In my table I have ID as primary key, it is just a meaningless unique code
as it is a primary key SQL Server 2017 made it clustered.
I have another column in my table called myTime this is a timestamp with non uinique non clustered index
Can I make the PK a non clustered and the index is clustered and how?
Yes you can. If you already have an existing table then you need to:
Drop the current clustered PRIMARY KEY
Create your CLUSTERED INDEX
Create a PRIMARY KEY NONCLUSTERED
For example:
IF OBJECT_ID('tempdb..#Test') IS NOT NULL
DROP TABLE #Test
CREATE TABLE #Test (
ID INT,
TimeStamp DATETIME,
CONSTRAINT PK_Test PRIMARY KEY (ID)) -- Clustered by default
ALTER TABLE #Test DROP PK_Test
CREATE CLUSTERED INDEX CI_Test_TimeStamp ON #Test (TimeStamp)
ALTER TABLE #Test ADD CONSTRAINT PK_Test PRIMARY KEY NONCLUSTERED (ID)
The only thing that will enforce uniqueness is the PRIMARY KEY constraint, you can still have a clustered index on repeated values, although it might raise an eyebrow for performance. See Eric's link for details.
Yes you can, by specifying the primary key be nonclustered.
ALTER TABLE TableName
ADD CONSTRAINT PK_name PRIMARY KEY NONCLUSTERED (ID);
You make another index clustered by specifying a clustered index.
CREATE CLUSTERED INDEX IX_Name
ON dbo.TableName (ColumnName);
Related
I want to change the clustered index on a table to a column combo other than the primary key.
How can I drop the clustered index, keep the same primary key I have, and add a new clustered index on a new set of columns.
Final result - Non-clustered indexed PK, clustered index on new set of columns.
The only way you can do this is:
1) First Drop the constraints with the below code:
ALTER TABLE TABLENAME DROP CONSTRAINT ConstrainName
2) Then Create the Clustered Index on the Column you want with the below code:
Create clustered index Index_Name on TableName (column1,column2)
3) Create a primary key on a column :
ALTER TABLE TableName
ADD CONSTRAINT constraint_name PRIMARY KEY (ColumnName);
I am new to SQL Server and while learning about clustered index, I got confused!
Is unique key clustered or a non-clustered index? Unique key holds only unique values in the column including null, so according to this concept, unique key should be a clustered index, right? But when I went through this article I got confused MSDN
When you create a UNIQUE constraint, a unique nonclustered index is
created to enforce a UNIQUE constraint by default. You can specify a
unique clustered index if a clustered index on the table does not
already exist.
Please help me to understand the concept in a better manner, Thank you.
There are three ways of enforcing uniqueness in SQL Server indexes.
Primary Key constraint
Unique constraint
Unique index (not constraint based)
Whether they are clustered or non clustered is orthogonal to whether or not the indexes are declared as unique using any of these methods.
All three methods can create a clustered or non clustered index.
By default the unique constraint and Unique index will create a non clustered index if you don't specify any different (and the PK will by default be created as CLUSTERED if no conflicting clustered index exists) but you can explicitly specify CLUSTERED/NONCLUSTERED for any of them.
Example syntax is
CREATE TABLE T
(
X INT NOT NULL,
Y INT NOT NULL,
Z INT NOT NULL
);
ALTER TABLE T ADD PRIMARY KEY NONCLUSTERED(X);
--Unique constraint NONCLUSTERED would be the default anyway
ALTER TABLE T ADD UNIQUE NONCLUSTERED(Y);
CREATE UNIQUE CLUSTERED INDEX ix ON T(Z);
DROP TABLE T;
For indexes that are not specified as unique SQL Server will silently make them unique any way. For clustered indexes this is done by appending a uniquefier to duplicate keys. For non clustered indexes the row identifier (logical or physical) is added to the key to guarantee uniqueness.
Unique index can be both clustered or non-clustered.
But if you have nullable column the NULL value should be unique (only 1 row where column is null).
