src/components/reports/CompareBudgetView.tsx

@@ -0,0 +1,47 @@
+import { useEffect, useState } from "react";
+import { useTranslation } from "react-i18next";
+import BudgetVsActualTable from "./BudgetVsActualTable";
+import { getBudgetVsActualData } from "../../services/budgetService";
+import type { BudgetVsActualRow } from "../../shared/types";
+
+export interface CompareBudgetViewProps {
+  year: number;
+  month: number;
+}
+
+export default function CompareBudgetView({ year, month }: CompareBudgetViewProps) {
+  const { t } = useTranslation();
+  const [rows, setRows] = useState<BudgetVsActualRow[]>([]);
+  const [error, setError] = useState<string | null>(null);
+
+  useEffect(() => {
+    let cancelled = false;
+    setError(null);
+    getBudgetVsActualData(year, month)
+      .then((data) => {
+        if (!cancelled) setRows(data);
+      })
+      .catch((e: unknown) => {
+        if (!cancelled) setError(e instanceof Error ? e.message : String(e));
+      });
+    return () => {
+      cancelled = true;
+    };
+  }, [year, month]);
+
+  if (error) {
+    return (
+      <div className="bg-[var(--negative)]/10 text-[var(--negative)] rounded-xl p-4">{error}</div>
+    );
+  }
+
+  if (rows.length === 0) {
+    return (
+      <div className="bg-[var(--card)] border border-[var(--border)] rounded-xl p-6 text-center text-[var(--muted-foreground)] italic">
+        {t("reports.bva.noData")}
+      </div>
+    );
+  }
+
+  return <BudgetVsActualTable data={rows} />;
+}

src/components/reports/CompareModeTabs.tsx

@@ -0,0 +1,38 @@
+import { useTranslation } from "react-i18next";
+import type { CompareMode } from "../../hooks/useCompare";
+
+export interface CompareModeTabsProps {
+  value: CompareMode;
+  onChange: (mode: CompareMode) => void;
+}
+
+export default function CompareModeTabs({ value, onChange }: CompareModeTabsProps) {
+  const { t } = useTranslation();
+
+  const modes: { id: CompareMode; labelKey: string }[] = [
+    { id: "mom", labelKey: "reports.compare.modeMoM" },
+    { id: "yoy", labelKey: "reports.compare.modeYoY" },
+    { id: "budget", labelKey: "reports.compare.modeBudget" },
+  ];
+
+  return (
+    <div className="inline-flex gap-1" role="tablist">
+      {modes.map(({ id, labelKey }) => (
+        <button
+          key={id}
+          type="button"
+          role="tab"
+          onClick={() => onChange(id)}
+          aria-selected={value === id}
+          className={`px-3 py-2 rounded-lg text-sm font-medium transition-colors ${
+            value === id
+              ? "bg-[var(--primary)] text-white"
+              : "bg-[var(--card)] border border-[var(--border)] text-[var(--foreground)] hover:bg-[var(--muted)]"
+          }`}
+        >
+          {t(labelKey)}
+        </button>
+      ))}
+    </div>
+  );
+}

