Assigning an index to a constraint - sql

Question

I have a script written for Oracle databases that I'm converting to work in SQL Server and I have two questions about a specific section of code.
In Oracle script I have this code:
CREATE UNIQUE INDEX "PK_PORTALROLES" ON "PORTAL_ROLE" ("ROLE_NAME");
ALTER TABLE "PORTAL_ROLE" ADD CONSTRAINT "PK_PORTALROLES"
PRIMARY KEY ("ROLE_NAME") USING INDEX ENABLE;
Question (1)
From the code above what is the USING INDEX command in the ALTER TABLE line doing? Is it assigning the UNIQUE INDEX created on the first line to the newly created CONSTRAINT or is the CONSTRAINT getting a new UNIQUE INDEX to use when it is created?
Question (2)
To duplicate this in SQL Server, I commented out the CREATE UNIQUE INDEX line like this:
--CREATE UNIQUE INDEX "PK_PORTALROLES" ON "PORTAL_ROLE" ("ROLE_NAME");
And then replaced the ALTER TABLE line with this:
ALTER TABLE "PORTAL_ROLE" ADD CONSTRAINT "PK_PORTALROLES" PRIMARY KEY ("ROLE_NAME");
I understand that when a PRIMARY KEY CONSTRAINT is created in SQL Server a UNIQUE INDEX is automatically made. So is the one line of SQL Server code directly above doing the same thing as the two lines of Oracle code above?
EDIT
One final question. Is there a way in Oracle and SQL Server to assign an existing INDEX to a CONSTRAINT?

Answer 1

In Oracle
If there are more than one indexes on the column on which you want to add PK constraint, we can selectively choose the index to be assoicated with the PK using “USING INDEX“. This clause can be used while:
Adding the PK constraint for the first time (using “ALTER TABLE” command).
CREATE TABLE tbl_test ( col_1 NUMBER,
col_2 NUMBER,
col_3 NUMBER);
CREATE INDEX idx_col_1_2 ON tbl_test(col_1, col_2);
CREATE INDEX idx_col_1_3 ON tbl_test(col_1, col_3);
CREATE UNIQUE INDEX idx_col_1 ON tbl_test(col_1);
-- Forcing oracle to use the unique index "IDX_COL_1"
ALTER TABLE tbl_test ADD CONSTRAINT tbl_test_pk PRIMARY KEY(col_1)
USING INDEX idx_col_1;
SELECT constraint_name, constraint_type, index_name
FROM user_constraints
WHERE table_name = 'TBL_TEST';
-- CONSTRAINT_NAME | CONSTRAINT_TYPE | INDEX_NAME
-- TBL_TEST_PK | P | IDX_COL_1
What if? If you don't use the USING INDEX clause
ALTER TABLE tbl_test ADD CONSTRAINT tbl_test_pk PRIMARY KEY(col_1);
-- Although an unique index exists, oracle has picked up the first index
SELECT constraint_name, constraint_type, index_name
FROM user_constraints
WHERE table_name = 'TBL_TEST';
-- CONSTRAINT_NAME | CONSTRAINT_TYPE | INDEX_NAME
-- TBL_TEST_PK | P | IDX_COL_1_2
That shows The index associated with the PK constraint needn’t be unique.
But in SQLServer implicitly CLUSTERED INDEX will be created when primary key defined on any column
So to your last question. In Oracle you can assign index which we created can be assigned to constraints which we will create in future.
In SQLServer i guess it is not possible.

Answer 2

In Oracle, you can do
alter table PORTAL_ROLE add constraint pk_portalroles primary key('ROLE_NAME');
And it will create unique index for you. But when you drop or disable(!) constraint, it will drop that index. To avoid that (or to give index some custom name, or to use non-unique index, etc), it is usually done in 2 steps as in your code (as recommended by ORACLE).
Q1: constraint will be using existing index.
Q2: Yes, it will be enough for SQL Server.
When you specify a unique constraint on one or more columns, Oracle implicitly creates an index on the unique key. If you are defining uniqueness for purposes of query performance, then Oracle recommends that you instead create the unique index explicitly using a CREATE UNIQUE INDEX statement. You can also use the CREATE UNIQUE INDEX statement to create a unique function-based index that defines a conditional unique constraint.