SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.short_description, 
  IF(
    cscart_product_descriptions.short_description = '' 
    OR cscart_product_descriptions.short_description IS NULL, 
    cscart_product_descriptions.full_description, 
    ''
  ) AS full_description, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21101, 21102, 21103, 21104, 21105, 21106, 
    21107, 21108, 21109, 21110, 21111, 
    21112, 21113, 21114, 21115, 21116
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00035

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_descriptions",
      "access_type": "range",
      "possible_keys": ["PRIMARY", "product_id", "idx_product_lang"],
      "key": "PRIMARY",
      "key_length": "9",
      "used_key_parts": ["product_id", "lang_code"],
      "rows": 16,
      "filtered": 100,
      "index_condition": "cscart_product_descriptions.product_id in (21101,21102,21103,21104,21105,21106,21107,21108,21109,21110,21111,21112,21113,21114,21115,21116) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}

Result

product_id short_description full_description unit_name
21101 Molding - rear bumper, left<p>Part number 86694T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21102 Cover-TAIL TRIM, left<p>Part number 866C9T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21103 Molding - rear bumper, right<p>Part number 86695T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21104 Cover-TAIL TRIM, right<p>Part number 866C0T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21105 Wiring Harness-Rear Bumper<p>Part number 91880T6040</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21106 Lamp assembly - back up, left<p>Part number 924A3T6010</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21107 Lamp assembly - back up, right<p>Part number 924A4T6010</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21108 screw-tapping<p>Part number 1249306257E</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21109 Rivet-Blind<p>Part number 81784B1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21110 Guard assembly - front wheel, left<p>Part number 86811T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21111 Guard assembly - front wheel, right<p>Part number 86812T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21112 Guard-rear wheel, left<p>Part number 86821T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21113 Guard-rear wheel, right<p>Part number 86822T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21114 Washer-Plain<p>Part number 1351004006K</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21115 Outside handle & lock assembly-trunk lid<p>Part number 81820T6000CA</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21116 Pad-Windshield<p>Part number 861142L000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>