diff --git a/CHANGELOG.md b/CHANGELOG.md
index de0a23124..48d97465f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## 2.0.1 under development
 
+- Chg #1142: Add `$constraintColumns` parameter to `upsert()`, `upsertReturning()`, and `upsertReturningPks()` methods
+  to allow explicit specification of columns for the `ON CONFLICT` clause. When `null` (default), the primary key or
+  the first matching unique constraint is used. This fixes invalid `ON CONFLICT` clause generation when a table has
+  multiple separate unique constraints (@dadansatria)
 - Bug #1127: Fix `AbstractSchema::hasTable()` and `AbstractSchema::hasView()` methods to support names quoted with curly
   brackets `{{%table}}` (@batyrmastyr)
 
diff --git a/src/Command/AbstractCommand.php b/src/Command/AbstractCommand.php
index 84cf33dca..b6b76bc88 100644
--- a/src/Command/AbstractCommand.php
+++ b/src/Command/AbstractCommand.php
@@ -499,9 +499,10 @@ public function upsert(
         string $table,
         array|QueryInterface $insertColumns,
         array|bool $updateColumns = true,
+        ?array $constraintColumns = null,
     ): static {
         $params = [];
-        $sql = $this->getQueryBuilder()->upsert($table, $insertColumns, $updateColumns, $params);
+        $sql = $this->getQueryBuilder()->upsert($table, $insertColumns, $updateColumns, $constraintColumns, $params);
         return $this->setSql($sql)->bindValues($params);
     }
 
@@ -509,16 +510,17 @@ public function upsertReturning(
         string $table,
         array|QueryInterface $insertColumns,
         array|bool $updateColumns = true,
+        ?array $constraintColumns = null,
         ?array $returnColumns = null,
     ): array {
         if ($returnColumns === []) {
-            $this->upsert($table, $insertColumns, $updateColumns)->execute();
+            $this->upsert($table, $insertColumns, $updateColumns, $constraintColumns)->execute();
             return [];
         }
 
         $params = [];
         $sql = $this->getQueryBuilder()
-            ->upsertReturning($table, $insertColumns, $updateColumns, $returnColumns, $params);
+            ->upsertReturning($table, $insertColumns, $updateColumns, $constraintColumns, $returnColumns, $params);
 
         $this->setSql($sql)->bindValues($params);
 
@@ -530,9 +532,10 @@ public function upsertReturningPks(
         string $table,
         array|QueryInterface $insertColumns,
         array|bool $updateColumns = true,
+        ?array $constraintColumns = null,
     ): array {
         $primaryKeys = $this->db->getSchema()->getTableSchema($table)?->getPrimaryKey() ?? [];
-        return $this->upsertReturning($table, $insertColumns, $updateColumns, $primaryKeys);
+        return $this->upsertReturning($table, $insertColumns, $updateColumns, $constraintColumns, $primaryKeys);
     }
 
     public function withDbTypecasting(bool $dbTypecasting = true): static
diff --git a/src/Command/CommandInterface.php b/src/Command/CommandInterface.php
index 397e30f02..4dc3bf4a4 100644
--- a/src/Command/CommandInterface.php
+++ b/src/Command/CommandInterface.php
@@ -894,6 +894,8 @@ public function update(
      * @param array|bool $updateColumns The column data (name => value) to update if it already exists.
      * If `true` is passed, the column data will be updated to match the insert column data.
      * If `false` is passed, no update will be performed if the column data already exist.
+     * @param string[]|null $constraintColumns The column names to use for the `ON CONFLICT` clause. If `null`,
+     * the primary key or the first matching unique constraint will be used.
      *
      * @throws Exception
      * @throws InvalidConfigException
@@ -910,6 +912,7 @@ public function upsert(
         string $table,
         array|QueryInterface $insertColumns,
         array|bool $updateColumns = true,
+        ?array $constraintColumns = null,
     ): static;
 
     /**
@@ -924,6 +927,8 @@ public function upsert(
      * @param array|bool $updateColumns The column data (name => value) to update if it already exists.
      * If `true` is passed, the column data will be updated to match the insert column data.
      * If `false` is passed, no update will be performed if the column data already exist.
+     * @param string[]|null $constraintColumns The column names to use for the `ON CONFLICT` clause. If `null`,
+     * the primary key or the first matching unique constraint will be used.
      * @param string[]|null $returnColumns The column names to return values from. `null` means all columns.
      *
      * @throws Exception
@@ -941,6 +946,7 @@ public function upsertReturning(
         string $table,
         array|QueryInterface $insertColumns,
         array|bool $updateColumns = true,
+        ?array $constraintColumns = null,
         ?array $returnColumns = null,
     ): array;
 