src/components/reports/ComparePeriodChart.tsx

@@ -0,0 +1,85 @@
+import { useTranslation } from "react-i18next";
+import {
+  BarChart,
+  Bar,
+  XAxis,
+  YAxis,
+  Cell,
+  ReferenceLine,
+  Tooltip,
+  ResponsiveContainer,
+} from "recharts";
+import type { CategoryDelta } from "../../shared/types";
+import { ChartPatternDefs, getPatternFill } from "../../utils/chartPatterns";
+
+export interface ComparePeriodChartProps {
+  rows: CategoryDelta[];
+}
+
+function formatCurrency(amount: number, language: string): string {
+  return new Intl.NumberFormat(language === "fr" ? "fr-CA" : "en-CA", {
+    style: "currency",
+    currency: "CAD",
+    maximumFractionDigits: 0,
+  }).format(amount);
+}
+
+export default function ComparePeriodChart({ rows }: ComparePeriodChartProps) {
+  const { t, i18n } = useTranslation();
+
+  if (rows.length === 0) {
+    return (
+      <div className="bg-[var(--card)] border border-[var(--border)] rounded-xl p-6 text-center text-[var(--muted-foreground)] italic">
+        {t("reports.empty.noData")}
+      </div>
+    );
+  }
+
+  const chartData = rows
+    .map((r, i) => ({
+      name: r.categoryName,
+      color: r.categoryColor,
+      delta: r.deltaAbs,
+      index: i,
+    }))
+    .sort((a, b) => a.delta - b.delta);
+
+  return (
+    <div className="bg-[var(--card)] border border-[var(--border)] rounded-xl p-4">
+      <ResponsiveContainer width="100%" height={Math.max(240, chartData.length * 32 + 40)}>
+        <BarChart data={chartData} layout="vertical" margin={{ top: 10, right: 20, bottom: 10, left: 10 }}>
+          <ChartPatternDefs
+            prefix="compare-delta"
+            categories={chartData.map((d) => ({ color: d.color, index: d.index }))}
+          />
+          <XAxis
+            type="number"
+            tickFormatter={(v) => formatCurrency(v, i18n.language)}
+            stroke="var(--muted-foreground)"
+            fontSize={11}
+          />
+          <YAxis type="category" dataKey="name" width={140} stroke="var(--muted-foreground)" fontSize={11} />
+          <ReferenceLine x={0} stroke="var(--border)" />
+          <Tooltip
+            formatter={(value) =>
+              typeof value === "number" ? formatCurrency(value, i18n.language) : String(value)
+            }
+            contentStyle={{
+              backgroundColor: "var(--card)",
+              border: "1px solid var(--border)",
+              borderRadius: "0.5rem",
+            }}
+          />
+          <Bar dataKey="delta">
+            {chartData.map((entry) => (
+              <Cell
+                key={entry.name}
+                fill={getPatternFill("compare-delta", entry.index, entry.color)}
+              />
+            ))}
+          </Bar>
+        </BarChart>
+      </ResponsiveContainer>
+    </div>
+  );
+}

src/components/reports/ComparePeriodTable.tsx

@@ -0,0 +1,118 @@
+import { useTranslation } from "react-i18next";
+import type { CategoryDelta } from "../../shared/types";
+
+export interface ComparePeriodTableProps {
+  rows: CategoryDelta[];
+  previousLabel: string;
+  currentLabel: string;
+}
+
+function formatCurrency(amount: number, language: string): string {
+  return new Intl.NumberFormat(language === "fr" ? "fr-CA" : "en-CA", {
+    style: "currency",
+    currency: "CAD",
+  }).format(amount);
+}
+
+function formatSignedCurrency(amount: number, language: string): string {
+  return new Intl.NumberFormat(language === "fr" ? "fr-CA" : "en-CA", {
+    style: "currency",
+    currency: "CAD",
+    signDisplay: "always",
+  }).format(amount);
+}
+
+function formatPct(pct: number | null, language: string): string {
+  if (pct === null) return "—";
+  return new Intl.NumberFormat(language === "fr" ? "fr-CA" : "en-CA", {
+    style: "percent",
+    maximumFractionDigits: 1,
+    signDisplay: "always",
+  }).format(pct / 100);
+}
+
+export default function ComparePeriodTable({
+  rows,
+  previousLabel,
+  currentLabel,
+}: ComparePeriodTableProps) {
+  const { t, i18n } = useTranslation();
+
+  return (
+    <div className="bg-[var(--card)] border border-[var(--border)] rounded-xl overflow-hidden">
+      <div className="overflow-x-auto">
+        <table className="w-full text-sm">
+          <thead>
+            <tr className="border-b border-[var(--border)]">
+              <th className="text-left px-3 py-2 font-medium text-[var(--muted-foreground)]">
+                {t("reports.highlights.category")}
+              </th>
+              <th className="text-right px-3 py-2 font-medium text-[var(--muted-foreground)]">
+                {previousLabel}
+              </th>
+              <th className="text-right px-3 py-2 font-medium text-[var(--muted-foreground)]">
+                {currentLabel}
+              </th>
+              <th className="text-right px-3 py-2 font-medium text-[var(--muted-foreground)]">
+                {t("reports.highlights.variationAbs")}
+              </th>
+              <th className="text-right px-3 py-2 font-medium text-[var(--muted-foreground)]">
+                {t("reports.highlights.variationPct")}
+              </th>
+            </tr>
+          </thead>
+          <tbody>
+            {rows.length === 0 ? (
+              <tr>
+                <td colSpan={5} className="px-3 py-4 text-center text-[var(--muted-foreground)] italic">
+                  {t("reports.empty.noData")}
+                </td>
+              </tr>
+            ) : (
+              rows.map((row) => (
+                <tr
+                  key={`${row.categoryId ?? "uncat"}-${row.categoryName}`}
+                  className="border-b border-[var(--border)] last:border-0 hover:bg-[var(--muted)]/40"
+                >
+                  <td className="px-3 py-2">
+                    <span className="flex items-center gap-2">
+                      <span
+                        className="w-2 h-2 rounded-full flex-shrink-0"
+                        style={{ backgroundColor: row.categoryColor }}
+                      />
+                      {row.categoryName}
+                    </span>
+                  </td>
+                  <td className="px-3 py-2 text-right tabular-nums">
+                    {formatCurrency(row.previousAmount, i18n.language)}
+                  </td>
+                  <td className="px-3 py-2 text-right tabular-nums">
+                    {formatCurrency(row.currentAmount, i18n.language)}
+                  </td>
+                  <td
+                    className="px-3 py-2 text-right tabular-nums font-medium"
+                    style={{
+                      color:
+                        row.deltaAbs >= 0 ? "var(--negative, #ef4444)" : "var(--positive, #10b981)",
+                    }}
+                  >
+                    {formatSignedCurrency(row.deltaAbs, i18n.language)}
+                  </td>
+                  <td
+                    className="px-3 py-2 text-right tabular-nums"
+                    style={{
+                      color:
+                        row.deltaAbs >= 0 ? "var(--negative, #ef4444)" : "var(--positive, #10b981)",
+                    }}
+                  >
+                    {formatPct(row.deltaPct, i18n.language)}
+                  </td>
+                </tr>
+              ))
+            )}
+          </tbody>
+        </table>
+      </div>
+    </div>
+  );
+}

src/hooks/useCompare.ts

@@ -1,21 +1,32 @@
-import { useReducer, useCallback } from "react";
+import { useReducer, useCallback, useEffect, useRef } from "react";
+import type { CategoryDelta } from "../shared/types";
+import { getCompareMonthOverMonth, getCompareYearOverYear } from "../services/reportService";
 import { useReportsPeriod } from "./useReportsPeriod";
 
 export type CompareMode = "mom" | "yoy" | "budget";
 
 interface State {
   mode: CompareMode;
+  year: number;
+  month: number;
+  rows: CategoryDelta[];
   isLoading: boolean;
   error: string | null;
 }
 
 type Action =
   | { type: "SET_MODE"; payload: CompareMode }
+  | { type: "SET_PERIOD"; payload: { year: number; month: number } }
   | { type: "SET_LOADING"; payload: boolean }
+  | { type: "SET_ROWS"; payload: CategoryDelta[] }
   | { type: "SET_ERROR"; payload: string };
 
+const today = new Date();
 const initialState: State = {
   mode: "mom",
+  year: today.getFullYear(),
+  month: today.getMonth() + 1,
+  rows: [],
   isLoading: false,
   error: null,
 };
@@ -24,8 +35,12 @@ function reducer(state: State, action: Action): State {
   switch (action.type) {
     case "SET_MODE":
       return { ...state, mode: action.payload };
+    case "SET_PERIOD":
+      return { ...state, year: action.payload.year, month: action.payload.month };
     case "SET_LOADING":
       return { ...state, isLoading: action.payload };
+    case "SET_ROWS":
+      return { ...state, rows: action.payload, isLoading: false, error: null };
     case "SET_ERROR":
       return { ...state, error: action.payload, isLoading: false };
     default:
@@ -36,11 +51,46 @@ function reducer(state: State, action: Action): State {
 export function useCompare() {
   const { from, to } = useReportsPeriod();
   const [state, dispatch] = useReducer(reducer, initialState);
+  const fetchIdRef = useRef(0);
+
+  const fetch = useCallback(async (mode: CompareMode, year: number, month: number) => {
+    if (mode === "budget") return; // Budget view uses BudgetVsActualTable directly
+    const id = ++fetchIdRef.current;
+    dispatch({ type: "SET_LOADING", payload: true });
+    try {
+      const rows =
+        mode === "mom"
+          ? await getCompareMonthOverMonth(year, month)
+          : await getCompareYearOverYear(year);
+      if (id !== fetchIdRef.current) return;
+      dispatch({ type: "SET_ROWS", payload: rows });
+    } catch (e) {
+      if (id !== fetchIdRef.current) return;
+      dispatch({ type: "SET_ERROR", payload: e instanceof Error ? e.message : String(e) });
+    }
+  }, []);
+
+  useEffect(() => {
+    fetch(state.mode, state.year, state.month);
+  }, [fetch, state.mode, state.year, state.month]);
+
+  // When the URL period changes, use the `to` date to infer the target year/month.
+  useEffect(() => {
+    const [y, m] = to.split("-").map(Number);
+    if (!Number.isFinite(y) || !Number.isFinite(m)) return;
+    if (y !== state.year || m !== state.month) {
+      dispatch({ type: "SET_PERIOD", payload: { year: y, month: m } });
+    }
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [to]);
 
   const setMode = useCallback((m: CompareMode) => {
     dispatch({ type: "SET_MODE", payload: m });
   }, []);
 
-  // Issue #73 will fetch via reportService.getCompareMonthOverMonth / ...YearOverYear
+  const setTargetPeriod = useCallback((year: number, month: number) => {
-  return { ...state, setMode, from, to };
+    dispatch({ type: "SET_PERIOD", payload: { year, month } });
+  }, []);
+
+  return { ...state, setMode, setTargetPeriod, from, to };
 }

src/i18n/locales/en.json

@@ -403,6 +403,11 @@
       "subviewGlobal": "Global flow",
       "subviewByCategory": "By category"
     },
+    "compare": {
+      "modeMoM": "Month vs previous month",
+      "modeYoY": "Year vs previous year",
+      "modeBudget": "Actual vs budget"
+    },
     "highlights": {
       "balances": "Balances",
       "netBalanceCurrent": "This month",

src/i18n/locales/fr.json

@@ -403,6 +403,11 @@
       "subviewGlobal": "Flux global",
       "subviewByCategory": "Par catégorie"
     },
+    "compare": {
+      "modeMoM": "Mois vs mois précédent",
+      "modeYoY": "Année vs année précédente",
+      "modeBudget": "Réel vs budget"
+    },
     "highlights": {
       "balances": "Soldes",
       "netBalanceCurrent": "Ce mois-ci",

src/pages/ReportsComparePage.tsx

@@ -1,11 +1,88 @@
+import { useState } from "react";
 import { useTranslation } from "react-i18next";
+import { Link } from "react-router-dom";
+import { ArrowLeft } from "lucide-react";
+import PeriodSelector from "../components/dashboard/PeriodSelector";
+import CompareModeTabs from "../components/reports/CompareModeTabs";
+import ComparePeriodTable from "../components/reports/ComparePeriodTable";
+import ComparePeriodChart from "../components/reports/ComparePeriodChart";
+import CompareBudgetView from "../components/reports/CompareBudgetView";
+import ViewModeToggle, { readViewMode, type ViewMode } from "../components/reports/ViewModeToggle";
+import { useCompare } from "../hooks/useCompare";
+import { useReportsPeriod } from "../hooks/useReportsPeriod";
+
+const STORAGE_KEY = "reports-viewmode-compare";
+
+const MONTH_NAMES_EN = [
+  "January", "February", "March", "April", "May", "June",
+  "July", "August", "September", "October", "November", "December",
+];
+const MONTH_NAMES_FR = [
+  "Janvier", "Février", "Mars", "Avril", "Mai", "Juin",
+  "Juillet", "Août", "Septembre", "Octobre", "Novembre", "Décembre",
+];
+
+function monthName(month: number, language: string): string {
+  return (language === "fr" ? MONTH_NAMES_FR : MONTH_NAMES_EN)[month - 1] ?? String(month);
+}
 
 export default function ReportsComparePage() {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const { period, setPeriod, from, to, setCustomDates } = useReportsPeriod();
+  const { mode, setMode, year, month, rows, isLoading, error } = useCompare();
+  const [viewMode, setViewMode] = useState<ViewMode>(() => readViewMode(STORAGE_KEY));
+
+  const preserveSearch = typeof window !== "undefined" ? window.location.search : "";
+
+  const previousLabel =
+    mode === "mom"
+      ? `${monthName(month === 1 ? 12 : month - 1, i18n.language)} ${month === 1 ? year - 1 : year}`
+      : `${year - 1}`;
+  const currentLabel =
+    mode === "mom" ? `${monthName(month, i18n.language)} ${year}` : `${year}`;
+
   return (
-    <div className="p-8 text-center text-[var(--muted-foreground)]">
+    <div className={isLoading ? "opacity-60" : ""}>
-      <h1 className="text-2xl font-bold mb-4">{t("reports.hub.compare")}</h1>
+      <div className="flex items-center gap-3 mb-4">
-      <p>{t("common.underConstruction")}</p>
+        <Link
+          to={`/reports${preserveSearch}`}
+          className="text-[var(--muted-foreground)] hover:text-[var(--foreground)] p-1 rounded-md hover:bg-[var(--muted)]"
+          aria-label={t("reports.hub.title")}
+        >
+          <ArrowLeft size={18} />
+        </Link>
+        <h1 className="text-2xl font-bold">{t("reports.hub.compare")}</h1>
+      </div>
+
+      <div className="flex flex-col sm:flex-row sm:items-center justify-between gap-4 mb-6 flex-wrap">
+        <PeriodSelector
+          value={period}
+          onChange={setPeriod}
+          customDateFrom={from}
+          customDateTo={to}
+          onCustomDateChange={setCustomDates}
+        />
+        <div className="flex gap-2 items-center flex-wrap">
+          <CompareModeTabs value={mode} onChange={setMode} />
+          {mode !== "budget" && (
+            <ViewModeToggle value={viewMode} onChange={setViewMode} storageKey={STORAGE_KEY} />
+          )}
+        </div>
+      </div>
+
+      {error && (
+        <div className="bg-[var(--negative)]/10 text-[var(--negative)] rounded-xl p-4 mb-6">
+          {error}
+        </div>
+      )}
+
+      {mode === "budget" ? (
+        <CompareBudgetView year={year} month={month} />
+      ) : viewMode === "chart" ? (
+        <ComparePeriodChart rows={rows} />
+      ) : (
+        <ComparePeriodTable rows={rows} previousLabel={previousLabel} currentLabel={currentLabel} />
+      )}
     </div>
   );
 }

src/services/reportService.test.ts

@@ -1,5 +1,10 @@
 import { describe, it, expect, vi, beforeEach } from "vitest";
-import { getCategoryOverTime, getHighlights } from "./reportService";
+import {
+  getCategoryOverTime,
+  getHighlights,
+  getCompareMonthOverMonth,
+  getCompareYearOverYear,
+} from "./reportService";
 
 // Mock the db module
 vi.mock("./db", () => ({
@@ -266,3 +271,68 @@ describe("getHighlights", () => {
     expect(result.topMovers[0].deltaAbs).toBe(120);
   });
 });
+
+describe("getCompareMonthOverMonth", () => {
+  it("passes current and previous month boundaries as parameters", async () => {
+    mockSelect.mockResolvedValueOnce([]);
+
+    await getCompareMonthOverMonth(2026, 4);
+
+    expect(mockSelect).toHaveBeenCalledTimes(1);
+    const sql = mockSelect.mock.calls[0][0] as string;
+    const params = mockSelect.mock.calls[0][1] as unknown[];
+    expect(sql).toContain("$1");
+    expect(sql).toContain("$4");
+    expect(params).toEqual(["2026-04-01", "2026-04-30", "2026-03-01", "2026-03-31"]);
+    expect(sql).not.toContain("'2026");
+  });
+
+  it("wraps to december of previous year when target month is january", async () => {
+    mockSelect.mockResolvedValueOnce([]);
+
+    await getCompareMonthOverMonth(2026, 1);
+
+    const params = mockSelect.mock.calls[0][1] as unknown[];
+    expect(params).toEqual(["2026-01-01", "2026-01-31", "2025-12-01", "2025-12-31"]);
+  });
+
+  it("converts raw rows into CategoryDelta with signed deltas", async () => {
+    mockSelect.mockResolvedValueOnce([
+      {
+        category_id: 1,
+        category_name: "Groceries",
+        category_color: "#10b981",
+        current_total: 500,
+        previous_total: 400,
+      },
+      {
+        category_id: 2,
+        category_name: "Restaurants",
+        category_color: "#f97316",
+        current_total: 120,
+        previous_total: 0,
+      },
+    ]);
+
+    const result = await getCompareMonthOverMonth(2026, 4);
+
+    expect(result).toHaveLength(2);
+    expect(result[0]).toMatchObject({
+      categoryName: "Groceries",
+      deltaAbs: 100,
+    });
+    expect(result[0].deltaPct).toBeCloseTo(25, 4);
+    expect(result[1].deltaPct).toBeNull(); // previous = 0
+  });
+});
+
+describe("getCompareYearOverYear", () => {
+  it("spans two full calendar years with parameterised boundaries", async () => {
+    mockSelect.mockResolvedValueOnce([]);
+
+    await getCompareYearOverYear(2026);
+
+    const params = mockSelect.mock.calls[0][1] as unknown[];
+    expect(params).toEqual(["2026-01-01", "2026-12-31", "2025-01-01", "2025-12-31"]);
+  });
+});

