diff --git a/common/workflow-operator/src/main/scala/org/apache/texera/amber/operator/aggregate/AggregationOperation.scala b/common/workflow-operator/src/main/scala/org/apache/texera/amber/operator/aggregate/AggregationOperation.scala
index 70105de9ef4..bec4b46c10d 100644
--- a/common/workflow-operator/src/main/scala/org/apache/texera/amber/operator/aggregate/AggregationOperation.scala
+++ b/common/workflow-operator/src/main/scala/org/apache/texera/amber/operator/aggregate/AggregationOperation.scala
@@ -222,7 +222,7 @@ class AggregationOperation {
       (partial1, partial2) =>
         if (AttributeTypeUtils.compare(partial1, partial2, attributeType) > 0) partial1
         else partial2,
-      partial => if (partial == AttributeTypeUtils.maxValue(attributeType)) null else partial
+      partial => if (partial == AttributeTypeUtils.minValue(attributeType)) null else partial
     )
   }
 
diff --git a/common/workflow-operator/src/test/scala/org/apache/texera/amber/operator/aggregate/AggregateOpSpec.scala b/common/workflow-operator/src/test/scala/org/apache/texera/amber/operator/aggregate/AggregateOpSpec.scala
index cb7925ec41d..f1e32c8bba5 100644
--- a/common/workflow-operator/src/test/scala/org/apache/texera/amber/operator/aggregate/AggregateOpSpec.scala
+++ b/common/workflow-operator/src/test/scala/org/apache/texera/amber/operator/aggregate/AggregateOpSpec.scala
@@ -23,6 +23,8 @@ import org.apache.texera.amber.core.tuple.{Attribute, AttributeType, Schema, Tup
 import org.apache.texera.amber.util.JSONUtils.objectMapper
 import org.scalatest.funsuite.AnyFunSuite
 
+import java.sql.Timestamp
+
 class AggregateOpSpec extends AnyFunSuite {
 
   /** Helpers */
@@ -408,6 +410,67 @@ class AggregateOpSpec extends AnyFunSuite {
     assert(result == maxValue)
   }
 
+  test("MAX aggregation finds largest INTEGER and returns null when given no values") {
+    val schema = makeSchema("temperature" -> AttributeType.INTEGER)
+    val tuple1 = makeTuple(schema, 10)
+    val tuple2 = makeTuple(schema, -2)
+    val tuple3 = makeTuple(schema, 5)
+
+    val operation = makeAggregationOp(AggregationFunction.MAX, "temperature", "max_temp")
+    val agg = operation.getAggFunc(AttributeType.INTEGER)
+
+    // Empty case: never iterate, just finalize init
+    val emptyPartial = agg.init()
+    val emptyResult = agg.finalAgg(emptyPartial)
+    assert(emptyResult == null)
+
+    // Non-empty case
+    var partial = agg.init()
+    partial = agg.iterate(partial, tuple1)
+    partial = agg.iterate(partial, tuple2)
+    partial = agg.iterate(partial, tuple3)
+
+    val result = agg.finalAgg(partial).asInstanceOf[Number].intValue()
+    assert(result == 10)
+  }
+
+  test("MAX aggregation returns null when all values are null") {
+    val schema = makeSchema("temperature" -> AttributeType.INTEGER)
+
+    val operation = makeAggregationOp(AggregationFunction.MAX, "temperature", "max_temp")
+    val agg = operation.getAggFunc(AttributeType.INTEGER)
+
+    var partial = agg.init()
+    Seq.fill(3)(makeTuple(schema, null)).foreach(tp => partial = agg.iterate(partial, tp))
+    assert(agg.finalAgg(partial) == null)
+  }
+
+  test("MAX aggregation keeps the type's maximum value when it is the true maximum") {
+    // Regression: the finalizer used to mistake a partial equal to
+    // maxValue(attributeType) for the "no value seen" sentinel and emit null.
