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 (
    21021, 21022, 21023, 21024, 21025, 21026, 
    21027, 21028, 21029, 21030, 21031, 
    21032, 21033, 21034, 21035, 21036
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00036

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 (21021,21022,21023,21024,21025,21026,21027,21028,21029,21030,21031,21032,21033,21034,21035,21036) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}

Result

product_id short_description full_description unit_name
21021 actuator assembly<p>Part number 97163E4000 </p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21022 Actuator - Temperature Door<p>Part number 97159D2000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21023 Lever-temperature door, left<p>Part number 97158T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21024 Actuator-mode door<p>Part number 97226B1500</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21025 Lever-temperature door, right<p>Part number 97158T1050</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21026 Actuator - Temperature<p>Part number 97159C1050</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21027 seal<p>Part number 971A3T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21028 seal<p>Part number 971A3T1050</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21029 Evaporator Assembly<p>Part number 97139T1001</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21030 Thermistor Assembly-Air Conditioner Evaporator<p>Part number 97614T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21031 Insulation-Evaporate, lower part<p>Part number 97258T1010</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21032 Cover Assembly - Air Filter<p>Part number 97129D2000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21033 Duct Assembly - Pre-Filter<p>Part number 97190T4100</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21034 FILTER-AIR<p>Part number 97133T6700</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21035 Sensor Assembly-Fine Dust<p>Part number 97253T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21036 Sensor Assembly-Incar<p>Part number 97270T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>