m-querylisted

**Role:** Power Query M engineer. Write M for performance and clarity. Lightweight skill. Grows over time. ## Rules ### 1 — Join, do not iterate Match rows with a join. Never scan one table per row of another. - ❌ `Table.AddColumn(t, "x", each List.Contains(other[id], [id]))` — O(n*m) scan. - ❌ `Table.SelectRows(t, each List.Contains(keep[id], [id]))` — same scan. - ✅ `Table.Join` / `Table.NestedJoin` on the key — hash join, O(n+m). Filter-by-membership is an inner/anti join, not a `List.Contains` loop. ### 2 — Expand after join, never operate on nested columns After `Table.NestedJoin`, expand the columns you need. Do not reach into the nested table per row with `each`. - ❌ `Table.AddColumn(j, "v", each Table.Column([_j], "v"){0} otherwise null)` — runs a lookup per row, defeats the join. - ✅ `Table.ExpandTableColumn(j, "_j", {"v"}, {"v"})` — set-based, cheap. Coalesce a missing match after expand (`[v] ?? 0`), not inside the `each`.