Data exploration and transformation remain a challenging prerequisite to the application of data analysis methods. The desired transformations are often ad-hoc so that existing end-user tools may not suffice, and plain programming may be necessary. We propose a guided query builder approach to reconcile expressivity and usability, i.e. to support the exploration of data, and the design of ad-hoc transformations, through data-user interaction only. This approach is available online as a client-side web application, named Dexteris. Its strengths and weaknesses are evaluated on a representative use case, and compared to plain programming and ChatGPT-assisted programming.