Local Temporary Table and Table Variable

There are lots of articles, blogs, forums discussed the differences and similarities between table variables and local temporary tables (ref. as temp tables hereafter). E.g.:

• Sql server table variable vs. local temporary table talks more about the difference from execution perspective.
• http://databases.aspfaq.com/database/should-i-use-a-temp-table-or-a-table-variable.html talks more about how to use them.
• And the recent article in SQLServerCentral.com

A summarization of table variable vs. temp table is attached in the appendix. Instead of repeating those well known facts, I'll focus on several differences that are hardly mentioned so far between table variable and temp table definitions.

XML Collection

The AdventureWorks sample database has an xml collection HumanResources.HRResumeSchemaCollection, which is used by the [Resume] column in the table HumanResources.JobCandidate.

The following script queries all the resumes into a table variable:

USE AdventureWorks
GO
DECLARE @TestXmls TABLE
(JobCandidateID int NOT NULL, [Resume] xml(HumanResources.HRResumeSchemaCollection) PRIMARY KEY CLUSTERED(JobCandidateID))

INSERT @TestXmls (JobCandidateID,[Resume])

SELECT JobCandidateID,[Resume] FROM HumanResources.JobCandidate
SELECT * FROM @TestXmls

GO

USE AdventureWorks
GO

It works perfectly. However, if you change the table variable to a temp table:

CREATE TABLE #TestXmls (JobCandidateID int NOT NULL,
  [Resume] xml(HumanResources.HRResumeSchemaCollection) 
  PRIMARY KEY CLUSTERED(JobCandidateID))

You will get the following error:

Msg 6314, Level 16, State 1, Line 2

Collection specified does not exist in metadata : 'HumanResources.HRResumeSchemaCollection'

This is because a temp table is created in TempDB, which does not have the xml collection HumanResources.HRResumeSchemaCollection. Though you can create the xml collection in the TempDB, it's not practical because:

• TempDB is recreated whenever SQL server restarts;
• TempDB is shared by all user/system databases and it is easily to have naming conflicts;

The work around is to use un-typed xml in the temporary table.

CREATE TABLE #TestXmls (JobCandidateID int NOT NULL,
  [Resume] xml PRIMARY KEY CLUSTERED(JobCandidateID))

INSERT #TestXmls (JobCandidateID,[Resume])

SELECT JobCandidateID,[Resume] FROM HumanResources.JobCandidate
SELECT * FROM #TestXmls
GO

User Defined DataTypes and User Defined Types (UDTs)

A user defined data type and UDT (CLR type) is similar to xml collection. It only resides in the database it is created. The AdventureWorks sample database has a user defined data type dbo.name, whose native data type is nvarchar(50). The following script queries all the address type names:

USE AdventureWorks
GO
DECLARE @TestUDTs TABLE (AddressTypeID int NOT NULL,
  [Name] dbo.Name NOT NULL PRIMARY KEY CLUSTERED(AddressTypeID))

INSERT @TestUDTs(AddressTypeID,[Name])

SELECT AddressTypeID,[Name]
 FROM Person.AddressType
SELECT * FROM @TestUDTs

It works fine. If you change to use temp table:

CREATE TABLE #TestUDTs (AddressTypeID int NOT NULL,
[Name] nvarchar(50) NOT NULL PRIMARY KEY CLUSTERED(AddressTypeID))

It will fail with the following error:

Msg 2715, Level 16, State 7, Line 1

Column, parameter, or variable #2: Cannot find data type dbo.Name.

The work around is to use the native data type of the user defined data type:

CREATE TABLE #TestUDTs (AddressTypeID int
NOT NULL,
[Name] nvarchar(50) NOT NULL PRIMARY KEY CLUSTERED(AddressTypeID))

UDTs are also scoped to a single database. If you use UDTs in a table variable, they are used in the same way as native data types. However since there is no equivalent native data type, if you need to use a UDT in a temp table, the UDT must be registered in tempdb the same way as for a user database, i.e. you need to CREATE the ASSEMLY and CREATE the TYPE in the TempDB. Due to the reasons mentioned in XML Collection

Collation

The collations for the string type (char, nchar, varchar, nvarchar) columns, if not given explicitly, will take the database collation of TempDB in a temp table, and inherit the collation of the current user database in a table variable. If not handled correctly, string comparing will fail.

