Difference between SQL Server codes?

Question

For a school assignment, I have to create a database and run reports. I've created code and a classmate has also created code and it runs the same thing, but his is in a format I've not seen and don't quite understand.

Here is mine:

SELECT
    Course.Name AS 'Course Name', 
    Program.Name AS 'Program Name'
FROM
    Course, Program, ProgramCourse
WHERE 
    ProgramCourse.CourseID = Course.ID
AND
    ProgramCourse.ProgramID = Program.ID
GO

And here's his:

CREATE VIEW NumberOfCoursePerProgram AS
        SELECT
                p.name AS ProgramName,  
                c.name AS CourseName
        FROM
                Program p
        JOIN
                ProgramCourse pc ON pc.ProgramID = p.ID
        JOIN 
                Course c ON c.ID = pc.CourseID
GO

I ran both queries using the data in the tables I've created. They return practically the same results, just in a slightly different order but it fulfills the assignment question. Anyway, if I delete the p from Program p from his code, it returns an error

The multi-part identifier "p.name" could not be bound.

So how is SQL Server able to accept p.name and p.ID, etc. when I haven't ever established these variables? I don't quite understand how the code is working on his. Mine seems simple and straightforward, and I definitely understand what's going on there. So can someone explain his?

Thanks

Answer 1

There's a few differences. First off, he's creating a VIEW rather than just a select statement:

CREATE VIEW NumberOfCoursePerProgram AS

Once the view is created, you can query the view just as you would a table:

SELECT * FROM NumberOfCoursePerProgram;

Second, he's using an ANSI JOIN rather than an implicit JOIN. His method is more modern and most likely considered more correct by today's standards:

JOIN ProgramCourse pc ON pc.ProgramID = p.ID
JOIN Course c ON c.ID= pc.CourseID

Rather than:

FROM Course, Program, ProgramCourse

Also, note he's assigning table aliases when he refers to a table:

FROM Program p

The p at the end allows you to substitute p rather than specify the entire table name of Program elsewhere in the query. For example, you can now say WHERE p.Foo > 5 rather than WHERE Program.Foo > 5. In this case, it's just a shortcut and saves a few characters. However, suppose you were referring to the same table twice (for example, JOINing in two different rows on the same table). In that case, you might have to provide aliases for each table to disambiguate which one is which.

Answer 2

These are called alias in SQL. Alias is basically created to give more readability and for better ease of writing code.

The readability of a SELECT statement can be improved by giving a table an alias, also known as a correlation name or range variable. A table alias can be assigned either with or without the AS keyword:

• table_name AS table alias
• table_name table_alias

So in your query p is an alias to Program so that means now you can refer your table Program by the name of p instead of writing the whole name Program everywhere.

Similarly you can access the names of the columns of your table Program by simply writing p with a dot and then the column name. Something like p.column. This technique is very useful when you using JOINS and some your tables have similar names of the columns.

EDIT:-

Although most of the points are covered in other's answer. I am just adding a point that you should avoid the habit of JOINING table the way you are doing it right now.

You may check

Bad habits to kick : using old-style JOINs

by Aaron Bertrand for reference.