diff --git a/src/test/kotlin/kestrel/WindowFunctionTest.kt b/src/test/kotlin/kestrel/WindowFunctionTest.kt
index 0640e99..0096c8c 100644
--- a/src/test/kotlin/kestrel/WindowFunctionTest.kt
+++ b/src/test/kotlin/kestrel/WindowFunctionTest.kt
@@ -779,3 +779,159 @@ class WindowFunctionTest {
         assertEquals(listOf(120L, 110L, 110L, 100L, 80L), result.rows.map { (it[1] as Value.IntegerValue).value })
     }
 
+    // ---- GROUPS frame unit ----
+    // sales (ordered by amount): id1=10, {id2=20, id3=20} tied, id4=30, id5=50 -> 4 distinct peer
+    // groups. GROUPS counts THESE groups, unlike ROWS (physical rows) or RANGE (value distance).
+
+    @Test
+    fun groupsBetweenPrecedingAndFollowingCountsPeerGroupsNotPhysicalRows() {
+        seedSales()
+        val groups = select(
+            "SELECT id, SUM(amount) OVER (ORDER BY amount GROUPS BETWEEN 1 PRECEDING AND 1 FOLLOWING) AS s " +
+                "FROM sales ORDER BY id ASC"
+        )
+        val rows = select(
+            "SELECT id, SUM(amount) OVER (ORDER BY amount ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING) AS s " +
+                "FROM sales ORDER BY id ASC"
+        )
+        // id2/id3's tied peer group makes the GROUPS frame span more physical rows than ROWS for
+        // their neighbours (id1 and id4), even though both ask for "1 PRECEDING AND 1 FOLLOWING".
+        assertEquals(listOf(50L, 80L, 80L, 120L, 80L), groups.rows.map { (it[1] as Value.IntegerValue).value })
+        assertEquals(listOf(30L, 50L, 70L, 100L, 80L), rows.rows.map { (it[1] as Value.IntegerValue).value })
+    }
+
+    @Test
+    fun groupsFrameBehavesExactlyLikeRowsFrameWhenNoRowsAreTied() {
+        run("CREATE TABLE prices (id INTEGER PRIMARY KEY, price DECIMAL(10, 2))")
+        run("INSERT INTO prices VALUES (1, 1.00), (2, 1.40), (3, 1.50), (4, 2.00)")
+        val groups = select(
+            "SELECT id, COUNT(*) OVER (ORDER BY price GROUPS BETWEEN 1 PRECEDING AND 1 FOLLOWING) AS c " +
+                "FROM prices ORDER BY id ASC"
+        )
+        val rows = select(
+            "SELECT id, COUNT(*) OVER (ORDER BY price ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING) AS c " +
+                "FROM prices ORDER BY id ASC"
+        )
+        assertEquals(rows.rows, groups.rows)
+    }
+
+    @Test
+    fun groupsCurrentRowExpandsToTheWholePeerGroupJustLikeRangeCurrentRow() {
+        seedSales()
+        val groups = select(
+            "SELECT id, SUM(amount) OVER (ORDER BY amount GROUPS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) AS s " +
+                "FROM sales ORDER BY id ASC"
+        )
+        val range = select(
+            "SELECT id, SUM(amount) OVER (ORDER BY amount RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) AS s " +
+                "FROM sales ORDER BY id ASC"
+        )
+        val rows = select(
+            "SELECT id, SUM(amount) OVER (ORDER BY amount ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) AS s " +
+                "FROM sales ORDER BY id ASC"
+        )
+        assertEquals(listOf(10L, 50L, 50L, 80L, 130L), groups.rows.map { (it[1] as Value.IntegerValue).value })
+        assertEquals(groups.rows, range.rows)
+        assertEquals(listOf(10L, 30L, 50L, 80L, 130L), rows.rows.map { (it[1] as Value.IntegerValue).value })
+    }
+
+    @Test
+    fun groupsFrameWorksWithAMultiColumnNonNumericOrderByUnlikeRange() {
+        seed()
+        val result = select(
+            "SELECT id, COUNT(*) OVER (ORDER BY deptId, name GROUPS BETWEEN 1 PRECEDING AND 1 FOLLOWING) AS c " +
+                "FROM employees ORDER BY deptId ASC, name ASC"
+        )
+        // ada, grace, linus (dept 1) then kay, meg, sam (dept 2) - every row is its own peer group
+        // since no two rows share the same (deptId, name), so this is exactly the sliding-window-of-3
+        // pattern (2 at each edge, 3 in the middle).
