CREATE TABLE `b_iblock_section_element` (
`IBLOCK_SECTION_ID` int(11) NOT NULL,
`IBLOCK_ELEMENT_ID` int(11) NOT NULL,
`ADDITIONAL_PROPERTY_ID` int(18) DEFAULT NULL,
UNIQUE KEY `ux_iblock_section_element` (`IBLOCK_SECTION_ID`,`IBLOCK_ELEMENT_ID`,`ADDITIONAL_PROPERTY_ID`),
KEY `UX_IBLOCK_SECTION_ELEMENT2` (`IBLOCK_ELEMENT_ID`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci

insert into b_iblock_section_element
(IBLOCK_SECTION_ID, IBLOCK_ELEMENT_ID)
select e.id, e.IBLOCK_SECTION_ID
from b_iblock_element e where e.IBLOCK_ID = 12 and e.IBLOCK_SECTION_ID is not null and e.id not in (select IBLOCK_ELEMENT_ID from b_iblock_section_element where IBLOCK_SECTION_ID = IBLOCK_SECTION_ID)