Understanding the Limitations of Quoted Identifier in Dynamic SQL

When working with dynamic SQL in T-SQL, there are certain limitations and gotchas that can catch developers off guard. In this article, we’ll explore one such limitation related to QUOTED_IDENTIFIER settings.

The Problem: Conditional Changes to QUOTED_IDENTIFIER

In a batch of dynamic SQL, it’s not possible to conditionally change the setting for QUOTED_IDENTIFIER. Any occurrence of SET QUOTED_IDENTIFIER within the batch will override the session’s current setting. This means that once you’ve set QUOTED_IDENTIFIER to ON or OFF in your script, it cannot be changed again.

Why is this the case?

The explanation lies in how T-SQL handles dynamic SQL and batches. When a batch of dynamic SQL is executed, T-SQL will parse it as if it were part of the session’s current setting for QUOTED_IDENTIFIER. Any changes made to this setting within the batch are effectively ignored.

Consequences of this Limitation

This limitation has significant implications for developers working with dynamic SQL. Here are some key consequences:

Inability to dynamically toggle QUOTED_IDENTIFIER: As explained earlier, it’s not possible to conditionally change QUOTED_IDENTIFIER settings within a batch.
Tight coupling between procedure creation and execution: The process of creating a stored procedure is tightly coupled with its execution. This can make it difficult to test or modify procedures in isolation.

Workarounds

While the limitation itself might seem restrictive, there are ways to work around it:

Run your “cursor” outside the procedure creation batch: Instead of executing your dynamic SQL within the batch where you create the procedure, consider running it as a separate command.
Dump contents to file and execute the script: Another approach is to read the backup definitions into a file, including any necessary settings, and then execute the file’s contents as a script.

Example of Running Dynamic SQL Outside Procedure Creation Batch

Consider the following pseudocode for running dynamic SQL outside the procedure creation batch:

DECLARE @cmdOn VARCHAR(MAX) = 'exec(''set quoted_identifier on; print @@options & 256;'')';
DECLARE @cmdOff VARCHAR(MAX) = 'exec(''set quoted_identifier off; print @@options & 256;'')';

DECLARE @both VARCHAR(MAX) = CONCAT(@cmdOn, CHAR(10), @cmdOff);

-- Run the command
EXEC (@both);

Example of Dumping Contents to File and Executing as Script

Here’s an example pseudocode for reading backup definitions into a file and executing it:

-- Take backup of sql_modules via ssms/whatever
-- Drop procedures via ssms/whatever

-- Open connection to SQL server
OPEN CONNECTION @server;

-- Read definition, settings from backup table
DECLARE @definition VARCHAR(MAX);
DECLARE @settings VARCHAR(MAX);

FETCH NEXT FROM BackupDefinitions INTO @definition, @settings;

WHILE @@FETCH_STATUS = 0
BEGIN
    -- Execute the command
    EXEC (@definition);
    
    FETCH NEXT FROM BackupDefinitions INTO @definition, @settings;
END

-- Close connection to SQL server
CLOSE CONNECTION @server;

In conclusion, while it might seem restrictive at first, understanding the limitations of QUOTED_IDENTIFIER in dynamic SQL can help you find creative workarounds and improve your overall coding skills.

Last modified on 2024-11-08