@@ -956,6 +962,8 @@ public function upsertReturning(
      * @param array|bool $updateColumns The column data (name => value) to update if it already exists.
      * If `true` is passed, the column data will be updated to match the insert column data.
      * If `false` is passed, no update will be performed if the column data already exist.
+     * @param string[]|null $constraintColumns The column names to use for the `ON CONFLICT` clause. If `null`,
+     * the primary key or the first matching unique constraint will be used.
      *
      * @throws Exception
      * @throws InvalidConfigException
@@ -971,6 +979,7 @@ public function upsertReturningPks(
         string $table,
         array|QueryInterface $insertColumns,
         array|bool $updateColumns = true,
+        ?array $constraintColumns = null,
     ): array;
 
     /**
diff --git a/src/Debug/CommandInterfaceProxy.php b/src/Debug/CommandInterfaceProxy.php
index 1e18f1078..47e7d9e56 100644
--- a/src/Debug/CommandInterfaceProxy.php
+++ b/src/Debug/CommandInterfaceProxy.php
@@ -498,6 +498,7 @@ public function upsert(
         string $table,
         array|QueryInterface $insertColumns,
         array|bool $updateColumns = true,
+        ?array $constraintColumns = null,
     ): static {
         return new self($this->decorated->{__FUNCTION__}(...func_get_args()), $this->collector);
     }
@@ -506,6 +507,7 @@ public function upsertReturning(
         string $table,
         array|QueryInterface $insertColumns,
         array|bool $updateColumns = true,
+        ?array $constraintColumns = null,
         ?array $returnColumns = null,
     ): array {
         /** @psalm-var array<string, mixed> */
@@ -516,6 +518,7 @@ public function upsertReturningPks(
         string $table,
         array|QueryInterface $insertColumns,
         array|bool $updateColumns = true,
+        ?array $constraintColumns = null,
     ): array {
         /** @var array */
         return $this->decorated->{__FUNCTION__}(...func_get_args());
diff --git a/src/QueryBuilder/AbstractDMLQueryBuilder.php b/src/QueryBuilder/AbstractDMLQueryBuilder.php
index db83ce010..c245a417b 100644
--- a/src/QueryBuilder/AbstractDMLQueryBuilder.php
+++ b/src/QueryBuilder/AbstractDMLQueryBuilder.php
@@ -25,12 +25,9 @@
 use function array_combine;
 use function array_diff;
 use function array_fill_keys;
-use function array_filter;
 use function array_key_exists;
 use function array_keys;
 use function array_map;
-use function array_merge;
-use function array_unique;
 use function array_values;
 use function count;
 use function get_object_vars;
@@ -149,6 +146,7 @@ public function upsert(
         string $table,
         array|QueryInterface $insertColumns,
         array|bool $updateColumns = true,
+        ?array $constraintColumns = null,
         array &$params = [],
     ): string {
         throw new NotSupportedException(__METHOD__ . ' is not supported by this DBMS.');
@@ -159,6 +157,7 @@ public function upsertReturning(
         string $table,
         array|QueryInterface $insertColumns,
         array|bool $updateColumns = true,
+        ?array $constraintColumns = null,
         ?array $returnColumns = null,
         array &$params = [],
     ): string {
@@ -474,6 +473,8 @@ protected function prepareUpsertSets(
      * Prepare column names and constraints for "upsert" operation.
      *
      * @param Index[] $constraints
+     * @param string[]|null $constraintColumns The column names to use for the conflict clause. If `null`,
+     * the primary key or the first matching unique constraint will be used.
      *
      * @psalm-param array<string, mixed>|QueryInterface $insertColumns
      *
@@ -485,6 +486,7 @@ protected function prepareUpsertColumns(
         array|QueryInterface $insertColumns,
         array|bool $updateColumns,
         array &$constraints = [],
+        ?array $constraintColumns = null,
     ): array {
         if ($insertColumns instanceof QueryInterface) {
             $insertNames = $this->getQueryColumnNames($insertColumns);
@@ -492,7 +494,13 @@ protected function prepareUpsertColumns(
             $insertNames = $this->getNormalizedColumnNames(array_keys($insertColumns));
         }
 
-        $uniqueNames = $this->getTableUniqueColumnNames($table, $insertNames, $constraints);
+        // Use provided constraint columns or auto-detect from table schema
+        if ($constraintColumns !== null) {
+            $uniqueNames = $this->getNormalizedColumnNames($constraintColumns);
+            $constraints = [];
+        } else {
+            $uniqueNames = $this->getTableUniqueColumnNames($table, $insertNames, $constraints);
+        }
 
         if ($updateColumns === true) {
             return [$uniqueNames, $insertNames, array_diff($insertNames, $uniqueNames)];
@@ -537,43 +545,29 @@ protected function getNormalizedColumnNames(array $columns): array
     }
 
