daffie
--- a/core/lib/Drupal/Core/Database/Query/Condition.php
+++ b/core/lib/Drupal/Core/Database/Query/Condition.php
    return $this;
  }

+  /**
+   * Compare two database fields with each other.
--- a/core/lib/Drupal/Core/Database/Query/Condition.php
+++ b/core/lib/Drupal/Core/Database/Query/Condition.php
+   *
+   * @param string $field
+   *   The name of the field to compare.
+   * @param string $field2
+   *   The name of the other field to compare.
+   * @param string|null $operator
+   *   (optional) The operator to use. The supported operators are: =, <>, <,
+   *   <=, >, >=, <>.
+   *
+   * @return $this
+   *   The called object.
+   *
+   * @throws \Drupal\Core\Database\InvalidQueryException
+   *   If passed invalid arguments, such as an empty array as $value.
+   */
+  public function compare(string $field, string $field2, ?string $operator = '=') {
-  public function compare(string $field, string $field2, ?string $operator = '=') {
+  public function compareColumns(string $columnLeft, string $operator, string $columnRight) {
--- a/core/lib/Drupal/Core/Database/Query/Condition.php
+++ b/core/lib/Drupal/Core/Database/Query/Condition.php
+   *   <=, >, >=, <>.
+   *
+   * @return $this
+   *   The called object.
+   *
+   * @throws \Drupal\Core\Database\InvalidQueryException
+   *   If passed invalid arguments, such as an empty array as $value.
+   */
+  public function compare(string $field, string $field2, ?string $operator = '=') {
+    if (!in_array($operator, ['=', '<', '>', '>=', '<=', '<>'], TRUE)) {
+      throw new InvalidQueryException(sprintf("In a query compare '%s %s %s' the operator must be one of the following: '=', '<', '>', '>=', '<=', '<>'.", $field, $operator, $field2));
+    }
+
+    $this->conditions[] = [
+      'field' => $field,
+      'field2' => $field2,
--- a/core/lib/Drupal/Core/Database/Query/Condition.php
+++ b/core/lib/Drupal/Core/Database/Query/Condition.php
+    $this->changed = TRUE;
+
+    return $this;
+  }
+
+  /**
+   * Update the alias placeholder in the condition and its children.
+   *
+   * @param string $placeholder
+   *   The value of the placeholder.
+   * @param string $alias
+   *   The value to replace the placeholder.
+   *
+   * @internal
+   */
+  public function updateAliasPlaceholder(string $placeholder, string $alias): void {
--- a/core/lib/Drupal/Core/Database/Query/Select.php
+++ b/core/lib/Drupal/Core/Database/Query/Select.php
   * {@inheritdoc}
   */
  public function addJoin($type, $table, $alias = NULL, $condition = NULL, $arguments = []) {
+    if (!empty($condition) && !$condition instanceof ConditionInterface) {
--- a/core/lib/Drupal/Core/Entity/Query/Sql/Tables.php
+++ b/core/lib/Drupal/Core/Entity/Query/Sql/Tables.php
      // Only the data table follows the entity language key, dedicated field
      // tables have a hard-coded 'langcode' column.
      $langcode_key = $entity_type->getDataTable() == $table ? $entity_type->getKey('langcode') : 'langcode';
-      $placeholder = ':langcode' . $this->sqlQuery->nextPlaceholder();
-      $join_condition .= ' AND [%alias].[' . $langcode_key . '] = ' . $placeholder;
-      $arguments[$placeholder] = $langcode;
+      if ($join_condition instanceof ConditionInterface) {
+        $join_condition->condition('%alias.' . $langcode_key, $langcode);
+      }
+      // Start of BC layer.
+      else {
+        $placeholder = ':langcode' . $this->sqlQuery->nextPlaceholder();
+        $join_condition .= ' AND [%alias].[' . $langcode_key . '] = ' . $placeholder;
+        $arguments[$placeholder] = $langcode;
+      }
+      // End of BC layer.
--- a/core/lib/Drupal/Core/Test/SimpletestTestRunResultsStorage.php
+++ b/core/lib/Drupal/Core/Test/SimpletestTestRunResultsStorage.php
    // Run a select query to return 'last_prefix' from {simpletest_test_id} and
    // 'test_class' from {simpletest}.
    $select = $this->connection->select($max_message_id_subquery, 'st_sub');
-    $select->join('simpletest', 'st', '[st].[message_id] = [st_sub].[max_message_id]');
-    $select->join('simpletest_test_id', 'sttid', '[st].[test_id] = [sttid].[test_id]');
+    $select->join('simpletest', 'st', $this->connection->condition('AND')->compare('st.message_id', 'st_sub.max_message_id'));
+    $select->join('simpletest_test_id', 'sttid', $this->connection->condition('AND')->compare('st.test_id', 'sttid.test_id'));
--- a/core/modules/file/src/Plugin/migrate/source/d6/Upload.php
+++ b/core/modules/file/src/Plugin/migrate/source/d6/Upload.php
    $query = $this->select('upload', 'u')
      ->distinct()
      ->fields('u', ['nid', 'vid']);
-    $query->innerJoin('node', 'n', static::JOIN);
+
+    // Start of BC layer.
+    if (is_string(static::JOIN)) {