+        assertEquals(
+            listOf(int(1), int(3), int(2), int(5), int(6), int(4)),
+            result.rows.map { it[0] }
+        )
+        assertEquals(listOf(2L, 3L, 3L, 3L, 3L, 2L), result.rows.map { (it[1] as Value.IntegerValue).value })
+    }
+
+    @Test
+    fun groupsFrameEntirelyPastThePartitionEdgeAggregatesToNullAndCountsZero() {
+        seedSales()
+        val result = select(
+            "SELECT id, SUM(amount) OVER (ORDER BY amount GROUPS BETWEEN 100 FOLLOWING AND 200 FOLLOWING) AS s, " +
+                "COUNT(*) OVER (ORDER BY amount GROUPS BETWEEN 100 FOLLOWING AND 200 FOLLOWING) AS c " +
+                "FROM sales ORDER BY id ASC"
+        )
+        assertEquals((1L..5L).map { listOf(int(it), Value.NullValue, int(0)) }, result.rows)
+    }
+
+    @Test
+    fun groupsFrameOffsetMustBeAnInteger() {
+        seedSales()
+        val error = assertFailsWith<KestrelException> {
+            run("SELECT SUM(amount) OVER (ORDER BY amount GROUPS BETWEEN 1.5 PRECEDING AND CURRENT ROW) FROM sales")
+        }
+        assertTrue(error.message!!.contains("integer"), error.message!!)
+    }
+
+    @Test
+    fun groupsFrameOffsetMustNotBeNegative() {
+        seedSales()
+        val error = assertFailsWith<KestrelException> {
+            run("SELECT SUM(amount) OVER (ORDER BY amount GROUPS BETWEEN -1 PRECEDING AND CURRENT ROW) FROM sales")
+        }
+        assertTrue(error.message!!.contains("negative"), error.message!!)
+    }
+
+    @Test
+    fun groupsFrameWithUnboundedPrecedingAndFollowingCoversTheWholePartitionJustLikeRowsAndRange() {
+        seedSales()
+        val groups = select(
+            "SELECT id, SUM(amount) OVER (ORDER BY amount GROUPS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) AS s " +
+                "FROM sales ORDER BY id ASC"
+        )
+        assertEquals(listOf(130L, 130L, 130L, 130L, 130L), groups.rows.map { (it[1] as Value.IntegerValue).value })
+    }
+
+    @Test
+    fun groupsFrameIsRejectedOnRankingAndOffsetWindowFunctions() {
+        seed()
+        val error = assertFailsWith<KestrelException> {
+            run("SELECT ROW_NUMBER() OVER (ORDER BY id GROUPS BETWEEN 1 PRECEDING AND CURRENT ROW) FROM employees")
+        }
+        assertTrue(error.message!!.contains("GROUPS"), error.message!!)
+    }
+
+    @Test
+    fun excludeGroupCombinesWithAGroupsFrameJustLikeItDoesWithRange() {
+        seedSales()
+        val groups = select(
+            "SELECT id, SUM(amount) OVER (" +
+                "ORDER BY amount GROUPS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING EXCLUDE GROUP" +
+                ") AS s FROM sales ORDER BY id ASC"
+        )
+        assertEquals(listOf(120L, 90L, 90L, 100L, 80L), groups.rows.map { (it[1] as Value.IntegerValue).value })
+    }
+
+    @Test
+    fun nestedWindowFunctionInAGroupsFrameOffsetIsRejected() {
+        seedSales()
+        val error = assertFailsWith<KestrelException> {
+            run(
+                "SELECT SUM(amount) OVER (ORDER BY amount GROUPS BETWEEN ROW_NUMBER() OVER (ORDER BY amount) PRECEDING AND CURRENT ROW) " +
+                    "FROM sales"
+            )
+        }
+        assertTrue(error.message!!.contains("nested"), error.message!!)
+    }
+
+    @Test
+    fun groupsFrameSurvivesViewDefinitionRoundTrip() {
+        seedSales()
+        run(
+            "CREATE VIEW groupTotals AS SELECT id, " +
+                "SUM(amount) OVER (ORDER BY amount GROUPS BETWEEN 1 PRECEDING AND 1 FOLLOWING) AS s " +
+                "FROM sales"
+        )
+        val result = select("SELECT id, s FROM groupTotals ORDER BY id ASC")
+        assertEquals(listOf(50L, 80L, 80L, 120L, 80L), result.rows.map { (it[1] as Value.IntegerValue).value })
+    }
+}