Primary key of temp table already exists - sql-server

Question

I have the following query
declare #var1 int, #var2 int
set #var1 = 110
set #var2 = 300
IF object_id('tempdb..#tbl_Contract') IS NOT NULL
BEGIN
ALTER TABLE #tbl_Contract drop constraint PK_#tbl_Contract
DROP TABLE #tbl_Contract
END
CREATE TABLE #tbl_Contract
( ContractID int NOT NULL
, PersonID int NOT NULL
, EndDate smalldatetime NULL
, CONSTRAINT [PK_#tbl_Contract] PRIMARY KEY CLUSTERED
(
ContractID ASC
)
)
...
But when I run this query the second time I'm getting an error:
Msg 2714, Level 16, State 5, Line 1 There is already an object named
'PK_#tbl_Contract' in the database. Msg 1750, Level 16, State 0,
Line 1 Could not create constraint. See previous errors.
What I'm doing wrong? Why my primary key was not removed?
Should I use GO after ALTER? But I can't because there are some varibles

Answer 1

You don't need to drop a constraint and then immediately drop the table. It is simpler to just drop the table. Also, there really is no need to name constraints in a temp table. You can greatly simplify your code like this.
IF object_id('tempdb..#tbl_Contract') IS NOT NULL
BEGIN
DROP TABLE #tbl_Contract
END
GO
CREATE TABLE #tbl_Contract
( ContractID int NOT NULL PRIMARY KEY CLUSTERED
, PersonID int NOT NULL
, EndDate smalldatetime NULL
)

Answer 2

For a composite primary key constraint on a temporary table it is much better not to name the primary key constraint, otherwise you will run into trouble with concurrent sessions. In the following way you can also add any number of indexes or check constraints without naming them:
IF object_id('tempdb..#tbl_Contract') IS NOT NULL
BEGIN
DROP TABLE #tbl_Contract
END
GO
CREATE TABLE #tbl_Contract
( ContractID int NOT NULL
, PersonID int NOT NULL
, EndDate smalldatetime NULL
, PRIMARY KEY CLUSTERED (ContractID ASC, PersonID ASC)
, CHECK (ContractID > 10 AND EndDate > N'2019-12-31')
)
This allows you to avoid the pitfall of #mpag's solution in that you can further use the temp table in your stored procedure. In #mpag's solution you cannot use #tbl_Contract outside the #Q statement.
You cannot create a primary key with the same name in different sessions, even on a temp table. If you do not name the primary key and other constraints, they are automatically given a different unique name by the system for each session.
Otherwise you will have the following error when concurrently running the procedure from different sessions:
Error number: 1750, error procedure:
dbo.ProcedureName, error line: XXX, error message:
Could not create constraint or index. See previous errors.

Answer 3

I had the same issue as #NReilingh raised in the comments (i.e. needing a composite primary key). So, lets assume we want a primary key on the combination of ContractID and PersonID. The following works in that circumstance (assuming you have appropriate permissions for EXEC):
IF object_id('tempdb..#tbl_Contract') IS NOT NULL
DROP TABLE #tbl_Contract
GO
DECLARE #Q as nvarchar(MAX) = N'
CREATE TABLE #tbl_Contract (
ContractID int NOT NULL
, PersonID int NOT NULL
, EndDate smalldatetime NULL
, CONSTRAINT
[PK_#tbl_Contract' + CAST(##SPID as nvarchar(6)) + N']
PRIMARY KEY CLUSTERED (
ContractID ASC,
PersonID ASC
)
)'
EXEC (#Q)
This works because each connection at any one time on a given server should have a unique SPID value. SPID values are typically a double-digit number greater than 50; but are of type smallint so could be up to 5 digits long, potentially signed.
Remember to double up any single quotes in within your table's DDL (i.e. replace ' with '')