3. Power Query: Append, Merge, and Group By

• Append Queries — stack tables one under another: January sales + February sales. Columns are matched by name;
• Merge Queries — bring in columns from another table by a key, like VLOOKUP: add the rep's city to every deal. The join kind is "Left Outer": every row of the first table stays, even if it has no match;
• Group By — totals by group, like a PivotTable: the sum by city.

In the trainer, the Power Query editor sits under the sheet. Pick a source, add steps with the "+ Step" button, and check the result after each step (click a step in the "Applied Steps" list). The M formula of the step is shown above the preview. Remove a wrong step with "Delete last step".