What role does a variable play in a query expression?

A variable in a query expression plays a critical role in allowing users to create more flexible and customizable data views. By using variables, users can modify the parameters of the query dynamically based on specific criteria or user input. This enables the generation of different outcomes from the same query structure, catering to varying analytical needs without the need for rewriting the entire query.

For instance, if a variable is used to filter data based on a user-selected date range or a specific product line, the query adapts to show only the relevant data. This adaptability enhances user experience and makes data analysis more efficient, as users can manipulate the variables to refine their queries according to what they wish to explore in the data set.

The other options are limiting in their descriptions; while a constant value can provide consistency in a query, it does not allow for customization. Similarly, performing data entries or locking data refers to actions that do not directly pertain to the dynamic nature of queries and the use of variables within them. The emphasis on user-defined data views highlights the significant advantage that variables offer in terms of flexibility and interactivity in query construction.