diff --git a/CHANGELOG.md b/CHANGELOG.md
index 38e1439..5608f70 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -13,6 +13,7 @@ Versioning follows Semantic Versioning with preview suffix `major.minor.patch-pr
 
 - **`having` now requires `groupBy`**: Queries that filter with `having` must also group their rows with `groupBy`. Previously the schema accepted `having` on its own, even though it has no meaning without groups. (#40)
 - **`groupBy` can no longer be empty**: `groupBy` must list at least one field. To skip grouping, leave the clause out.
+- **Grouped queries select only single values**: When a query has `groupBy`, `select` accepts only aggregates, scalars, parameters and arithmetic over them. The `groupBy` fields now appear in the result automatically as the first columns, so remove them from `select`. Fields that aren't grouped and per-row `each*` columns are rejected, because a group has no single value for them. (#52)
 
 ---
 
diff --git a/CLAUDE.md b/CLAUDE.md
index 5a89603..025e0a7 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -48,7 +48,7 @@ Every operator in the schema belongs to one of two parallel families:
 | `where` | single-value boolean **or** per-row boolean (per-row is the common case) |
 | `join.on` | single-value boolean **or** per-row boolean (per-row equi-join is the common case) |
 | `having` | single-value boolean **only** — operands must reduce to one value per group |
-| `select` | any value-returning expression, including per-row computed columns |
+| `select` | any value-returning expression, including per-row computed columns — **single-value only when `groupBy` is present** |
 | `groupBy` / `orderBy` | field references only |
 
 ### Fields are `arrayReturning`
@@ -120,6 +120,10 @@ Only the root `from` expression supports an `alias`. Joined entities are always
 
 Not select expressions — just plain `{ entity, field, type }` field references. No aliases, no operators.
 
+### Grouped `select` is single-value only
+
+When `groupBy` is present, every `select` item must be `singleValueReturning` (enforced via root `dependentSchemas`). Group keys are emitted automatically as the leading result columns, so **never repeat `groupBy` fields in `select`** — the schema rejects them, along with any non-grouped field or `each*` column.
+
 ## Workflow rules
 
 - Never commit directly to `main`. Always create a new branch and open a pull request.
diff --git a/PureQL-Specification.json b/PureQL-Specification.json
index b683849..0ebea87 100644
--- a/PureQL-Specification.json
+++ b/PureQL-Specification.json
@@ -3265,6 +3265,17 @@
       "groupBy"
     ]
   },
+  "dependentSchemas": {
+    "groupBy": {
+      "properties": {
+        "select": {
+          "items": {
+            "$ref": "#/definitions/singleValueReturning"
+          }
+        }
+      }
+    }
+  },
   "title": "PureQL specification",
   "type": "object"
 }
diff --git a/README.md b/README.md
index edf6e94..895bac1 100644
--- a/README.md
+++ b/README.md
@@ -10,7 +10,7 @@ PureQL is a JSON-based declarative query language for relational data. Queries a
 | `select`   | yes      | Array of expressions to return |
 | `where`    | no       | Boolean filter applied before grouping |
 | `joins`    | no       | Array of join clauses |
-| `groupBy`  | no       | Fields to group rows by (at least one) |
+| `groupBy`  | no       | Fields to group rows by (at least one); group keys are output automatically |
 | `having`   | no       | Boolean filter applied after grouping; requires `groupBy` |
 | `orderBy`  | no       | Fields to order results by |
 | `pagination` | no     | `skip` and `take` for paging |
@@ -91,6 +91,21 @@ Parameters are named placeholders resolved at execution time, analogous to prepa
 
 Each item in `select` is a value-returning expression (field, scalar, aggregate, arithmetic, boolean expression) with an optional `alias`.
 