src/services/reportService.ts

@@ -6,6 +6,7 @@ import type {
   CategoryOverTimeItem,
   HighlightsData,
   HighlightMover,
+  CategoryDelta,
   MonthBalance,
   RecentTransaction,
 } from "../shared/types";
@@ -340,3 +341,107 @@ export async function getHighlights(
     topTransactions: recentRows,
   };
 }
+
+// --- Compare (Issue #73) ---
+
+interface RawDeltaRow {
+  category_id: number | null;
+  category_name: string;
+  category_color: string;
+  current_total: number | null;
+  previous_total: number | null;
+}
+
+function rowsToDeltas(rows: RawDeltaRow[]): CategoryDelta[] {
+  return rows.map((r) => {
+    const current = Number(r.current_total ?? 0);
+    const previous = Number(r.previous_total ?? 0);
+    const deltaAbs = current - previous;
+    const deltaPct = previous === 0 ? null : (deltaAbs / previous) * 100;
+    return {
+      categoryId: r.category_id,
+      categoryName: r.category_name,
+      categoryColor: r.category_color,
+      previousAmount: previous,
+      currentAmount: current,
+      deltaAbs,
+      deltaPct,
+    };
+  });
+}
+
+function monthBoundaries(year: number, month: number): { start: string; end: string } {
+  const mm = String(month).padStart(2, "0");
+  const endDate = new Date(Date.UTC(year, month, 0));
+  const dd = String(endDate.getUTCDate()).padStart(2, "0");
+  return { start: `${year}-${mm}-01`, end: `${year}-${mm}-${dd}` };
+}
+
+function previousMonth(year: number, month: number): { year: number; month: number } {
+  if (month === 1) return { year: year - 1, month: 12 };
+  return { year, month: month - 1 };
+}
+
+/**
+ * Month-over-month expense delta by category. All SQL parameterised.
+ */
+export async function getCompareMonthOverMonth(
+  year: number,
+  month: number,
+): Promise<CategoryDelta[]> {
+  const db = await getDb();
+  const { start: curStart, end: curEnd } = monthBoundaries(year, month);
+  const prev = previousMonth(year, month);
+  const { start: prevStart, end: prevEnd } = monthBoundaries(prev.year, prev.month);
+
+  const rows = await db.select<RawDeltaRow[]>(
+    `SELECT
+        t.category_id,
+        COALESCE(c.name, 'Uncategorized') AS category_name,
+        COALESCE(c.color, '#9ca3af')      AS category_color,
+        COALESCE(SUM(CASE WHEN t.date >= $1 AND t.date <= $2 THEN ABS(t.amount) ELSE 0 END), 0) AS current_total,
+        COALESCE(SUM(CASE WHEN t.date >= $3 AND t.date <= $4 THEN ABS(t.amount) ELSE 0 END), 0) AS previous_total
+      FROM transactions t
+      LEFT JOIN categories c ON t.category_id = c.id
+      WHERE t.amount < 0
+        AND (
+          (t.date >= $1 AND t.date <= $2)
+          OR (t.date >= $3 AND t.date <= $4)
+        )
+      GROUP BY t.category_id, category_name, category_color
+      ORDER BY ABS(current_total - previous_total) DESC`,
+    [curStart, curEnd, prevStart, prevEnd],
+  );
+  return rowsToDeltas(rows);
+}
+
+/**
+ * Year-over-year expense delta by category. All SQL parameterised.
+ */
+export async function getCompareYearOverYear(year: number): Promise<CategoryDelta[]> {
+  const db = await getDb();
+  const curStart = `${year}-01-01`;
+  const curEnd = `${year}-12-31`;
+  const prevStart = `${year - 1}-01-01`;
+  const prevEnd = `${year - 1}-12-31`;
+
+  const rows = await db.select<RawDeltaRow[]>(
+    `SELECT
+        t.category_id,
+        COALESCE(c.name, 'Uncategorized') AS category_name,
+        COALESCE(c.color, '#9ca3af')      AS category_color,
+        COALESCE(SUM(CASE WHEN t.date >= $1 AND t.date <= $2 THEN ABS(t.amount) ELSE 0 END), 0) AS current_total,
+        COALESCE(SUM(CASE WHEN t.date >= $3 AND t.date <= $4 THEN ABS(t.amount) ELSE 0 END), 0) AS previous_total
+      FROM transactions t
+      LEFT JOIN categories c ON t.category_id = c.id
+      WHERE t.amount < 0
+        AND (
+          (t.date >= $1 AND t.date <= $2)
+          OR (t.date >= $3 AND t.date <= $4)
+        )
+      GROUP BY t.category_id, category_name, category_color
+      ORDER BY ABS(current_total - previous_total) DESC`,
+    [curStart, curEnd, prevStart, prevEnd],
+  );
+  return rowsToDeltas(rows);
+}

src/shared/types/index.ts

@@ -278,7 +278,7 @@ export interface RecentTransaction {
 
 export type ReportTab = "trends" | "byCategory" | "overTime" | "budgetVsActual";
 
-export interface HighlightMover {
+export interface CategoryDelta {
   categoryId: number | null;
   categoryName: string;
   categoryColor: string;
@@ -288,6 +288,9 @@ export interface HighlightMover {
   deltaPct: number | null; // null when previous is 0
 }
 
+// Historical alias — used by the highlights hub. Shape identical to CategoryDelta.
+export type HighlightMover = CategoryDelta;
+
 export interface MonthBalance {
   month: string; // "YYYY-MM"
   netBalance: number;