+    val cases: Seq[(AttributeType, Seq[Any], Any)] = Seq(
+      (AttributeType.INTEGER, Seq(1, 5, Int.MaxValue), Int.MaxValue),
+      (AttributeType.LONG, Seq(1L, 5L, Long.MaxValue), Long.MaxValue),
+      (AttributeType.DOUBLE, Seq(1.0, 5.0, Double.MaxValue), Double.MaxValue),
+      (
+        AttributeType.TIMESTAMP,
+        Seq(new Timestamp(1L), new Timestamp(Long.MaxValue)),
+        new Timestamp(Long.MaxValue)
+      )
+    )
+
+    for ((attrType, values, expected) <- cases) {
+      val schema = makeSchema("v" -> attrType)
+      val operation = makeAggregationOp(AggregationFunction.MAX, "v", "max_v")
+      val agg = operation.getAggFunc(attrType)
+
+      var partial = agg.init()
+      values.foreach(v => partial = agg.iterate(partial, makeTuple(schema, v)))
+
+      assert(agg.finalAgg(partial) == expected, s"MAX over $attrType must keep $expected")
+    }
+  }
+
   test("AVERAGE aggregation ignores nulls and returns null when all values are null") {
     val schema = makeSchema("price" -> AttributeType.DOUBLE)
     val tuple1 = makeTuple(schema, 10.0)
diff --git a/common/workflow-operator/src/test/scala/org/apache/texera/amber/operator/aggregate/AggregationOperationSpec.scala b/common/workflow-operator/src/test/scala/org/apache/texera/amber/operator/aggregate/AggregationOperationSpec.scala
index 4ebba05fbc2..b11bc3e9596 100644
--- a/common/workflow-operator/src/test/scala/org/apache/texera/amber/operator/aggregate/AggregationOperationSpec.scala
+++ b/common/workflow-operator/src/test/scala/org/apache/texera/amber/operator/aggregate/AggregationOperationSpec.scala
@@ -173,6 +173,35 @@ class AggregationOperationSpec extends AnyFlatSpec {
     assert(finalSum == 36, "single-pass SUM(1+2+3+10+20) == 36")
   }
 
+  it should
+    "keep the type's maximum value when partial MAX results are re-aggregated via getFinal" in {
+    // Regression: the local stage used to finalize a partial equal to
+    // maxValue(attributeType) to null, so the final stage reported the largest
+    // value from the other worker instead of the true maximum.
+    val workerOp = op(AggregationFunction.MAX, attribute = "v", resultAttribute = "max_v")
+    val workerAgg = workerOp.getAggFunc(AttributeType.INTEGER)
+
+    val w1State = Seq[AnyRef](Int.box(1), Int.box(Int.MaxValue))
+      .map(v => tupleOf("v", AttributeType.INTEGER, v))
+      .foldLeft(workerAgg.init())(workerAgg.iterate)
+    val w1Out = workerAgg.finalAgg(w1State)
+    assert(w1Out == Int.box(Int.MaxValue), "worker 1's true maximum is Int.MaxValue")
+
+    val w2State = Seq[AnyRef](Int.box(5), Int.box(2))
+      .map(v => tupleOf("v", AttributeType.INTEGER, v))
+      .foldLeft(workerAgg.init())(workerAgg.iterate)
+    val w2Out = workerAgg.finalAgg(w2State)
+    assert(w2Out == Int.box(5))
+
+    val finalOp = workerOp.getFinal
+    assert(finalOp.aggFunction == AggregationFunction.MAX)
+    val finalAgg = finalOp.getAggFunc(AttributeType.INTEGER)
+    val finalState = Seq(w1Out, w2Out)
+      .map(p => tupleOf("max_v", AttributeType.INTEGER, p))
+      .foldLeft(finalAgg.init())(finalAgg.iterate)
+    assert(finalAgg.finalAgg(finalState) == Int.box(Int.MaxValue))
+  }
+
   // --- AveragePartialObj -----------------------------------------------------
 
   "AveragePartialObj" should "expose its sum and count fields and support value equality" in {
@@ -183,4 +212,227 @@ class AggregationOperationSpec extends AnyFlatSpec {
     assert(a == b)
     assert(a.hashCode == b.hashCode)
   }
+<<<<<<< HEAD
+=======
+
+  // --- getAggFunc: the accepting side of the four-clause type guard ----------
+
+  // SUM/MIN/MAX each guard on
+  // `!= INTEGER && != DOUBLE && != LONG && != TIMESTAMP`. The existing tests
+  // only drive the rejecting side plus INTEGER/DOUBLE, so the chain is never
+  // walked to its later clauses; TIMESTAMP in particular has to pass all four.
