Working with Dynamic SQL Queries

Dynamic SQL queries are a powerful tool in SQL Server that allow you to construct and execute SQL statements dynamically at runtime. This can be useful when you need to perform complex queries that involve variables or user-input data.

In this tutorial, we will cover the basics of dynamic SQL queries, including how to declare variables, construct SQL statements, and execute them using the EXEC command or the sp_executesql stored procedure.

Declaring Variables

Before we can start working with dynamic SQL queries, we need to declare some variables to hold our data. In SQL Server, you can declare variables using the DECLARE statement, followed by the variable name and its data type.

For example:

DECLARE @theDate DATETIME;
SET @theDate = '2010-01-01';

In this example, we declare a variable called @theDate with a data type of DATETIME, and then set its value to '2010-01-01'.

Constructing SQL Statements

Once we have our variables declared, we can start constructing our SQL statement. We can use string concatenation to build our query, using the + operator to concatenate strings.

For example:

DECLARE @SQL VARCHAR(MAX);
SET @SQL = 'SELECT * FROM tblWO WHERE OrderDate BETWEEN ''' + CONVERT(VARCHAR(10), @theDate, 120) + ''' AND ''' + CONVERT(VARCHAR(19), DATEADD(SECOND, -1, DATEADD(DAY, 1, @theDate)), 120) + '''';

In this example, we declare a variable called @SQL with a data type of VARCHAR(MAX), and then set its value to a SQL statement that selects all columns from the tblWO table where the OrderDate is between the start and end dates.

Executing Dynamic SQL Queries

Once we have our SQL statement constructed, we can execute it using the EXEC command or the sp_executesql stored procedure.

For example:

EXEC (@SQL);

This will execute the SQL statement contained in the @SQL variable.

Alternatively, we can use the sp_executesql stored procedure to execute our query, which allows us to pass parameters to the query and avoid SQL injection attacks.

DECLARE @sqlCommand nvarchar(1000);
DECLARE @columnList varchar(75);
DECLARE @city varchar(75);

SET @columnList = 'CustomerID, ContactName, City';
SET @city = 'London';

SET @sqlCommand = 'SELECT ' + @columnList + ' FROM customers WHERE City = @city';

EXECUTE sp_executesql @sqlCommand, N'@city nvarchar(75)', @city = @city;

In this example, we declare a variable called @sqlCommand with a data type of nvarchar(1000), and then set its value to a SQL statement that selects all columns from the customers table where the City is equal to the @city parameter. We then execute the query using the sp_executesql stored procedure, passing in the @city parameter.

Best Practices

When working with dynamic SQL queries, it’s essential to follow best practices to avoid SQL injection attacks and ensure that your code is secure and efficient. Here are some tips:

• Always use parameters when executing dynamic SQL queries, rather than concatenating user-input data into the query.
• Use the sp_executesql stored procedure instead of the EXEC command whenever possible.
• Avoid using dynamic SQL queries whenever possible, and opt for static queries instead.
• Always validate user-input data before passing it to a dynamic SQL query.

By following these best practices and understanding how to work with dynamic SQL queries, you can write more efficient and secure code that takes advantage of the power of SQL Server.