If you want to store more then 1 NULLs you can create the index with filter "where columnName is not null".
well all the answers provided was very helpful, but still i would like to add some detailed answer so that i would be helpful for some others as well
A table can contain only one clustered index and a primary key can
be a clustered / non-clustered index.
Unique Key can be a clustered/non-clustered index as well,
below are some of the examples
Scenario 1 : Primary Key will default to Clustered Index
In this case we will create only Primary Key and when we check the kind of index created on the table we will notice that it has created clustered index automatically over it.
USE TempDB
GO
-- Create table
CREATE TABLE TestTable
(ID INT NOT NULL PRIMARY KEY,
Col1 INT NOT NULL)
GO
-- Check Indexes
SELECT OBJECT_NAME(OBJECT_ID) TableObject,
[name] IndexName,
[Type_Desc] FROM sys.indexes
WHERE OBJECT_NAME(OBJECT_ID) = 'TestTable'
GO
-- Clean up
DROP TABLE TestTable
GO
Scenario 2: Primary Key is defined as a Non-clustered Index
In this case we will explicitly defined Primary Key as a non-clustered index and it will create it as a non-clustered index. It proves that Primary Key can be non-clustered index.
USE TempDB
GO
-- Create table
CREATE TABLE TestTable
(ID INT NOT NULL PRIMARY KEY NONCLUSTERED,
Col1 INT NOT NULL)
GO
-- Check Indexes
SELECT OBJECT_NAME(OBJECT_ID) TableObject,
[name] IndexName,
[Type_Desc] FROM sys.indexes
WHERE OBJECT_NAME(OBJECT_ID) = 'TestTable'
GO
-- Clean up
DROP TABLE TestTable
GO
Scenario 3: Primary Key defaults to Non-Clustered Index with another column defined as a Clustered Index
In this case we will create clustered index on another column, SQL Server will automatically create a Primary Key as a non-clustered index as clustered index is specified on another column.
-- Case 3 Primary Key Defaults to Non-clustered Index
USE TempDB
GO
-- Create table
CREATE TABLE TestTable
(ID INT NOT NULL PRIMARY KEY,
Col1 INT NOT NULL UNIQUE CLUSTERED)
GO
-- Check Indexes
SELECT OBJECT_NAME(OBJECT_ID) TableObject,
[name] IndexName,
[Type_Desc] FROM sys.indexes
WHERE OBJECT_NAME(OBJECT_ID) = 'TestTable'
GO
-- Clean up
DROP TABLE TestTable
GO
Scenario 4: Primary Key defaults to Clustered Index with other index defaults to Non-clustered index
In this case we will create two indexes on the both the tables but we will not specify the type of the index on the columns. When we check the results we will notice that Primary Key is automatically defaulted to Clustered Index and another column as a Non-clustered index.
-- Case 4 Primary Key and Defaults
USE TempDB
GO
-- Create table
CREATE TABLE TestTable
(ID INT NOT NULL PRIMARY KEY,
Col1 INT NOT NULL UNIQUE)
GO
-- Check Indexes
SELECT OBJECT_NAME(OBJECT_ID) TableObject,
[name] IndexName,
[Type_Desc] FROM sys.indexes
WHERE OBJECT_NAME(OBJECT_ID) = 'TestTable'
GO
-- Clean up
DROP TABLE TestTable
GO
reference:the above details is been refrenced from this article
I am creating a table Brands with the following schema :
UserId
CarId
Brand
The UserId references the id of an user in the user table
The CarId references the id of a car in the car table
The only query that I will use is a search bases on these 2 columns, to get the corresponding brand.
So my question was about the constraint part, as I am a beginner, I would like to know which type of constraint to use (index, primary key, clustered or non clustered, on each field or on the 2 fields together) to have my query the more optimized possible.
