Differences between SQL Server temporary tables and table variables

Temporary Tables

Temporary tables are created in tempdb.  The name "temporary" is slightly misleading, for even though the tables are instantiated in tempdb, they are backed by physical disk and are even logged into the transaction log.  They act like regular tables in that you can query their data via SELECT queries and modify their data via UPDATE, INSERT, and DELETE statements.  If created inside a stored procedure they are destroyed upon completion of the stored procedure.  Furthermore, the scope of any particular temporary table is the session in which it is created; meaning it is only visible to the current user.  Multiple users could create a temp table named #TableX and any queries run simultaneously would not affect one another - they would remain autonomous transactions and the tables would remain autonomous objects.  You may notice that my sample temporary table name started with a "#" sign.  This is the identifier for SQL Server that it is dealing with a temporary table.

The syntax for creating a temporary table is identical to creating a physical table in Microsoft SQL Server with the exception of the aforementioned pound sign (#):

CREATE TABLE #TempTable 

(

   ID INT NOT NULL, 

   Name VARCHAR(10), 

   DOB DATETIME

)



Table Variables

The syntax for creating table variables is quite similar to creating either regular or temporary tables.  The only differences involve a naming convention unique to variables in general, and the need to declare the table variable as you would any other local variable in Transact SQL:

DECLARE @TableVariable TABLE (

   ID INT NOT NULL, 

   Name VARCHAR(10), 

   DOB DATETIME

)


As you can see the syntax bridges local variable declaration (DECLARE @variable_name variable_data_type) and table creation (column_name, data_type, nullability).  As with any other local variable in T-SQL, the table variable must be prefixed with an "@" sign.  Unlike temporary or regular table objects, table variables have certain clear limitations. 

  • Table variables can not have Non-Clustered Indexes
  • You can not create constraints in table variables
  • You can not create default values on table variable columns
  • Statistics can not be created against table variables


Similarities with temporary tables include:
  • Instantiated in tempdb
  • Clustered indexes can be created on table variables and temporary tables
  • Both are logged in the transaction log
  • Just as with temp and regular tables, users can perform all Data Modification Language (DML) queries against a table variable:  SELECT, INSERT, UPDATE, and DELETE.
 
Differences between SQL Server temporary tables and table variables

There are three major theoretical differences between temporary tables And table variables
  • The first difference is that transaction logs are not recorded for the table variables. Hence, they are out of scope of the transaction mechanism

    After declaring our temporary table #T and our table-variable @T, we assign each one with the same "old value" string. Then, we begin a transaction that updates their contents. At this point, both will now contain the same "new value" string. But when we rollback the transaction, as you can see, the table-variable @T retained its value instead of reverting back to the "old value" string. This happened because, even though the table-variable was updated within the transaction, it is not a part of the transaction itself.

  • The second major difference is that any procedure with a temporary table cannot be pre-compiled, while an execution plan of procedures with table variables can be statically compiled in advance. Pre-compiling a script gives a major advantage to its speed of execution. This advantage can be dramatic for long procedures, where recompilation can be too pricy.

  • Finally, table variables exist only in the same scope as variables. Contrary to the temporary tables, they are not visible in inner stored procedures and in exec(string) statements. Also, they cannot be used in an insert/exec statement.