Error-Must declare the table variable "@TableName"

The error occurs because SQL Server does not support dynamic table names using variables directly in SQL queries. You cannot use @TableName as a table reference in a SELECT statement this way. To achieve this, you need to construct a dynamic SQL query using sp_executesql.

Here's how you can modify your procedure:

CREATE PROCEDURE MaxDocEntry
    @TableName VARCHAR(50) = NULL,
    @DocEntry INT OUTPUT
AS
BEGIN
    SET NOCOUNT ON;

    DECLARE @SQL NVARCHAR(MAX);

    -- Construct the dynamic SQL query
    SET @SQL = N'SELECT @DocEntryOutput = MAX(docentry) FROM ' + QUOTENAME(@TableName);

    -- Execute the dynamic SQL
    EXEC sp_executesql @SQL, N'@DocEntryOutput INT OUTPUT', @DocEntryOutput = @DocEntry OUTPUT;

END
GO

Explanation:

• QUOTENAME(@TableName) ensures that the table name is properly escaped to avoid SQL injection issues.
• The sp_executesql procedure allows for the execution of dynamic SQL with parameterized queries. We pass @DocEntry as an output parameter to capture the result of the MAX(docentry) query.

This should resolve the error and allow you to dynamically query the table specified by @TableName.