Check If Temporary Table Exists

- March 29, 2008

When working with temporary tables in SQL Server, the common conundrum is often determining whether a temp table exists before performing operations on it. In this blog post, we’ll delve into the nuances of checking the existence of temp tables and explore the potential pitfalls and solutions.

The Basics: Local Temp Tables

To check if a local temp table exists, one commonly used approach is:

IF OBJECT_ID('tempdb..#temp') IS NOT NULL
BEGIN
    PRINT '#temp exists!'
END
ELSE
BEGIN
    PRINT '#temp does not exist!'
END

This method utilizes the OBJECT_ID function to verify the existence of the temp table in the tempdb database.

The Undocumented Second Parameter

An alternative way involves an undocumented optional second parameter:

IF OBJECT_ID('tempdb..#temp','u') IS NOT NULL
BEGIN
    PRINT '#temp exists!'
END
ELSE
BEGIN
    PRINT '#temp does not exist!'
END

However, it’s important to note that using 'u' is undocumented, and relying on undocumented features always comes with a level of caution.

Local Temp Table Caveat

Beware of inadvertently checking the local database:

--Don't do this because this checks the local DB and will return does not exist
IF OBJECT_ID('tempdb..#temp','local') IS NOT NULL
BEGIN
    PRINT '#temp exists!'
END
ELSE
BEGIN
    PRINT '#temp does not exist!'
END

This approach may lead to false negatives as it checks the local database, not the tempdb.

Global Temp Tables Unveiled

Let’s now explore global temp tables, denoted by the double pound sign (##). Creating a global temp table involves:

CREATE TABLE ##temp(id INT)

Notice the 2 pound signs, that’s how you create a global variable`

To check its existence:

-- Check if it exists
IF OBJECT_ID('tempdb..##temp') IS NOT NULL
BEGIN
    PRINT '##temp exists!'
END
ELSE
BEGIN
    PRINT '##temp does not exist!'
END

Global temp tables persist beyond the scope of the creating session.

Testing the Waters: Multiple Windows

Now, let’s open a new Query Analyzer window (CTRL + N) and run the same code:

`-- Check if it exists
IF OBJECT_ID('tempdb..##temp') IS NOT NULL
BEGIN
    PRINT '##temp exists!'
END
ELSE
BEGIN
    PRINT '##temp does not exist!'
END`

Surprisingly, it exists! Global temp tables are visible across different Query Analyzer windows.

Recap and Caution

In summary, checking the existence of temp tables involves the judicious use of OBJECT_ID. Local temp tables are session-specific, and the check should be made in the tempdb database. Global temp tables, on the other hand, persist across sessions.

Understanding these nuances ensures accurate checks and prevents potential pitfalls when working with temporary tables in SQL Server. Happy querying!