ Monday
123457   .............    2    ............ Tuesday
123458   .............    1    ............ Friday

Now I would like to output the record having an order_id = 123458, however, I don't want the output to be :

123458   .............    1    ............ Friday

I want it to display the name Alex instead of value 1.

I want it like this:

Order_id .............    Name    ............ Day
123458   .............    Alex    ............ Friday

For example, if I do:

SELECT * FROM ORDERS WHERE ORDER_ID = 123458 it will display this output:

123458   .............    1    ............ Friday

As said before, I want the name Alex to be displayed instead of value 1.

As d_onimic say is better if you start with a sql tutorial. Here Is a sample of using INNER JOIN and creating as Alias for the table names

SELECT O.OrderID, U.Name, O.Day
FROM ORDERS as O
INNER JOIN Users as U
  ON O.UserID = U.ID
WHERE ORDER_ID = 123458