The must declare the scalar variable error is undoubtedly one of the most commonly seen problems for any coder despite their expertise. There are a wide range of causes behind the problem. 

In this case, the Declare statement doesn’t declare the query’s function. There is a local variable in the script and function, which can also be stored as a procedure for the statement Declare.

This variable declaration is used to specify the data type’s name. Then, it will allocate the storage. Plus, the declaration assigns the initial value of the variable and imposes the constraint named Not Null. 

This way, the variable can keep its values without internal components and change the value. 

Scalar Variable Warning Reasons

The problem  arises when you execute an undeclared variable. 

Use An Undeclared Variable

The warning message will surely pop up when you fail to declare the variable. For example:

PRINT @AuthorName

The problem declares the scalar @AuthorName variable. The main cause for this error is the execution of the variable in the statement Print without its declaration. 

This approach is not allowed in the SQL server.

Go Statement Before Local Declared Variable

This case will also show an error:

DECLARE @AuthorName VARCHAR(100) = ‘Vishal Balasubramanian’

PRINT @AuthorName

GO

PRINT @AuthorName

Output:

Must declare the scalar variable “@AuthorName.”

In this case, the @AuthorName variable goes after the GO statement batch separator in the statement. This means it is not declared to the local variables’ scope in the batch.

How To Fix The must Declare The Scalar Variable Issue

Here are some methods to fix it and make your code run smoothly.

Solve The Undeclared Variable

 It is one of the most simple approaches, which only require you to declare the variable before passing the statement. It is worth noting down any local name will start with @. Without this syntax necessity, you will not define the type of data.

Also, if you fail to make the initial value, the variable will be assigned as a Null.

Method 2: Batch Separator And Go Statement Fix

As a rule of thumb, only use the variable in the statement Print after redeclaration and Go Statement. In this approach, employing a Go statement enables users to create new branches.

Each branch shows declared variables which are invisible past this statement. This way, those at the file’s top are not accessible without executing the Go Statement.

Despite twice variable declaration, the code will still work smoothly. This is because the SQL server sends the statements’ first batch to the engine for execution. 

Then, the second batch will be delivered to the SQL engine after the Go statement when the previous execution is off.

Conclusion

There are various methods to fix the must declare the scalar variable error. Check the article above again to choose a suitable approach.