Add multiple primary keys to a table

Question

I have a table with a primary key available; I want to add another one in primary but can't add it.

SRNO	TRNDATE	RATE	AMT
1	08-02-2022	120	120
2	09-02-2022	170	170
3	10-02-2022	180	180

I want to add column Trndate to the primary key.

When I was try to add in primary following code

ALTER TABLE dbo.avgEnt 
ADD CONSTRAINT PK_avgent PRIMARY KEY NONCLUSTERED (srno, trndate);

I get an error:

Msg 1779, Level 16, State 0, Line 1
Table 'avgEnt' already has a primary key defined on it.

Msg 1750, Level 16, State 0, Line 1
Could not create constraint or index. See previous errors.

When I open the design of table and add primary key (SRNO, TRNDATE) then it works fine.

When I was trying to add by using the query, it does not work.

What happened when design windows save that time script i want.

My database version is SQL Server 2014.

Answer 1

A table cannot have multiple PRIMARY KEY s, this is simply not allowed, there is not work around. It is by design.

A table can, however, have multiple UNIQUE INDEX es or UNIQUE CONSTRAINT s. These are not the same as a PRIMARY KEY but do offer similar functionality, and may well provide the solution you need. A column (or set of columns) that are part of a UNIQUE INDEX can also be the target of a FOREIGN KEY , which if your "need" for a second PRIMARY KEY is so you can use it as a FOREIGN KEY then this would also fulfil that requirement.

As an example, you might have something like this:

CREATE TABLE dbo.PrimaryTable (ID int IDENTITY,
                               SomeValue varchar(10) NULL,
                               AnotherIdentifier uniqueidentifier NOT NULL
                               CONSTRAINT PK_PrimaryTable PRIMARY KEY (ID));

CREATE UNIQUE INDEX UQ_PrimaryTable_AnotherIdentifier ON dbo.PrimaryTable (AnotherIdentifier);
GO

CREATE TABLE dbo.SecondaryTable (ID int IDENTITY,
                                 PrimaryID int NOT NULL,
                                 AnotherValue decimal(12,2) NOT NULL,
                                 CONSTRAINT PK_SecondaryTable PRIMARY KEY (ID),
                                 CONSTRAINT FK_SecondaryTable_PrimaryTable FOREIGN KEY (PrimaryID) REFERENCES dbo.PrimaryTable (ID));
GO

CREATE TABLE dbo.AnotherTable (ID int IDENTITY,
                               AnotherID uniqueidentifier NOT NULL,
                               OtherValue int NOT NULL,
                               CONSTRAINT PK_AnotherTable PRIMARY KEY (ID),
                               CONSTRAINT FK_SecondaryTable_PrimaryTable_AnotherIdentifier FOREIGN KEY (AnotherID) REFERENCES dbo.PrimaryTable (AnotherIdentifier));
GO

So here you can see that dbo.SecondaryTable uses the ID column of dbo.PrimaryTable as it's FOREIGN KEY , however, dbo.AnotherTable uses the column AnotherIdentifier even though it isn't the PRIMARY KEY . This is because the column has a UNIQUE INDEX on it, meaning that data integrity can be maintained.

Otherwise, as I mentioned in the comments, you could split the table into 2 tables, and then have a 3rd table with a composite key to manage the relationship. So using the dbo.PrimaryTable you might have something like this:

CREATE TABLE dbo.PrimaryTable (ID int IDENTITY,
                               SomeValue varchar(10) NULL,
                               CONSTRAINT PK_PrimaryTable PRIMARY KEY (ID));

CREATE TABLE dbo.AnotherPrimaryTable (AnotherIdentifier uniqueidentifier NOT NULL,
                                      CONSTRAINT PK_AnotherPrimaryTable PRIMARY KEY (AnotherIdentifier));
GO
CREATE TABLE dbo.PrimaryTableLink (ID int NOT NULL,
                                   AnotherIdentifier uniqueidentifier NOT NULL,
                                   CONSTRAINT PK_PrimaryTableLink PRIMARY KEY (ID,AnotherIdentifier),
                                   CONSTRAINT FK_PrimaryTableLink_PrimaryTable FOREIGN KEY (ID) REFERENCES dbo.PrimaryTable (ID),
                                   CONSTRAINT FK_PrimaryTableLink_AnotherPrimaryTable FOREIGN KEY (AnotherIdentifier) REFERENCES dbo.AnotherPrimaryTable (AnotherIdentifier));

Which you use is up to you. It more depends on your design and requirements. I would likely only use the latter if you needed a many to many relationship, as with a one to one the UNIQUE INDEX serves the goal fine. If you wanted to use the multiple table solution for a one to one relationship, you'd likely need to also add UNIQUE INDEX es to the ID and AnotherIdentifier columns on dbo.PrimaryTableLink , which poses the question of why didn't you create said INDEX on the original column in dbo.PrimaryTable first?

Answer 2

I find one solution.

select OBJECT_NAME(OBJECT_ID) AS NameofConstraint
FROM sys.objects
where OBJECT_NAME(parent_object_id)='avgEnt'
and type_desc LIKE '%CONSTRAINT'

Find constraint of table PK_avgent

After drop primary on table

-- drop current primary key constraint
ALTER TABLE dbo.avgEnt
DROP CONSTRAINT PK_avgent;
GO

After add two column

-- create new primary key constraint
ALTER TABLE dbo.avgEnt
ADD CONSTRAINT PK_avgent PRIMARY KEY NONCLUSTERED (SrNo, TrnDate);
GO

it is work form me.

Answer 3

You can not alter the primary key. The only way is dropping it and then recreating it.

ALTER TABLE avgEnt DROP PRIMARY KEY, ADD PRIMARY KEY(srno,trndate);

https://www.db-fiddle.com/f/BQuEjw2pthMDb1z8NTdHv/0 check aswell