Adding Primary Keys in T-SQL - tsql

Question

In a T-SQL tutorial I´ve seen that there are two different ways (at least) of creating or adding a primary key constraint to a table, one of them being making an alteration on the pre-existing table (Customer):
USE RoomReservation;
GO
ALTER TABLE dbo.Customer ADD CONSTRAINT
PK_Customer PRIMARY KEY CLUSTERED (CustomerId);
and the other one being at the moment of creation of the Customer table itself:
USE RoomReservation;
GO
CREATE TABLE dbo.Customer (
CustomerId INT NOT NULL CONSTRAINT PK_Customer PRIMARY KEY,
FirstName NVARCHAR(50) NOT NULL,
LastName NVARCHAR(50) NOT NULL);
My question is :
Are CustomerId and PK_Customer 2 different names for the same primary key atribute?
Are they 2 different attributes that are copies of the same value?
And, either in name or in value, what is the use for this kind of redundance?

Answer 1

Are CustomerId and PK_Customer 2 different names for the same primary key atribute?
No they are not the same. Actually they are two different objects (column != constraint):
CustomerId -- name of column
PK_Customer -- name of constraint
For example if you omit name of PRIMARY KEY constraint it will be autogenerated:
CREATE TABLE dbo.Customer (
CustomerId INT NOT NULL CONSTRAINT PK_Customer PRIMARY KEY,
FirstName NVARCHAR(50) NOT NULL,
LastName NVARCHAR(50) NOT NULL);
SELECT name, object_id, parent_object_id, type_desc
FROM sys.objects
WHERE parent_object_id = OBJECT_ID('dbo.Customer');
┌─────────────┬───────────┬──────────────────┬────────────────────────┐
│ name │ object_id │ parent_object_id │ type_desc │
├─────────────┼───────────┼──────────────────┼────────────────────────┤
│ PK_Customer │ 917578307 │ 901578250 │ PRIMARY_KEY_CONSTRAINT │
└─────────────┴───────────┴──────────────────┴────────────────────────┘
DROP TABLE dbo.Customer;
CREATE TABLE dbo.Customer (
CustomerId INT NOT NULL PRIMARY KEY,
FirstName NVARCHAR(50) NOT NULL,
LastName NVARCHAR(50) NOT NULL);
SELECT name, object_id, parent_object_id, type_desc
FROM sys.objects
WHERE parent_object_id = OBJECT_ID('dbo.Customer');
┌────────────────────────────────┬───────────┬──────────────────┬────────────────────────┐
│ name │ object_id │ parent_object_id │ type_desc │
├────────────────────────────────┼───────────┼──────────────────┼────────────────────────┤
│ PK__Customer__A4AE64D8749AE915 │ 949578421 │ 933578364 │ PRIMARY_KEY_CONSTRAINT │
└────────────────────────────────┴───────────┴──────────────────┴────────────────────────┘
DBFiddle Demo
Are they 2 different attributes that are copies of the same value?
No, they are not copies at all. I guess that it may be confusing because of inline PRIMARY KEY definition. You could make it separate:
CREATE TABLE dbo.Customer (
CustomerId INT NOT NULL ,
FirstName NVARCHAR(50) NOT NULL,
LastName NVARCHAR(50) NOT NULL,
CONSTRAINT PK_Customer PRIMARY KEY -- out of the line
);
Or even on multiple columns(not possible with inline syntax):
CREATE TABLE dbo.InvoiceLine (
InvoiceId INT,
InvoiceLine INT,
Description VARCHAR(100),
CONSTRAINT PK_InvoiceLine PRIMARY KEY(InvoiceId, InvoiceLine)
);