Let's first find out the collation of the TempDB:

SELECT collation_name FROM sys.databases
WHERE database_id = DB_ID('tempdb')

To list all available collations:

SELECT * FROM fn_helpcollations()

Now let's create a test database, remember to use a different collation than the TempDB:

CREATE DATABASE CollateTest COLLATE Albanian_BIN
GO
USE CollateTest
GO

Create a table and populate some data:

CREATE TABLE dbo.Test1 (C2 int NOT NULL, C1 nvarchar(100) NOT NULL PRIMARY KEY CLUSTERED(C2))

GO

INSERT dbo.Test1 (C1,C2) SELECT N'R1',100 UNION ALL SELECT N'R2',200

GO

Now use a table variable to join with the table:

DECLARE @TestCollations TABLE (RID int identity not null, [Name] nvarchar(100) NOT NULL
    PRIMARY KEY CLUSTERED(RID))

INSERT @TestCollations ([Name]) VALUES (N'R2')
INSERT @TestCollations ([Name]) VALUES (N'R3')

SELECT A.* FROM dbo.Test1 A, @TestCollations B WHERE A.C1=B.[Name]
GO

It works fine. If you change to use temp table:

CREATE TABLE #TestCollations (RID int identity not null, [Name] nvarchar(100) NOT NULL
    PRIMARY KEY CLUSTERED(RID))

INSERT #TestCollations ([Name]) VALUES (N'R2')
INSERT #TestCollations ([Name]) VALUES (N'R3')

SELECT A.* FROM dbo.Test1 A, #TestCollations B WHERE A.C1=B.[Name]
GO

You get the following error:

Msg 468, Level 16, State 9, Line 6

Cannot resolve the collation conflict between "Latin1_General_CI_AI" and "Albanian_BIN" in the equal to operation.

There are two workarounds. One is to force the collation conversion in the string comparing:

SELECT A.* 
 FROM dbo.Test1 A, #TestCollations B 
  WHERE A.C1=B.[Name] collate database_default

However it's a little bit annoying if you have lots of string comparing in your database. A better solution is to explicitly give the collation for the string column in the temp table:

IF object_id('tempdb..#TestCollations') IS NOT NULL
 DROP TABLE #TestCollations
GO
CREATE TABLE #TestCollations (
  RID int identity not null, 
  [Name] nvarchar(100) collate database_default NOT NULL 
  PRIMARY KEY CLUSTERED(RID))

INSERT #TestCollations ([Name]) VALUES (N'R2')
INSERT #TestCollations ([Name]) VALUES (N'R3')

SELECT A.* FROM dbo.Test1 A, #TestCollations B WHERE A.C1=B.[Name]
GO

The "collate database_default" clause in the column definition forces SQL Server to take current database(CollateTest)'s collation instead of TempDB's for the temp table.

If a SQL Server hosts multiple databases, or you need to restore external databases to the server, you can easily run into the collation conflict issue if the user databases use temp tables and string columns do not declare collations properly.

Conclusion

If you use table variables, you are free to use user defined data types, user defined types (UDTs), and xml collections that are defined in your database. You don't need to worry about collation conflicts. If you have to use a temp table due to various reasons, please keep in mind the following best practices:

• Always attach "collate database_default" clause for string columns in a temp table.
• Always use un-typed xml for xml columns in a temp table.
• Always use the equivalent native data type for a user defined data type in a temp table.
• If you need to use UDTs in a temp table, you have to register the type in TempDB.

 Appendix

Table Variable vs. Temp Table Summary: