diff --git a/.changeset/dataset-authoring-form.md b/.changeset/dataset-authoring-form.md
new file mode 100644
index 0000000000..f8bd5ce73e
--- /dev/null
+++ b/.changeset/dataset-authoring-form.md
@@ -0,0 +1,20 @@
+---
+"@objectstack/spec": minor
+---
+
+feat(spec): dataset authoring form + derived measures without a dummy aggregate
+
+`dataset` was the only UI-authorable metadata type without a `defineForm`
+layout, so Studio's create surface fell back to the auto-generated flat layout
+(free-text `object`, no grouping). Adds `dataset.form.ts` (registered in
+`METADATA_FORM_REGISTRY`): sectioned Basics / Source / Dimensions / Measures
+with an `object` picker (`ref:object`) and guidance — matching the sibling
+`report` editor.
+
+Also makes `DatasetMeasureSchema.aggregate` optional. A derived measure
+(`derived: { op, of }`) combines other measures by name and `aggregate` is
+ignored for it at compile time, but the schema still required it — so a derived
+measure failed validation unless you added a meaningless aggregate. `aggregate`
+is now required only for non-derived measures (enforced in the existing
+`superRefine`). Backward compatible: existing measures that carry an aggregate
+stay valid.
diff --git a/.changeset/dataset-join-ambiguous-column.md b/.changeset/dataset-join-ambiguous-column.md
new file mode 100644
index 0000000000..e909465afd
--- /dev/null
+++ b/.changeset/dataset-join-ambiguous-column.md
@@ -0,0 +1,13 @@
+---
+"@objectstack/service-analytics": patch
+---
+
+fix(analytics): qualify base-object columns in joined dataset queries
+
+A dataset that joins a related object (`include` + a `relationship.field`
+dimension/measure) emitted BARE base-table columns in SELECT/GROUP BY while the
+joined columns were alias-qualified. When the base and joined tables share a
+column name (e.g. both have `status`), the query failed at runtime with
+"ambiguous column name". `NativeSQLStrategy` now qualifies plain base-column
+identifiers with the base table when the cube has joins; single-object cubes
+are unchanged (byte-for-byte identical SQL).
diff --git a/packages/services/service-analytics/src/__tests__/native-sql-rls.test.ts b/packages/services/service-analytics/src/__tests__/native-sql-rls.test.ts
index a2aca608d8..34d46bb15c 100644
--- a/packages/services/service-analytics/src/__tests__/native-sql-rls.test.ts
+++ b/packages/services/service-analytics/src/__tests__/native-sql-rls.test.ts
@@ -108,3 +108,51 @@ describe('NativeSQLStrategy — D-C RLS hardening', () => {
     expect(sql).toContain('$2');
   });
 });
+
+describe('NativeSQLStrategy — base-column qualification under joins', () => {
+  // Regression: a joined dataset whose base table and joined table share a
+  // column name (e.g. `status`) produced `GROUP BY status` → "ambiguous column
+  // name" at runtime. Base columns must be qualified with the base table when
+  // the cube can join. (Found by dogfooding a joined dataset in Studio.)
+  const joinCube: Cube = {
+    name: 'sales',
+    title: 'Sales',
+    sql: 'opportunity',
+    measures: { revenue: { name: 'revenue', label: 'Revenue', type: 'sum', sql: 'amount' } },
+    dimensions: {
+      status: { name: 'status', label: 'Status', type: 'string', sql: 'status' },
+      region: { name: 'region', label: 'Region', type: 'string', sql: 'account.region' },
+    },
+    joins: { account: { name: 'account', relationship: 'many_to_one', sql: '' } },
+    public: false,
+  };
+
+  it('qualifies a base-table dimension with the base table when the cube has joins', async () => {
+    const strategy = new NativeSQLStrategy();
+    const ctx = ctxWith({
+      getCube: (n) => (n === 'sales' ? joinCube : undefined),
+      getAllowedRelationships: () => new Set(['account']),
+    });
+    const q: AnalyticsQuery = { cube: 'sales', measures: ['revenue'], dimensions: ['status', 'region'], timezone: 'UTC' };
+    const { sql } = await strategy.generateSql(q, ctx);
+    expect(sql).toContain('"opportunity"."status"'); // base column qualified (was bare `status`)
+    expect(sql).toContain('"account"."region"');      // relationship column still qualified
+    expect(sql).toContain('LEFT JOIN "account"');
+    expect(sql).not.toMatch(/GROUP BY\s+status\b/);    // no bare, ambiguous base column
+  });
+
+  it('leaves base columns BARE for a single-object cube (no joins) — generated SQL unchanged', async () => {
+    const soloCube: Cube = {
+      name: 'tasks', title: 'Tasks', sql: 'task',
+      measures: { c: { name: 'c', label: 'Count', type: 'count', sql: '*' } },
+      dimensions: { status: { name: 'status', label: 'Status', type: 'string', sql: 'status' } },
+      public: false,
+    };
+    const strategy = new NativeSQLStrategy();
+    const ctx = ctxWith({ getCube: (n) => (n === 'tasks' ? soloCube : undefined) });
+    const q: AnalyticsQuery = { cube: 'tasks', measures: ['c'], dimensions: ['status'], timezone: 'UTC' };
+    const { sql } = await strategy.generateSql(q, ctx);
+    expect(sql).toContain('GROUP BY status'); // bare — unchanged for single-object cubes
+    expect(sql).not.toContain('"task"."status"');
+  });
+});
diff --git a/packages/services/service-analytics/src/dataset-compiler.ts b/packages/services/service-analytics/src/dataset-compiler.ts
index 083573d7eb..da273e5cb7 100644
--- a/packages/services/service-analytics/src/dataset-compiler.ts
+++ b/packages/services/service-analytics/src/dataset-compiler.ts
@@ -58,6 +58,11 @@ export type RelationshipResolver = (
 
 /** Map a dataset measure's aggregate to the Cube metric `type`. */
 function aggregateToMetricType(m: DatasetMeasure): Metric['type'] {
+  // Only reached for non-derived measures, where the spec refinement guarantees
+  // an aggregate; guard defensively so the type narrows from `optional`.
+  if (!m.aggregate) {
+    throw new Error(`[dataset-compiler] non-derived measure "${m.name}" has no aggregate`);
+  }
   if (UNSUPPORTED_AGGREGATES.has(m.aggregate)) {
     throw new Error(
       `[dataset-compiler] measure "${m.name}" uses aggregate "${m.aggregate}" which is ` +
diff --git a/packages/services/service-analytics/src/strategies/native-sql-strategy.ts b/packages/services/service-analytics/src/strategies/native-sql-strategy.ts
index 03668fa328..d25aff5a52 100644
--- a/packages/services/service-analytics/src/strategies/native-sql-strategy.ts
+++ b/packages/services/service-analytics/src/strategies/native-sql-strategy.ts
@@ -234,7 +234,19 @@ export class NativeSQLStrategy implements AnalyticsStrategy {
     joins: Map<string, string>,
     cube?: Cube,
   ): string {
-    if (!rawSql.includes('.')) return rawSql;
+    if (!rawSql.includes('.')) {
+      // Base-table column. When the cube can join other tables, a bare column
+      // that also exists on a joined table (e.g. base `status` vs joined
+      // `account.status`) makes the SQL engine raise "ambiguous column name".
+      // Qualify plain identifiers with the base table; leave SQL expressions
+      // and `*` untouched. Single-object cubes (no joins) keep bare columns so
+      // their generated SQL is byte-for-byte unchanged.
+      const canJoin = !!cube?.joins && Object.keys(cube.joins).length > 0;
+      if (canJoin && /^[A-Za-z_][A-Za-z0-9_]*$/.test(rawSql)) {
+        return `"${parentTable}"."${rawSql}"`;
+      }
+      return rawSql;
+    }
     // Only the first dotted hop is supported (single-level relation).
     const [alias, ...rest] = rawSql.split('.');
     if (!alias || rest.length === 0) return rawSql;
diff --git a/packages/spec/api-surface.json b/packages/spec/api-surface.json
index 43d8ce822c..78b495f336 100644
--- a/packages/spec/api-surface.json
+++ b/packages/spec/api-surface.json
@@ -3342,6 +3342,7 @@
     "actionForm (const)",
     "appForm (const)",
     "dashboardForm (const)",
+    "datasetForm (const)",
     "defineAction (function)",
     "defineApp (function)",
     "defineDataset (function)",
diff --git a/packages/spec/src/system/metadata-form-registry.ts b/packages/spec/src/system/metadata-form-registry.ts
index 0d81b5234f..fb3114fb0c 100644
--- a/packages/spec/src/system/metadata-form-registry.ts
+++ b/packages/spec/src/system/metadata-form-registry.ts
@@ -37,6 +37,7 @@ import {
     viewForm,
     appForm,
     dashboardForm,
+    datasetForm,
     actionForm,
     pageForm,
     reportForm,
@@ -67,6 +68,7 @@ export const METADATA_FORM_REGISTRY: Readonly<Record<string, FormView>> = Object
     field: fieldForm,
     hook: hookForm,
     report: reportForm,
+    dataset: datasetForm,
     view: viewForm,
     app: appForm,
     dashboard: dashboardForm,
diff --git a/packages/spec/src/ui/dataset.form.ts b/packages/spec/src/ui/dataset.form.ts
new file mode 100644
index 0000000000..63267aa22f
--- /dev/null
+++ b/packages/spec/src/ui/dataset.form.ts
@@ -0,0 +1,63 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+import { defineForm } from './view.zod';
+
+/**
+ * Form layout for the Dataset metadata editor (ADR-0021 analytics semantic layer).
+ *
+ * Bound to {@link DatasetSchema}. Until this entry existed, `dataset` was the
+ * only UI-authorable metadata type WITHOUT a registered {@link FormView}, so the
+ * metadata-admin create surface fell back to the auto-generated single-section
+ * layout. That fallback (a) silently DROPPED the optional `include` (joins) and
+ * `filter` (intrinsic scope) fields — making joined / scoped datasets
+ * un-authorable online — and (b) rendered the base `object` and dimension/
+ * measure `field`s as bare free-text inputs with no object context.
+ *
+ * This layout restores `include` + `filter`, groups the surface into sections
+ * with guidance, and uses pickers (`ref:object`, `filter-builder` scoped to the
+ * base object) so a business user can author a dataset without memorising
+ * machine names. Mirrors {@link reportForm} — the sibling analytics editor.
+ */
+export const datasetForm = defineForm({
+  schemaId: 'dataset',
+  type: 'simple',
+  sections: [
+    {
+      name: 'basics',
+      label: 'Basics',
+      description: 'Dataset identity.',
+      columns: 2,
+      fields: [
+        { field: 'name', type: 'text', colSpan: 1, required: true, immutable: true, helpText: 'snake_case unique identifier' },
+        { field: 'label', type: 'text', colSpan: 1, required: true, helpText: 'Display name' },
+        { field: 'description', type: 'textarea', colSpan: 2, helpText: 'What this dataset measures' },
+      ],
+    },
+    {
+      name: 'source',
+      label: 'Source',
+      description: 'The base object, the relationships to join, and the dataset’s intrinsic scope. Joins are derived from the object graph — pick relationship (lookup / master_detail) names, never write an ON clause.',
+      fields: [
+        { field: 'object', widget: 'ref:object', required: true, helpText: 'Base object — the FROM' },
+        { field: 'include', widget: 'string-tags', helpText: 'Relationship (lookup / master_detail) field names to join — enables `relationship.field` dimensions/measures (e.g. include "account" → group by account.region)' },
+        { field: 'filter', widget: 'filter-builder', dependsOn: 'object', helpText: 'Intrinsic scope filter (e.g. exclude soft-deleted records), ANDed into every query' },
+      ],
+    },
+    {
+      name: 'dimensions',
+      label: 'Dimensions',
+      description: 'Groupable axes. Use a base field, or `relationship.field` (e.g. account.region) for a relationship included above.',
+      fields: [
+        { field: 'dimensions', type: 'repeater', required: true, helpText: 'Each: name (referenced by presentations), field, type, and — for dates — a default bucketing granularity' },
+      ],
+    },
+    {
+      name: 'measures',
+      label: 'Measures',
+      description: 'Aggregatable values defined once and referenced by name. A measure is sum/avg/count/… of a field; a derived measure combines other measures (ratio/sum/difference/product). Measure-scoped filters and derived ops are edited per-row in the dataset designer.',
+      fields: [
+        { field: 'measures', type: 'repeater', required: true, helpText: 'Each: name, aggregate, field (optional for count), display format/currency, and a “certified” governance flag' },
+      ],
+    },
+  ],
+});
diff --git a/packages/spec/src/ui/dataset.test.ts b/packages/spec/src/ui/dataset.test.ts
index 88cf52d16c..b8d3264612 100644
--- a/packages/spec/src/ui/dataset.test.ts
+++ b/packages/spec/src/ui/dataset.test.ts
@@ -88,3 +88,26 @@ describe('DatasetSchema', () => {
     expect(d).toBe(base);
   });
 });
+
+describe('DatasetSchema — derived measure aggregate optionality', () => {
+  // Regression: a derived measure had to carry a meaningless `aggregate` or
+  // validation failed, even though `aggregate` is ignored for derived measures.