     /**
-     * Returns all column names belonging to constraints enforcing uniqueness (`PRIMARY KEY`, `UNIQUE INDEX`, etc.)
-     * for the named table removing constraints which didn't cover the specified column list.
-     *
-     * The column list will be unique by column names.
+     * Returns column names of the first constraint enforcing uniqueness (`PRIMARY KEY`, `UNIQUE INDEX`, etc.)
+     * for the named table that is covered by the specified column list.
      *
      * @param string $name The table name, may contain schema name if any. Don't quote the table name.
      * @param string[] $columns Source column list.
-     * @param Index[] $indexes This parameter optionally receives a matched index list.
-     * The constraints will be unique by their column names.
+     * @param Index[] $indexes This parameter optionally receives the first matched index,
+     * or an empty array if no matching index is found.
      *
-     * @return string[] The column names.
+     * @return string[] The column names of the first matching constraint.
     */
     private function getTableUniqueColumnNames(string $name, array $columns, array &$indexes = []): array
     {
         $indexes = $this->schema->getTableUniques($name);
-        $columnNames = [];
-
-        // Remove all indexes which don't cover the specified column list.
-        $indexes = array_values(
-            array_filter(
-                $indexes,
-                static function (Index $index) use ($columns, &$columnNames): bool {
-                    $result = empty(array_diff($index->columnNames, $columns));
 
-                    if ($result) {
-                        $columnNames[] = $index->columnNames;
-                    }
-
-                    return $result;
-                },
-            ),
-        );
-
-        if (empty($columnNames)) {
-            return [];
+        // Find the first index that is fully covered by the specified column list.
+        foreach ($indexes as $index) {
+            if (empty(array_diff($index->columnNames, $columns))) {
+                $indexes = [$index];
+                return $index->columnNames;
+            }
         }
 
-        return array_unique(array_merge(...$columnNames));
+        $indexes = [];
+        return [];
     }
 }
diff --git a/src/QueryBuilder/AbstractQueryBuilder.php b/src/QueryBuilder/AbstractQueryBuilder.php
index 34db88f17..37a2a5e20 100644
--- a/src/QueryBuilder/AbstractQueryBuilder.php
+++ b/src/QueryBuilder/AbstractQueryBuilder.php
@@ -575,19 +575,21 @@ public function upsert(
         string $table,
         array|QueryInterface $insertColumns,
         array|bool $updateColumns = true,
+        ?array $constraintColumns = null,
         array &$params = [],
     ): string {
-        return $this->dmlBuilder->upsert($table, $insertColumns, $updateColumns, $params);
+        return $this->dmlBuilder->upsert($table, $insertColumns, $updateColumns, $constraintColumns, $params);
     }
 
     public function upsertReturning(
         string $table,
         array|QueryInterface $insertColumns,
         array|bool $updateColumns = true,
+        ?array $constraintColumns = null,
         ?array $returnColumns = null,
         array &$params = [],
     ): string {
-        return $this->dmlBuilder->upsertReturning($table, $insertColumns, $updateColumns, $returnColumns, $params);
+        return $this->dmlBuilder->upsertReturning($table, $insertColumns, $updateColumns, $constraintColumns, $returnColumns, $params);
     }
 
     public function withTypecasting(bool $typecasting = true): static
diff --git a/src/QueryBuilder/DMLQueryBuilderInterface.php b/src/QueryBuilder/DMLQueryBuilderInterface.php
index 75d7e7a8c..66d86072e 100644
--- a/src/QueryBuilder/DMLQueryBuilderInterface.php
+++ b/src/QueryBuilder/DMLQueryBuilderInterface.php
@@ -226,6 +226,8 @@ public function update(
      * @param array|bool $updateColumns The column data (name => value) to update if they already exist. If `true`
      * is passed, the column data will be updated to match the insert column data. If `false` is passed, no update will
      * be performed if the column data already exist.
+     * @param string[]|null $constraintColumns The column names to use for the `ON CONFLICT` clause. If `null`,
+     * the primary key or the first matching unique constraint will be used.
      * @param array $params The binding parameters that will be generated by this method. They should be bound to the DB
      * command later.
      *
@@ -243,6 +245,7 @@ public function upsert(
         string $table,
         array|QueryInterface $insertColumns,
         array|bool $updateColumns = true,
+        ?array $constraintColumns = null,
         array &$params = [],
     ): string;
 
@@ -258,6 +261,8 @@ public function upsert(
      * @param array|bool $updateColumns The column data (name => value) to update if they already exist. If `true`
      * is passed, the column data will be updated to match the insert column data. If `false` is passed, no update will
      * be performed if the column data already exist.
+     * @param string[]|null $constraintColumns The column names to use for the `ON CONFLICT` clause. If `null`,
+     * the primary key or the first matching unique constraint will be used.
      * @param string[]|null $returnColumns The column names to return values from. `null` means all columns.
      * @param array $params The binding parameters that will be generated by this method. They should be bound to the DB
      * command later.
@@ -274,6 +279,7 @@ public function upsertReturning(
         string $table,
         array|QueryInterface $insertColumns,
         array|bool $updateColumns = true,
+        ?array $constraintColumns = null,
         ?array $returnColumns = null,
         array &$params = [],
     ): string;