code: public function getTablesUsedInFrom()
      {
          if (empty($this->from)) {
              return [];
          }
  
          if (is_array($this->from)) {
              $tableNames = $this->from;
          } elseif (is_string($this->from)) {
              $tableNames = preg_split('/\s*,\s*/', trim($this->from), -1, PREG_SPLIT_NO_EMPTY);
          } elseif ($this->from instanceof Expression) {
              $tableNames = [$this->from];
          } else {
              throw new InvalidConfigException(gettype($this->from) . ' in $from is not supported.');
          }
  
          return $this->cleanUpTableNames($tableNames);
      }
docs: Returns table names used in [[from]] indexed by aliases.
  Both aliases and names are enclosed into {{ and }}.
  @return string[] table names indexed by aliases
  @throws \yii\base\InvalidConfigException
  @since 2.0.12