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In the SQL Server world, dynamic SQL is often viewed with a mixture of suspicion and caution. This is largely due to the potential security risks associated with its misuse - most notably SQL injection attacks. However, when used correctly and safely, dynamic SQL can be an incredibly powerful tool, offering greater flexibility than static SQL. In this post, we will be discussing how to use dynamic SQL safely with the stored procedure sp_executesql, a feature available in SQL Server 2016 and SQL Server 2017.

sp_executesql is a system stored procedure that executes a Transact-SQL statement or batch that can be reused many times, or one that has been built dynamically. The main advantage of sp_executesql over the EXECUTE statement is its ability to accept parameters. This makes it a safer and more efficient choice for executing dynamic SQL.

DECLARE @SQL NVARCHAR(500)
DECLARE @ParamDefinition NVARCHAR(100)
DECLARE @City NVARCHAR(50)

SET @City = 'London'
SET @SQL = N'SELECT * FROM Customers WHERE City = @City'
SET @ParamDefinition = N'@City NVARCHAR(50)'

EXECUTE sp_executesql @SQL, @ParamDefinition, @City = @City

In the code snippet above, we define a dynamic SQL query that selects all records from the Customers table where the City is 'London'. The @City parameter is defined in the @ParamDefinition variable and passed to the sp_executesql stored procedure, which safely executes the dynamic SQL query.

While sp_executesql is specific to SQL Server, other databases offer similar functionalities. In MySQL 5.7, you can use the PREPARE and EXECUTE statements to safely run dynamic SQL. Here's an example:

SET @s = 'SELECT * FROM Customers WHERE City = ?';
PREPARE stmt FROM @s;
SET @city = 'London';
EXECUTE stmt USING @city;
DEALLOCATE PREPARE stmt;

In DB2 11.1, you can use the PREPARE and EXECUTE statements as well to safely run dynamic SQL. Here's an example:

DECLARE @s VARCHAR(100);
SET @s = 'SELECT * FROM Customers WHERE City = ?';
PREPARE stmt FROM @s;
SET @city = 'London';
EXECUTE stmt USING @city;

For those using Azure SQL, you'll be pleased to know that the sp_executesql stored procedure is available here too. It works in the same way as it does in SQL Server, providing a safe and efficient way to execute dynamic SQL.

In conclusion, when used properly, dynamic SQL can be an extremely useful tool in your SQL toolkit, providing you with the flexibility to construct queries dynamically. However, it's crucial to always prioritize security and use features like SQL Server's sp_executesql, MySQL's PREPARE and EXECUTE statements, or DB2's PREPARE and EXECUTE statements to safely execute dynamic SQL and guard against SQL injection attacks.