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 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    87902, 84728, 84729, 84730, 84731, 84732, 
    84733, 87921, 87923, 87933, 87935, 
    84720, 84723, 84727, 84939, 84734, 
    84735, 84938, 84719, 84722, 84724, 
    84744, 84718, 84721, 84937, 84936, 
    87922, 87928, 87934, 84856
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00046

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "13.52"
    },
    "table": {
      "table_name": "cscart_product_descriptions",
      "access_type": "range",
      "possible_keys": [
        "PRIMARY",
        "product_id"
      ],
      "key": "PRIMARY",
      "used_key_parts": [
        "product_id",
        "lang_code"
      ],
      "key_length": "9",
      "rows_examined_per_scan": 30,
      "rows_produced_per_join": 30,
      "filtered": "100.00",
      "cost_info": {
        "read_cost": "10.52",
        "eval_cost": "3.00",
        "prefix_cost": "13.52",
        "data_read_per_join": "114K"
      },
      "used_columns": [
        "product_id",
        "lang_code",
        "short_description",
        "full_description"
      ],
      "attached_condition": "((`nuie_scalesta_net`.`cscart_product_descriptions`.`product_id` in (87902,84728,84729,84730,84731,84732,84733,87921,87923,87933,87935,84720,84723,84727,84939,84734,84735,84938,84719,84722,84724,84744,84718,84721,84937,84936,87922,87928,87934,84856)) and (`nuie_scalesta_net`.`cscart_product_descriptions`.`lang_code` = 'en'))"
    }
  }
}

Result

product_id short_description full_description
84718 Old London Richmond 500mm Basin & Pedestal 1 Tap Hole
84719 Old London Richmond 560mm Basin & Pedestal 1 Tap Hole
84720 Old London Richmond 600mm Basin & Pedestal 1 Tap Hole
84721 Old London Richmond 500mm Basin & Pedestal 2 Tap Hole
84722 Old London Richmond 560mm Basin & Pedestal 2 Tap Hole
84723 Old London Richmond 600mm Basin & Pedestal 2 Tap Hole
84724 Old London Richmond 560mm Basin & Pedestal 3 Tap Hole
84727 Old London Richmond 600mm Basin & Pedestal 3 Tap Hole
84728 Old London Richmond 600mm 3 Tap Hole Basin & Comfort Height Pedestal
84729 Old London Richmond 600mm 2 Tap Hole Basin & Comfort Height Pedestal
84730 Old London Richmond 600mm 1 Tap Hole Basin & Comfort Height Pedestal
84731 Old London Richmond 560mm 3 Tap Hole Basin & Comfort Height Pedestal
84732 Old London Richmond 560mm 2 Tap Hole Basin & Comfort Height Pedestal
84733 Old London Richmond 560mm 1 Tap Hole Basin & Comfort Height Pedestal
84734 Old London Richmond 500mm 2 Tap Hole Basin & Comfort Height Pedestal
84735 Old London Richmond 500mm 1 Tap Hole Basin & Comfort Height Pedestal
84744 nuie Carlton 560mm Basin & Pedestal 2 Tap Hole
84856 Hudson Reed Luna 420mm Basin & Semi Pedestal
84936 nuie Ambrose 450mm Basin & Pedestal
84937 nuie Ambrose 500mm Basin & Pedestal
84938 nuie Ambrose 450mm Basin & Semi Pedestal
84939 nuie Ambrose 500mm Basin & Semi Pedestal
87902 nuie Round Vessel 350 x 350 x 120mm
87921 nuie Luxe Oval Vessel 565 x 350 x 145mm - Matt White
87922 nuie Luxe Rectangular Vessel 365 x 235 x 120mm - Matt White
87923 nuie Luxe Rectangular Vessel 465 x 235 x 120mm - Matt White
87928 nuie Luxe Rectangular Vessel 365 x 235 x 120mm - Matt Grey
87933 nuie Luxe Oval Vessel 565 x 350 x 145mm - Matt Black
87934 nuie Luxe Rectangular Vessel 365 x 235 x 120mm - Matt Black
87935 nuie Luxe Rectangular Vessel 465 x 235 x 120mm - Matt Black