+  private val supportedAggTypes = Seq(
+    AttributeType.INTEGER,
+    AttributeType.DOUBLE,
+    AttributeType.LONG,
+    AttributeType.TIMESTAMP
+  )
+
+  private val guardedAggregations = Seq(
+    AggregationFunction.SUM -> "sum",
+    AggregationFunction.MIN -> "min",
+    AggregationFunction.MAX -> "max"
+  )
+
+  it should "accept every supported attribute type on SUM, MIN and MAX" in {
+    for ((func, name) <- guardedAggregations; t <- supportedAggTypes)
+      assert(op(func).getAggFunc(t) != null, s"$name should accept $t")
+  }
+
+  it should "reject unsupported attribute types on SUM, MIN and MAX, naming the aggregation and the type" in {
+    val unsupported =
+      Seq(AttributeType.STRING, AttributeType.BOOLEAN, AttributeType.BINARY)
+    for ((func, name) <- guardedAggregations; t <- unsupported) {
+      val ex = intercept[UnsupportedOperationException](op(func).getAggFunc(t))
+      assert(ex.getMessage == s"Unsupported attribute type for $name aggregation: $t")
+    }
+  }
+
+  // --- AVERAGE over TIMESTAMP: the timestamp branch of getNumericalValue -----
+
+  // Everywhere else AVERAGE is driven over DOUBLE, which takes the
+  // `value.toString.toDouble` path. A TIMESTAMP column is the only route into
+  // the `parseTimestamp(...).getTime` branch.
+  private val earlier = Timestamp.valueOf("2020-03-05 10:00:00")
+  private val later = Timestamp.valueOf("2020-03-05 11:00:00")
+  private val midpointMillis = (earlier.getTime + later.getTime) / 2.0
+
+  "AVERAGE over a TIMESTAMP column" should "average the values' epoch milliseconds" in {
+    val agg = op(AggregationFunction.AVERAGE).getAggFunc(AttributeType.TIMESTAMP)
+    val state = Seq(earlier, later)
+      .map(ts => tupleOf("v", AttributeType.TIMESTAMP, ts))
+      .foldLeft(agg.init())(agg.iterate)
+
+    assert(agg.finalAgg(state).asInstanceOf[java.lang.Double] == midpointMillis)
+  }
+
+  it should "combine per-worker partials through merge" in {
+    val agg = op(AggregationFunction.AVERAGE).getAggFunc(AttributeType.TIMESTAMP)
+    val p1 = agg.iterate(agg.init(), tupleOf("v", AttributeType.TIMESTAMP, earlier))
+    val p2 = agg.iterate(agg.init(), tupleOf("v", AttributeType.TIMESTAMP, later))
+
+    val merged = agg.merge(p1, p2)
+
+    assert(agg.finalAgg(merged).asInstanceOf[java.lang.Double] == midpointMillis)
+  }
+
+  it should "ignore null timestamps and return null when every value is null" in {
+    val agg = op(AggregationFunction.AVERAGE).getAggFunc(AttributeType.TIMESTAMP)
+
+    val mixed = Seq[AnyRef](earlier, null, later)
+      .map(ts => tupleOf("v", AttributeType.TIMESTAMP, ts))
+      .foldLeft(agg.init())(agg.iterate)
+    assert(agg.finalAgg(mixed).asInstanceOf[java.lang.Double] == midpointMillis)
+
+    val allNull =
+      agg.iterate(agg.init(), tupleOf("v", AttributeType.TIMESTAMP, null))
+    assert(agg.finalAgg(allNull) == null)
+  }
+
+  // --- merge: the partial-combination lambda of each aggregation --------------
+
+  // `AggregateOpSpec` drives init/iterate/finalAgg for every aggregation but only
+  // ever merges AVERAGE/CONCAT partials. The `merge` lambdas of SUM, COUNT, MIN
+  // and MAX are what the global stage calls when several workers report in, so
+  // each one is exercised here directly and cross-checked against the equivalent
+  // single-pass aggregation.
