code: public function buildExistsCondition($operator, $operands, &$params)
      {
          array_unshift($operands, $operator);
          return $this->buildCondition($operands, $params);
      }
docs: Creates an SQL expressions with the `EXISTS` operator.
  @param string $operator the operator to use (e.g. `EXISTS` or `NOT EXISTS`)
  @param array $operands contains only one element which is a [[Query]] object representing the sub-query.
  @param array $params the binding parameters to be populated
  @return string the generated SQL expression
  @throws InvalidArgumentException if the operand is not a [[Query]] object.
  @deprecated since 2.0.14. Use `buildCondition()` instead.