diff --git a/env/spreadsheet.go b/env/spreadsheet.go
index b5fd2558..365534d1 100644
--- a/env/spreadsheet.go
+++ b/env/spreadsheet.go
@@ -222,7 +222,7 @@ func (s Spreadsheet) GetRowValue(column string, rrow SpreadsheetRow) (any, error
 		}
 	}
 	if index < 0 {
-		return "", nil
+		return "", fmt.Errorf("column %s not found", column)
 	}
 	return rrow.Values[index], nil
 }
diff --git a/evaldo/builtins_spreadsheet.go b/evaldo/builtins_spreadsheet.go
index cf657037..12b95315 100644
--- a/evaldo/builtins_spreadsheet.go
+++ b/evaldo/builtins_spreadsheet.go
@@ -713,6 +713,52 @@ var Builtins_spreadsheet = map[string]*env.Builtin{
 			}
 		},
 	},
+	"group-by": {
+		Argsn: 3,
+		Doc:   "Groups a spreadsheet by the given column and (optional) aggregations.",
+		Fn: func(ps *env.ProgramState, arg0 env.Object, arg1 env.Object, arg2 env.Object, arg3 env.Object, arg4 env.Object) (res env.Object) {
+			switch spr := arg0.(type) {
+			case env.Spreadsheet:
+				switch aggBlock := arg2.(type) {
+				case env.Block:
+					if len(aggBlock.Series.S)%2 != 0 {
+						return MakeBuiltinError(ps, "Aggregation block must contain pairs of column name and function for each aggregation.", "group-by")
+					}
+					aggregations := make(map[string][]string)
+					for i := 0; i < len(aggBlock.Series.S); i += 2 {
+						col := aggBlock.Series.S[i]
+						fun, ok := aggBlock.Series.S[i+1].(env.Word)
+						if !ok {
+							return MakeBuiltinError(ps, "Aggregation function must be a word", "group-by")
+						}
+						colStr := ""
+						switch col := col.(type) {
+						case env.Tagword:
+							colStr = ps.Idx.GetWord(col.Index)
+						case env.String:
+							colStr = col.Value
+						default:
+							return MakeBuiltinError(ps, "Aggregation column must be a word or string", "group-by")
+						}
+						funStr := ps.Idx.GetWord(fun.Index)
+						aggregations[colStr] = append(aggregations[colStr], funStr)
+					}
+					switch col := arg1.(type) {
+					case env.Word:
+						return GroupBy(ps, spr, ps.Idx.GetWord(col.Index), aggregations)
+					case env.String:
+						return GroupBy(ps, spr, col.Value, aggregations)
+					default:
+						return MakeArgError(ps, 2, []env.Type{env.WordType, env.StringType}, "group-by")
+					}
+				default:
+					return MakeArgError(ps, 3, []env.Type{env.BlockType}, "group-by")
+				}
+			default:
+				return MakeArgError(ps, 1, []env.Type{env.SpreadsheetType}, "group-by")
+			}
+		},
+	},
 }
 
 func GenerateColumn(ps *env.ProgramState, s env.Spreadsheet, name env.Word, extractCols env.Block, code env.Block) env.Object {
@@ -759,7 +805,7 @@ func GenerateColumnRegexReplace(ps *env.ProgramState, s *env.Spreadsheet, name e
 		// get value from current row
 		val, err := s.GetRowValue(ps.Idx.GetWord(fromColName.Index), row)
 		if err != nil {
-			return MakeError(ps, "Couldn't retrieve value at row "+strconv.Itoa(ix))
+			return MakeError(ps, fmt.Sprintf("Couldn't retrieve value at row %d (%s)", ix, err))
 		}
 
 		var newVal any
@@ -1041,10 +1087,10 @@ func LeftJoin(ps *env.ProgramState, s1 env.Spreadsheet, s2 env.Spreadsheet, col1
 		}
 	}
 	nspr := env.NewSpreadsheet(combinedCols)
-	for _, row1 := range s1.GetRows() {
+	for i, row1 := range s1.GetRows() {
 		val1, err := s1.GetRowValue(col1, row1)
 		if err != nil {
-			return MakeError(ps, "Couldn't retrieve value at row")
+			return MakeError(ps, fmt.Sprintf("Couldn't retrieve value at row %d (%s)", i, err))
 		}
 		newRow := make([]any, len(combinedCols))
 