+When `groupBy` is present, `select` accepts **single-value expressions only** (aggregates, scalars, parameters, arithmetic over them). Fields and per-row `each*` columns are rejected by the schema, because a group has no single value for them. The `groupBy` fields are output automatically as the leading result columns, in `groupBy` order and named after the field, followed by the `select` entries:
+
+```json
+"select": [
+  { "operator": "count", "arg": { "entity": "orders", "field": "id", "type": { "name": "uuid" } }, "alias": "order_count" }
+],
+"groupBy": [
+  { "entity": "orders", "field": "user_id", "type": { "name": "uuid" } }
+]
+```
+
+Result columns: `user_id`, `order_count`.
+
+Ungrouped example:
+
 ```json
 "select": [
   { "entity": "users", "field": "name",  "type": { "name": "string" } },
@@ -142,7 +157,7 @@ Each join specifies its type (`inner`, `left`, `right`, `full`), the entity to j
 
 ### `groupBy` / `orderBy`
 
-`groupBy` accepts an array of field references. `orderBy` accepts an array of `orderByItem` objects, each pairing a `field` with an optional `direction` (`"asc"` | `"desc"`, default `"asc"`).
+`groupBy` accepts an array of field references. Group keys are added to the result automatically, so they are not repeated in `select`. `orderBy` accepts an array of `orderByItem` objects, each pairing a `field` with an optional `direction` (`"asc"` | `"desc"`, default `"asc"`).
 
 ```json
 "groupBy": [
@@ -179,7 +194,7 @@ Where each family fits:
 |---|---|
 | `where` / `join.on` | per-row boolean (typical) or single-value boolean |
 | `having` | single-value boolean only — non-aggregated fields are structurally rejected |
-| `select` | any value-returning expression, including per-row computed columns |
+| `select` | any value-returning expression, including per-row computed columns; single-value only when `groupBy` is present |
 | `sum.arg` / `min_*.arg` / `max_*.arg` / `average_*.arg` | any array-returning expression (field, per-row computation) |
 | right operand of any `each*` comparison | matching `*Returning` (broadcast scalar) or `*ArrayReturning` (element-wise) |
 
diff --git a/samples/07_group_by.json b/samples/07_group_by.json
index 76d3068..53d489a 100644
--- a/samples/07_group_by.json
+++ b/samples/07_group_by.json
@@ -1,7 +1,6 @@
 {
   "from": { "entity": "order_items" },
   "select": [
-    { "entity": "order_items", "field": "product_id", "type": { "name": "uuid" } },
     {
       "operator": "count",
       "arg": { "entity": "order_items", "field": "id", "type": { "name": "uuid" } },
diff --git a/samples/08_having.json b/samples/08_having.json
index 408c45b..3417635 100644
--- a/samples/08_having.json
+++ b/samples/08_having.json
@@ -1,7 +1,6 @@
 {
   "from": { "entity": "orders" },
   "select": [
-    { "entity": "orders", "field": "user_id", "type": { "name": "uuid" } },
     {
       "operator": "count",
       "arg": { "entity": "orders", "field": "id", "type": { "name": "uuid" } },
diff --git a/samples/12_complex_query.json b/samples/12_complex_query.json
index 6ac4a34..ee0afc0 100644
--- a/samples/12_complex_query.json
+++ b/samples/12_complex_query.json
@@ -1,10 +1,6 @@
 {
   "from": { "entity": "orders", "alias": "o" },
   "select": [
-    { "entity": "users",    "field": "id",           "type": { "name": "uuid" },   "alias": "customer_id" },
-    { "entity": "users",    "field": "name",          "type": { "name": "string" }, "alias": "customer_name" },
-    { "entity": "users",    "field": "email",         "type": { "name": "string" } },
-    { "entity": "users",    "field": "tier",          "type": { "name": "string" }, "alias": "membership_tier" },
     {
       "operator": "count",
       "arg": { "entity": "o", "field": "id", "type": { "name": "uuid" } },
diff --git a/samples/18_aggregate_of_each_multiply.json b/samples/18_aggregate_of_each_multiply.json
index 3b6acfb..3ef47a5 100644
--- a/samples/18_aggregate_of_each_multiply.json
+++ b/samples/18_aggregate_of_each_multiply.json
@@ -1,7 +1,6 @@
 {
   "from": { "entity": "order_items", "alias": "oi" },
   "select": [
-    { "entity": "orders", "field": "user_id", "type": { "name": "uuid" } },
     {
       "operator": "sum",
       "arg": {