+
+  private def aggregateAll(
+      agg: DistributedAggregation[Object],
+      t: AttributeType,
+      values: Seq[AnyRef]
+  ): Object =
+    agg.finalAgg(values.map(v => tupleOf("v", t, v)).foldLeft(agg.init())(agg.iterate))
+
+  "SUM aggregation merge" should "add two partials, matching a single-pass SUM" in {
+    val agg = op(AggregationFunction.SUM).getAggFunc(AttributeType.LONG)
+    val left = Seq[AnyRef](Long.box(1L), Long.box(2L))
+    val right = Seq[AnyRef](Long.box(10L), Long.box(20L))
+
+    val p1 = left.map(v => tupleOf("v", AttributeType.LONG, v)).foldLeft(agg.init())(agg.iterate)
+    val p2 = right.map(v => tupleOf("v", AttributeType.LONG, v)).foldLeft(agg.init())(agg.iterate)
+
+    assert(agg.finalAgg(agg.merge(p1, p2)) == Long.box(33L))
+    assert(agg.finalAgg(agg.merge(p1, p2)) == aggregateAll(agg, AttributeType.LONG, left ++ right))
+    // merging with an untouched (zero) partial must be a no-op
+    assert(agg.finalAgg(agg.merge(p1, agg.init())) == Long.box(3L))
+  }
+
+  "COUNT aggregation merge" should "add the per-worker counts" in {
+    val agg = op(AggregationFunction.COUNT).getAggFunc(AttributeType.INTEGER)
+    val p1 = Seq[AnyRef](Int.box(1), null, Int.box(3))
+      .map(v => tupleOf("v", AttributeType.INTEGER, v))
+      .foldLeft(agg.init())(agg.iterate)
+    val p2 = Seq[AnyRef](Int.box(4))
+      .map(v => tupleOf("v", AttributeType.INTEGER, v))
+      .foldLeft(agg.init())(agg.iterate)
+
+    // COUNT(v) skips the null, so 2 + 1 == 3
+    assert(agg.finalAgg(agg.merge(p1, p2)) == Int.box(3))
+    assert(agg.finalAgg(agg.merge(agg.init(), agg.init())) == Int.box(0))
+  }
+
+  "MIN and MAX aggregation merge" should "pick the smaller and larger partial respectively" in {
+    val minAgg = op(AggregationFunction.MIN).getAggFunc(AttributeType.DOUBLE)
+    val maxAgg = op(AggregationFunction.MAX).getAggFunc(AttributeType.DOUBLE)
+    val left = Seq[AnyRef](Double.box(4.0), Double.box(9.0))
+    val right = Seq[AnyRef](Double.box(-1.5), Double.box(2.0))
+
+    def partialOf(agg: DistributedAggregation[Object], values: Seq[AnyRef]): Object =
+      values.map(v => tupleOf("v", AttributeType.DOUBLE, v)).foldLeft(agg.init())(agg.iterate)
+
+    assert(
+      minAgg.finalAgg(minAgg.merge(partialOf(minAgg, left), partialOf(minAgg, right)))
+        == Double.box(-1.5)
+    )
+    // merge must be symmetric
+    assert(
+      minAgg.finalAgg(minAgg.merge(partialOf(minAgg, right), partialOf(minAgg, left)))
+        == Double.box(-1.5)
+    )
+    assert(
+      maxAgg.finalAgg(maxAgg.merge(partialOf(maxAgg, left), partialOf(maxAgg, right)))
+        == Double.box(9.0)
+    )
+    assert(
+      maxAgg.finalAgg(maxAgg.merge(partialOf(maxAgg, right), partialOf(maxAgg, left)))
+        == Double.box(9.0)
+    )
+  }
+
+  "MIN aggregation merge" should "stay neutral when one side saw no values" in {
+    val agg = op(AggregationFunction.MIN).getAggFunc(AttributeType.INTEGER)
+    val seen = agg.iterate(agg.init(), tupleOf("v", AttributeType.INTEGER, Int.box(7)))
+
+    // An empty partial is the sentinel maxValue, so it must lose the comparison.
