# M Query

> Power Query M conventions for pbiflow models. Triggers when user mentions "m-query", "power query", "m language", ".m model", "powerquery", or edits `**/*.m` files. Read before writing or reviewing M code.

- Skill: `thestratcore/m-query` (Agent Skill)
- Install (CLI): `npx skillmds@latest add thestratcore/m-query`
- Raw SKILL.md: https://api.skillmd.com/api/skills/thestratcore/m-query/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: AI & ML
- Author: thestratcore (https://skillmd.com/u/thestratcore)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/thestratcore/m-query

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**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`.