This is my script right now :
IF NOT EXISTS (SELECT * FROM sys.objects WHERE object_id = OBJECT_ID(N'[Brands]') AND type in (N'U'))
BEGIN
CREATE TABLE [Brands]
(
UserId BIGINT NOT NULL
CONSTRAINT [FK_Brands_Users]
FOREIGN KEY REFERENCES [Users] (UserId),
CarId BIGINT NOT NULL
CONSTRAINT [FK_Brands_Cars]
FOREIGN KEY REFERENCES [Cars] (CarId),
Brand nvarchar(15),
);
END
GO
ALTER TABLE [Brands]
ADD CONSTRAINT [Unique_UserId_BrandId] UNIQUE(UserId, BrandId);
GO
I would create a primary key including both fields. When you define a primary key it automatically create a clustered index. Also your primary key has a unique constraint build in. Your table is now a heap the way you wrote it here above which is not good. You can additionally create an extra non-clustered index on CarId. Having an additional non-clustered index on UserId is not usefull I think. The column UserId can use the clustered index because it's the first field in the clustered index but I'm not sure about that.
Is it possible to create a clustered index from a create table statement in SQL Server 2008 that is not a primary key?
The purpose of this is for a table in SQL Azure, so it is not an option for me to first create the table, and then create the clustered index on the table.
Edit: Apparently it was FluentMigrator that was causing my problems, it's version table does not have a clustered index so it was erroring trying to create the versioning table not my table.
Yes, it is possible to create a clustered index that is not the primary key. Just use a CREATE CLUSTERED INDEX statement.
CREATE TABLE dbo.myTable (
myTableId int PRIMARY KEY NONCLUSTERED
myColumn int NOT NULL
)
CREATE CLUSTERED INDEX myIndex ON dbo.myTable(myColumn)
Prior to version Azure SQL Database v12, you had to have a clustered index before you could insert any data to a table. As of Azure SQL Database v12, heaps (tables without a clustered index) are now supported.
If your database was created prior to June 2016, here are the instructions for upgrading to version 12.
CREATE TABLE dbo.Table_1
(
Id int NOT NULL IDENTITY (1, 1) PRIMARY KEY NONCLUSTERED,
SomeOtherUniqueColumn int NOT NULL CONSTRAINT Item4 UNIQUE CLUSTERED
) ON [PRIMARY]
note the specification of nonclustered on the primary key
This will still work.
CREATE TABLE dbo.Table_1
(
SomeOtherUniqueColumn int NOT NULL CONSTRAINT Item4 UNIQUE CLUSTERED
) ON [PRIMARY]
The code below is compatible with Azure. It creates a primary key non-clustered and a clustered index in a single create table statement. This syntax also allows for specifying more than one column in your key.
CREATE TABLE MyTable (
ID uniqueidentifier NOT NULL,
UserID uniqueidentifier NOT NULL,
EntryDate DATETIME NOT NULL,
CONSTRAINT PK_MyPrimaryKey_Name PRIMARY KEY NONCLUSTERED (ID),
CONSTRAINT UCI_MyClusteredIndexName UNIQUE CLUSTERED (UserID ASC,EntryDate ASC,ID ASC)
);
In order to change a tables clustered index, the clusteredd index must be dropped, which converts the table into a heap and then the new clustered index is applied. Because Azure does not support heaps (tables without clustered indexes) it is not possible to change the clustered index without dropping the table and recreating it. In Azure you can not specify a clustered index in any other place other than the table create statement.
I would like create a table and add to it a Primary Key.
As for my understanding MS SQL add a clustered Index on the Primary Key and will name it with a default name.
I would like to know if is possible create a table and ASSIGN a custom name for the index created by default or how can i change the default name after the table as been created.
Thanks!
Sure - you can define the PRIMARY KEY constraint in your CREATE TABLE statement.
This will generate the default PRIMARY KEY
CREATE TABLE dbo.Table
(ID INT IDENTITY PRIMARY KEY,
.......)
but you can totally define the name of the constraint, too:
CREATE TABLE dbo.Table2
(ID INT IDENTITY CONSTRAINT PK_Table2 PRIMARY KEY,
......)
When you create a Clustered Primary Key, you do´nt create a index, but a Table organized as index.
Clustered Primary key is default option when you create a table with primary key on SqlServer.
http://msdn.microsoft.com/en-us/library/aa933131(SQL.80).aspx