+  it('accepts a derived measure with NO aggregate', () => {
+    expect(() =>
+      DatasetSchema.parse({
+        ...base,
+        measures: [
+          { name: 'won_amount', aggregate: 'sum', field: 'amount' },
+          { name: 'win_rate', derived: { op: 'ratio', of: ['won_amount', 'revenue'] } },
+          { name: 'revenue', aggregate: 'sum', field: 'amount' },
+        ],
+      }),
+    ).not.toThrow();
+  });
+
+  it('still rejects a NON-derived measure with no aggregate', () => {
+    expect(() =>
+      DatasetSchema.parse({ ...base, measures: [{ name: 'revenue', field: 'amount' }] }),
+    ).toThrowError(/requires `aggregate`/);
+  });
+});
diff --git a/packages/spec/src/ui/dataset.zod.ts b/packages/spec/src/ui/dataset.zod.ts
index 9a9ea54a92..0f8f3570b9 100644
--- a/packages/spec/src/ui/dataset.zod.ts
+++ b/packages/spec/src/ui/dataset.zod.ts
@@ -65,7 +65,7 @@ export const DatasetMeasureSchema = lazySchema(() => z.object({
   name: SnakeCaseIdentifierSchema.describe('Measure name — e.g. "revenue"; defined once'),
   label: I18nLabelSchema.optional(),
   /** Aggregation function — reuses the canonical query.zod enum. */
-  aggregate: AggregationFunction.describe('Aggregation (sum/avg/count/...)'),
+  aggregate: AggregationFunction.optional().describe('Aggregation (sum/avg/count/...); omit when `derived` is set'),
   /** Base or `relationship.field`. Optional for `count` (count(*)). */
   field: z.string().optional().describe('Aggregated field; optional for count(*)'),
   /** Measure-scoped filter (e.g. only won deals for "won_amount"). */
@@ -152,8 +152,16 @@ export const DatasetSchema = lazySchema(() => z.object({
   // Derived measures may only reference OTHER measures declared in this dataset.
   for (const m of ds.measures) {
     if (!m.derived) {
-      // A non-derived measure needs a field unless it is a plain count.
-      if (!m.field && m.aggregate !== 'count') {
+      // A non-derived measure must declare an aggregate (a derived measure
+      // omits it — it combines other measures by name instead).
+      if (!m.aggregate) {
+        ctx.addIssue({
+          code: 'custom',
+          message: `measure "${m.name}" requires \`aggregate\` (or a \`derived\` spec)`,
+          path: ['measures'],
+        });
+      } else if (!m.field && m.aggregate !== 'count') {
+        // A non-derived measure needs a field unless it is a plain count.
         ctx.addIssue({
           code: 'custom',
           message: `measure "${m.name}" requires \`field\` (only \`count\` may omit it)`,
diff --git a/packages/spec/src/ui/index.ts b/packages/spec/src/ui/index.ts
index c1ea9636cc..2b537db3e2 100644
--- a/packages/spec/src/ui/index.ts
+++ b/packages/spec/src/ui/index.ts
@@ -22,6 +22,7 @@ export { reportForm } from './report.form';
 export { viewForm } from './view.form';
 export { appForm } from './app.form';
 export { dashboardForm } from './dashboard.form';
+export { datasetForm } from './dataset.form';
 export { actionForm } from './action.form';
 export { pageForm } from './page.form';
 export * from './action.zod';