SQL Server - executing direct query vs. executing with sp_executesql

In SQL Server, executing a query directly and executing it using sp_executesql are two different approaches for executing dynamic SQL. Dynamic SQL refers to SQL statements that are constructed or modified at runtime and are not known at compile time.

1. Executing a Query Directly: When you execute a query directly, you simply send the complete SQL statement to the SQL Server for execution. For example:

   sqlCopy code
   DECLARE @FirstName NVARCHAR(50) = 'John';
   DECLARE @LastName NVARCHAR(50) = 'Doe';
   
   -- Direct execution
   SELECT * FROM Customers WHERE FirstName = @FirstName AND LastName = @LastName;
   

   The query is executed as it is, and SQL Server directly processes it. This method is straightforward and often used when you have a fixed SQL statement without any parameters.

   Advantages:

   • Simplicity: There's no additional complexity in the execution process.
   • Suitable for fixed queries: Ideal for fixed, static queries that don't change frequently.

   Disadvantages:

   • Vulnerable to SQL injection: If the input values are not properly sanitized, it can be vulnerable to SQL injection attacks.
   • No query plan reuse: The SQL Server has to generate a new query plan for each execution, which can impact performance for frequently executed dynamic queries.

2. Executing with sp_executesql: sp_executesql is a system stored procedure in SQL Server that allows you to execute dynamic SQL statements with parameters. It's designed to help mitigate SQL injection risks and enhance performance for frequently executed dynamic queries.

   sqlCopy code
   DECLARE @FirstName NVARCHAR(50) = 'John';
   DECLARE @LastName NVARCHAR(50) = 'Doe';
   DECLARE @Sql NVARCHAR(MAX);
   
   -- Dynamic SQL execution with sp_executesql
   SET @Sql = N'SELECT * FROM Customers WHERE FirstName = @FirstName AND LastName = @LastName;';
   EXEC sp_executesql @Sql, N'@FirstName NVARCHAR(50), @LastName NVARCHAR(50)', @FirstName, @LastName;
   

   In this approach, you construct the SQL statement as a string, and the parameters are specified separately as part of the sp_executesql call. The stored procedure handles parameterization and query plan reuse.

   Advantages:

   • Parameterization: sp_executesql allows for parameterized queries, reducing the risk of SQL injection attacks.
   • Query plan reuse: SQL Server can reuse the query plan, improving performance for frequently executed dynamic queries.

   Disadvantages:

   • Slightly more complex: It requires constructing the SQL statement as a string and specifying parameters separately.
   • Not suitable for all scenarios: sp_executesql might not be suitable for very complex dynamic SQL queries with changing structure.

In general, it's recommended to use sp_executesql when executing dynamic SQL, especially when the SQL statement depends on user input or external data. It helps to mitigate security risks and can improve performance by allowing for query plan reuse. However, if you have a fixed SQL query without parameters, executing it directly is straightforward and appropriate.