+    assert(agg.finalAgg(agg.merge(seen, agg.init())) == Int.box(7))
+    assert(agg.finalAgg(agg.merge(agg.init(), seen)) == Int.box(7))
+    // Two empty partials still finalize to null (no rows anywhere).
+    assert(agg.finalAgg(agg.merge(agg.init(), agg.init())) == null)
+  }
+
+  "MAX aggregation merge" should "stay neutral when one side saw no values" in {
+    val agg = op(AggregationFunction.MAX).getAggFunc(AttributeType.INTEGER)
+    val seen = agg.iterate(agg.init(), tupleOf("v", AttributeType.INTEGER, Int.box(7)))
+
+    // An empty partial is the sentinel minValue, so it must lose the comparison.
+    assert(agg.finalAgg(agg.merge(seen, agg.init())) == Int.box(7))
+    assert(agg.finalAgg(agg.merge(agg.init(), seen)) == Int.box(7))
+    // Two empty partials still finalize to null (no rows anywhere).
+    assert(agg.finalAgg(agg.merge(agg.init(), agg.init())) == null)
+  }
+
+  // --- CONCAT: a null first value seeds the partial with an empty string ------
+
+  "CONCAT aggregation" should "swallow leading nulls but keep interior ones as empty slots" in {
+    // AggregateOpSpec only ever concatenates a null in the middle of the stream.
+    // A null on the very first tuple takes the `partial == ""` side of the branch
+    // and leaves the partial empty, so — unlike an interior null — it does not
+    // occupy a slot in the comma-joined output.
+    val agg = op(AggregationFunction.CONCAT).getAggFunc(AttributeType.STRING)
+    val result = Seq[AnyRef](null, "red", null, "blue")
+      .map(v => tupleOf("v", AttributeType.STRING, v))
+      .foldLeft(agg.init())(agg.iterate)
+
+    assert(agg.finalAgg(result) == "red,,blue")
+  }
+
+  it should "return an empty string when every value is null" in {
+    val agg = op(AggregationFunction.CONCAT).getAggFunc(AttributeType.STRING)
+    val onlyNull = agg.iterate(agg.init(), tupleOf("v", AttributeType.STRING, null))
+
+    assert(agg.finalAgg(onlyNull) == "")
+  }
+
+  it should "stringify non-string values it is pointed at" in {
+    // CONCAT is schema-restricted to STRING columns in the UI, but the executor
+    // only ever calls `.toString`, so an INTEGER column still concatenates.
+    val agg = op(AggregationFunction.CONCAT).getAggFunc(AttributeType.STRING)
+    val result = Seq[AnyRef](Int.box(1), Int.box(2))
+      .map(v => tupleOf("v", AttributeType.INTEGER, v))
+      .foldLeft(agg.init())(agg.iterate)
+
+    assert(agg.finalAgg(result) == "1,2")
+  }
+
+  // --- getAggregationAttribute / getFinal: message and identity guarantees ----
+
+  "getAggregationAttribute" should "name the unknown aggregation function in its error" in {
+    val ex = intercept[RuntimeException](op(null).getAggregationAttribute(AttributeType.INTEGER))
+    assert(ex.getMessage == "Unknown aggregation function: null")
+  }
+
+  "getFinal" should "produce a detached copy that re-reads the result column" in {
+    val original = op(AggregationFunction.MAX, attribute = "src", resultAttribute = "dst")
+    val copy = original.getFinal
+
+    assert(copy ne original)
+    assert(copy.aggFunction == AggregationFunction.MAX)
+    // the final stage reads and writes the same (result) column
+    assert(copy.attribute == "dst")
+    assert(copy.resultAttribute == "dst")
+    // the original is left untouched
+    assert(original.attribute == "src")
+  }
+>>>>>>> b0dd3ffc5 (fix(workflow-operator): MAX aggregation checks the wrong empty-group sentinel (#7532))
 }