How to explain "inner join Vehicle v on v.Id = d.VehicleId"? - sql

Question

I hope that i can explain it right, because i'm not a native english speaker.
I have 2 tables- Document and Vehicle.
As you can see in the SELECT query, the columns Id(Vehicle) and VehicleId(Document) are common.
The columns SeriePolita,NumarPolita,d.Status are from the Document table and v.CascoCategorieVehicul,v.NumarInmatriculare from the Vehicle table.
This code was written some time ago ( is working - no problem in the syntax ) by someone who left the company and my colleagues are using this code and weren't able to explain it to me.
My question is -
Can some explain or put a link with some info, for the Vehicle v on v.Id = d.VehicleId. What is the meanning of vehicle v and how is using d.Status different that Status, without the d.
select
SeriePolita,
NumarPolita,
d.Status,
v.CascoCategorieVehicul,
v.NumarInmatriculare
from Document d
inner join Vehicle v on v.Id = d.VehicleId
where EndDate>'2015-06-30' and d.Class='CASCO' and Status in ('Polita', 'Anulat')

Answer 1

Vehicle v is a reference to the Vehicle table by the alias v. The reason for doing this is so you don't have to type, for instance inner join Vehicle on Vehicle.Id = Document.VehicleId - it's shortened and more concise and you can refer to the alias in the select and where clause.
Now suppose that there is a Status column in both tables, without referring to it by either table name or alias, you would get an ambiguous column error as the DB engine would not know which table column you are referring to. If there is only only Status column then your query will run fine, although it is unclear which table the column actually belongs to!
See more on this in the documentation:
Using Table Aliases

Answer 2

the code you have can be rewritten as follows:
select
d.SeriePolita,
d.NumarPolita,
d.Status,
v.CascoCategorieVehicul,
v.NumarInmatriculare
from Document AS d
inner join Vehicle AS v on v.Id = d.VehicleId
where EndDate>'2015-06-30'
and d.Class='CASCO'
and Status in ('Polita', 'Anulat');
mind the AS keyword between Document and d: that is the keyword to tell the system that the object Document from now on will be 'known as' d.
it is a keyword that is not mandatory, may be omitted safely as in your code.
on msdn you can find details about table aliasing.

Answer 3

„Vehicle v” is a shorthand for „Vehicle as v”
Here the “v” is just an alias for the table “Vehicle” so you can just access columns of the table using this alias.
v.NumarInmatriculare has excactly same semantic meaning that Vehicle.NumarInmatriculare
Regards
Jan

Answer 4

Here Vehicle is a table and v is a table alias.
see this for more info https://technet.microsoft.com/en-us/library/ms187455%28v=sql.105%29.aspx
http://www.w3schools.com/sql/sql_alias.asp
Vehicle v on v.Id = d.VehicleId
v.Id means Id column from Vehicle table and d.VehicleId means VehicleId from document table

Answer 5

Can some explain or put a link with some info, for the Vehicle v on
v.Id = d.VehicleId. What is the meanning of vehicle v and how is using
d.Status different that Status, without the d.
TLDR: vehicle v means that v is an alias for vehicle and anywhere we use v.columnname would mean vehicle.columnname
Detail explanation:
Often in SQL queries we create an alias for table names, so that we can refer to them using that alias instead of original table name. There are multtiple reasons to do so. Let me introduce to two common ones.
When resolving ambiguity in column name Like when you want to join two tables say Employee and Spouse and both have same columns ID and name. And you want to select both names in your output. So query syntax is like
SELECT name,name FROM Employee JOIN Spouse on ID=ID
This query can not execute as both ID columns and name columns are in both tables and cannot be resolved to a single table correctly. Correct way to write query is
SELECT Employee.name,Spouse.name FROM Employee JOIN Spouse on Employee.ID=Spouse.ID
Note that though this query executes, still the output will be like
name | name
________________
John | Jane
Margaret | Dave
Still you cannot guess spouse or Employee from output. Good way is to use Alias like this
SELECT Employee.name as EmployeeName,Spouse.name as SpouseName FROM Employee JOIN Spouse on Employee.ID=Spouse.ID
which gives you output like
EmployeeName | SpouseName
___________________________
John | Jane
Margaret | Dave
This syntax columnname as aliasname allows us to use aliasname instead of original column name, clearly any ambiguity or renaming column so that expected output makes sense especially in case of join
Now if the example had both spouse and employees stored in same table say Person, so we had create self join
SELECT Person.name as EmployeeName,Person.name as SpouseName FROM person JOIN person on person.ID=person.ID
now again we have the same problem as both person.ID and Person.name repeat even though they are qualified by their table names, as there are two table names person and person (left and right side of JOIN).
In this case to resolve ambiguity we need to give alias to table names and refer to column using those alias like below:
SELECT Employee.name as EmployeeName,Spouse.name as SpouseName FROM person as Employee JOIN person as Spouse on Employee.ID=Spouse.ID
note that as keyword can be omitted. Most DBAs do not use it. So this below query is same as before.
SELECT Employee.name EmployeeName,Spouse.name SpouseName FROM person Employee JOIN person Spouse on Employee.ID=Spouse.ID
When you need to use a shorter, less typo prone name in query
Now wouldn't it be better that instead of writing long names again and again we can use shorter alias. Well we can. So both queries below produce same output.
This is same as
SELECT Employee.name as EmployeeName,Spouse.name as SpouseName FROM Employee JOIN Spouse on Employee.ID=Spouse.ID
this
SELECT E.name EmployeeName,S.name SpouseName FROM Employee E JOIN Spouse S on E.ID=S.ID