@@ -1057,13 +1103,13 @@ func LeftJoin(ps *env.ProgramState, s1 env.Spreadsheet, s2 env.Spreadsheet, col1
 				s2RowId = rowIds[0]
 			}
 		} else {
-			for i, row2 := range s2.GetRows() {
+			for j, row2 := range s2.GetRows() {
 				val2, err := s2.GetRowValue(col2, row2)
 				if err != nil {
-					return MakeError(ps, "Couldn't retrieve value at row")
+					return MakeError(ps, fmt.Sprintf("Couldn't retrieve value at row %d (%s)", j, err))
 				}
 				if val1.(env.Object).Equal(val2.(env.Object)) {
-					s2RowId = i
+					s2RowId = j
 					break
 				}
 			}
@@ -1077,11 +1123,100 @@ func LeftJoin(ps *env.ProgramState, s1 env.Spreadsheet, s2 env.Spreadsheet, col1
 				newRow[i+len(s1.Cols)] = v
 			}
 		} else {
-			for i := range s2.Cols {
-				newRow[i+len(s1.Cols)] = env.Void{}
+			for k := range s2.Cols {
+				newRow[k+len(s1.Cols)] = env.Void{}
 			}
 		}
 		nspr.AddRow(*env.NewSpreadsheetRow(newRow, nspr))
 	}
 	return *nspr
 }
+
+func GroupBy(ps *env.ProgramState, s env.Spreadsheet, col string, aggregations map[string][]string) env.Object {
+	if !slices.Contains(s.Cols, col) {
+		return MakeBuiltinError(ps, "Column not found.", "group-by")
+	}
+
+	aggregatesByGroup := make(map[string]map[string]float64)
+	countByGroup := make(map[string]int)
+	for i, row := range s.Rows {
+		groupingVal, err := s.GetRowValue(col, row)
+		if err != nil {
+			return MakeError(ps, fmt.Sprintf("Couldn't retrieve value at row %d (%s)", i, err))
+		}
+		groupValStr, ok := groupingVal.(env.String)
+		if !ok {
+			return MakeBuiltinError(ps, "Grouping column value must be a string", "group-by")
+		}
+
+		if _, ok := aggregatesByGroup[groupValStr.Value]; !ok {
+			aggregatesByGroup[groupValStr.Value] = make(map[string]float64)
+		}
+		groupAggregates := aggregatesByGroup[groupValStr.Value]
+
+		for aggCol, funs := range aggregations {
+			for _, fun := range funs {
+				colAgg := aggCol + "_" + fun
+				if fun == "count" {
+					if aggCol != col {
+						return MakeBuiltinError(ps, "Count aggregation can only be applied on the grouping column", "group-by")
+					}
+					groupAggregates[colAgg]++
+					continue
+				}
+				valObj, err := s.GetRowValue(aggCol, row)
+				if err != nil {
+					return MakeError(ps, fmt.Sprintf("Couldn't retrieve value at row %d (%s)", i, err))
+				}
+				var val float64
+				switch valObj := env.ToRyeValue(valObj).(type) {
+				case env.Integer:
+					val = float64(valObj.Value)
+				case env.Decimal:
+					val = valObj.Value
+				default:
+					return MakeBuiltinError(ps, "Aggregation column value must be a number", "group-by")
+				}
+				switch fun {
+				case "sum":
+					groupAggregates[colAgg] += val
+				case "avg":
+					groupAggregates[colAgg] += val
+					countByGroup[groupValStr.Value]++
+				case "min":
+					if min, ok := groupAggregates[colAgg]; !ok || val < min {
+						groupAggregates[colAgg] = val
+					}
+				case "max":
+					if max, ok := groupAggregates[colAgg]; !ok || val > max {
+						groupAggregates[colAgg] = val
+					}
+				default:
+					return MakeBuiltinError(ps, fmt.Sprintf("Unknown aggregation function: %s", fun), "group-by")
+				}
+			}
+		}
+	}
+	newCols := []string{col}
+	for aggCol, funs := range aggregations {
+		for _, fun := range funs {
+			newCols = append(newCols, aggCol+"_"+fun)
+		}
+	}
+	newS := env.NewSpreadsheet(newCols)
+	for groupVal, groupAggregates := range aggregatesByGroup {
+		newRow := make([]any, len(newCols))
+		newRow[0] = *env.NewString(groupVal)
+		for i, col := range newCols[1:] {
+			if strings.HasSuffix(col, "_count") {
+				newRow[i+1] = *env.NewInteger(int64(groupAggregates[col]))
+			} else if strings.HasSuffix(col, "_avg") {
+				newRow[i+1] = *env.NewDecimal(groupAggregates[col] / float64(countByGroup[groupVal]))
+			} else {
+				newRow[i+1] = *env.NewDecimal(groupAggregates[col])
+			}
+		}
+		newS.AddRow(*env.NewSpreadsheetRow(newRow, newS))
+	}
+	return *newS
+}
diff --git a/tests/structures.rye b/tests/structures.rye
index fab6707a..419181a5 100644
--- a/tests/structures.rye
+++ b/tests/structures.rye
@@ -653,6 +653,23 @@ section "Spreadsheet related functions"
 				names .inner-join houses 'id 'id 
 		} spreadsheet { "id" "name" "id_2" "house" } { 1 "Paul" 1 "Atreides" 3 "Vladimir" 3 "Harkonnen" }
 	}
+
+	group "group by"
+	mold\nowrap ?group-by
+	{ { block } }
+	{
+		equal { spreadsheet { "name" "val" } { "a" 1 "b" 2 } |group-by 'name { } |sort-col! 'name
+		} spreadsheet { "name" } { "a" "b" }
+
+		equal { spreadsheet { "name" "val" } { "a" 1 "b" 6 "a" 5 "b" 10 "a" 7 } 
+				|group-by 'name { 'name count 'val sum 'val min 'val max 'val avg } 
+				|sort-col! 'name
+		} spreadsheet { "name" "name_count" "val_sum" "val_min" "val_max" "val_avg" } 
+					{ 
+						"a" 3 13.0 1.0 7.0 4.333333333333333
+						"b" 2 16.0 6.0 10.0 8.0
+					}
+	}
 }