Monday, April 27, 2009

Where are temporary tables stored in SQL Server?

Good question right? :)

There are many cases where we need to create temporary tables in SQL Server and for various reasons.
Such reasons may include:

  • Breaking the logic of a large and complex SQL Statement in smaller portions of code.
  • Increase the performance of a SQL query, etc.

But what types of temporary tables does SQL Server provide and what is the meaning of each type? Last but not least, where are temporary tables stored and how can we get schema information about them?

There are two types of temporary tables:

  • Local temporary tables:
    • Only available to the current connection to the database for the current login
    • They are dropped when the connection is closed
  • Global temporary tables:
    • Available to any connection upon their creation
    • They are dropped when the last connection using them is closed

Code Example – Local Temporary Table
CREATE TABLE #table_name
(
column_name [DATATYPE]
)


Code Example – Global Temporary Table
CREATE TABLE ##table_name
(
column_name
[DATATYPE]
)


So, consider as an example that you create the following temporary table:

CREATE TABLE #temp_table
(
id INT,
name VARCHAR(50)
)

Then let’s say you want to find schema information regarding the above table. Where can you find this information in SQL Server?

The answer is that temporary tables (local and global) are stored in the tempDB database.

So, if you want to find schema information for the temporary table named temp_table you can use the following queries:

--Query 1(a): Get the exact name of the temporary table you are looking for
DECLARE @table_name AS VARCHAR(300)

SET @table_name = (SELECT TOP 1 [name]
FROM tempdb..sysobjects
WHERE name LIKE '#temp_table%')

Explanation: When you declare a temporary table, SQL Sever adds some additional characters on its name in order to provide a unique system name for it and then it stores it in tempDB in the sysobjects table. Even though you can query the temporary table with its logical name, internally is known with the exact name SQL Server has set. To this end, you need to execute the above query for finding the exact name of the temporary table.

--Query 1(b): Get column information for the temporary table
-- by using the sp_columns stored procedure

EXEC tempdb..sp_columns

@table_name

Explanation: The sp_columns stored procedure returns column information for the specified tables or views that can be queried in the current environment.


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