diff --git a/aggregation.go b/aggregation.go
new file mode 100644
index 0000000..e4462cf
--- /dev/null
+++ b/aggregation.go
@@ -0,0 +1,79 @@
+package norm
+
+import "context"
+
+type Merge[M1, M2 any] struct {
+	L M1
+	R []M2
+}
+
+type Keyer[K comparable] interface {
+	Key() K
+}
+
+// Lookup performs left outer join.
+func Lookup[
+	M1 ~[]T1,
+	M2 ~[]T2,
+	T1, T2 Keyer[K],
+	A1, A2 any,
+	K comparable,
+](
+	ctx context.Context,
+	lhs Reader[M1, A1], lhsArgs A1,
+	rhs Reader[M2, A2], rhsArgs A2,
+) (
+	merge []Merge[T1, T2],
+	err error,
+) {
+	l, err := lhs.Read(ctx, lhsArgs)
+	if err != nil {
+		return nil, err
+	}
+
+	r, err := rhs.Read(ctx, rhsArgs)
+	if err != nil {
+		return nil, err
+	}
+
+	return _lookup[M1, M2, T1, T2, K](l, r), nil
+}
+
+func _lookup[
+	M1 ~[]T1,
+	M2 ~[]T2,
+	T1 Keyer[K],
+	T2 Keyer[K],
+	K comparable,
+](
+	lhs M1,
+	rhs M2,
+) (
+	out []Merge[T1, T2],
+) {
+	out = make([]Merge[T1, T2], 0, len(lhs))
+	m := make(map[K][]int, len(rhs))
+
+	for i, v := range rhs {
+		k := v.Key()
+		m[k] = append(m[k], i)
+	}
+
+	for _, l := range lhs {
+		merge := Merge[T1, T2]{
+			L: l,
+			R: nil,
+		}
+
+		rii, ok := m[l.Key()]
+		if ok {
+			for _, i := range rii {
+				merge.R = append(merge.R, rhs[i])
+			}
+		}
+
+		out = append(out, merge)
+	}
+
+	return out
+}
diff --git a/aggregation_test.go b/aggregation_test.go
new file mode 100644
index 0000000..28f812e
--- /dev/null
+++ b/aggregation_test.go
@@ -0,0 +1,155 @@
+package norm
+
+import (
+	"context"
+	"reflect"
+	"testing"
+)
+
+type kint int
+
+func (v kint) Key() int {
+	return int(v)
+}
+
+type mem struct {
+	v []kint
+}
+
+func (m mem) Read(ctx context.Context, args struct{}) (value []kint, err error) {
+	return []kint(m.v), nil
+}
+
+func TestLookup(t *testing.T) {
+	type args struct {
+		ctx     context.Context
+		lhs     Reader[[]kint, struct{}]
+		lhsArgs struct{}
+		rhs     Reader[[]kint, struct{}]
+		rhsArgs struct{}
+	}
+	tests := []struct {
+		name      string
+		args      args
+		wantMerge []Merge[kint, kint]
+		wantErr   bool
+	}{
+		{
+			name: "with mathes",
+			args: args{
+				ctx:     context.Background(),
+				lhs:     mem{[]kint{1, 2, 3, 4, 5}},
+				lhsArgs: struct{}{},
+				rhs:     mem{[]kint{5, 5, 2, 2, 4}},
+				rhsArgs: struct{}{},
+			},
+			wantMerge: []Merge[kint, kint]{
+				{1, nil},
+				{2, []kint{2, 2}},
+				{3, nil},
+				{4, []kint{4}},
+				{5, []kint{5, 5}},
+			},
+			wantErr: false,
+		},
+		{
+			name: "no match",
+			args: args{
+				ctx:     context.Background(),
+				lhs:     mem{[]kint{1, 2, 3, 4, 5}},
+				lhsArgs: struct{}{},
+				rhs:     mem{[]kint{10, 11, 12, 13, 14}},
+				rhsArgs: struct{}{},
+			},
+			wantMerge: []Merge[kint, kint]{
+				{1, nil},
+				{2, nil},
+				{3, nil},
+				{4, nil},
+				{5, nil},
+			},
+			wantErr: false,
+		},
+		{
+			name: "right is empty",
+			args: args{
+				ctx:     context.Background(),
+				lhs:     mem{[]kint{1, 2, 3, 4, 5}},
+				lhsArgs: struct{}{},
+				rhs:     mem{[]kint{}},
+				rhsArgs: struct{}{},
+			},
+			wantMerge: []Merge[kint, kint]{
+				{1, nil},
+				{2, nil},
+				{3, nil},
+				{4, nil},
+				{5, nil},
+			},
+			wantErr: false,
+		},
+		{
+			name: "right is nil",
+			args: args{
+				ctx:     context.Background(),
+				lhs:     mem{[]kint{1, 2, 3, 4, 5}},
+				lhsArgs: struct{}{},
+				rhs:     mem{},
+				rhsArgs: struct{}{},
+			},
+			wantMerge: []Merge[kint, kint]{
+				{1, nil},
+				{2, nil},
+				{3, nil},
+				{4, nil},
+				{5, nil},
+			},
+			wantErr: false,
+		},
+		{
+			name: "left is empty",
+			args: args{
+				ctx:     context.Background(),
+				lhs:     mem{[]kint{}},
+				lhsArgs: struct{}{},
+				rhs:     mem{[]kint{1, 2, 3, 4, 5}},
+				rhsArgs: struct{}{},
+			},
+			wantMerge: []Merge[kint, kint]{},
+			wantErr:   false,
+		},
+		{
+			name: "left is nil",
+			args: args{
+				ctx:     context.Background(),
+				lhs:     mem{},
+				lhsArgs: struct{}{},
+				rhs:     mem{[]kint{1, 2, 3, 4, 5}},
+				rhsArgs: struct{}{},
+			},
+			wantMerge: []Merge[kint, kint]{},
+			wantErr:   false,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			gotMerge, err := Lookup[
+				[]kint, []kint,
+				kint, kint,
+				struct{}, struct{},
+				int,
+			](
+				tt.args.ctx,
+				tt.args.lhs, tt.args.lhsArgs,
+				tt.args.rhs, tt.args.rhsArgs,
+			)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("Lookup() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+			if !reflect.DeepEqual(gotMerge, tt.wantMerge) {
+				t.Errorf("Lookup() = %v, want %v", gotMerge, tt.wantMerge)
+			}
+		})
+	}
+}