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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 (204) 
  AND cscart_product_descriptions.lang_code = 'ja'

Query time 0.00004

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_descriptions",
      "access_type": "const",
      "possible_keys": ["PRIMARY", "product_id"],
      "key": "PRIMARY",
      "key_length": "9",
      "used_key_parts": ["product_id", "lang_code"],
      "ref": ["const", "const"],
      "rows": 1,
      "filtered": 100
    }
  }
}

Result

product_id	short_description	full_description
204		<p> ポリスのドラマー、スチュワート・コープランドのソロ・キャリアの中から代表曲を集めたコンピレーションアルバム。</p> <p> 収録内容:<br /> 1. Too Kool To Kalypso <br /> 2. Don't Box Me In<br /> 3. Tulsa Tango<br /> 4. Koteja <br /> 5. Serengeti Long Walk<br /> 6. Equalizer Main Title<br /> 7. Anacott Steel<br /> 8. Slither<br /> 9. Night Drive <br /> 10. Taxi Ride Home<br /> 11. Bill Is Dead<br /> 12. Wield the Spade <br /> 13. Look Up <br /> 14. Rain<br /> 15. Childhood Friends<br /> 16. Mud Lions <br /> 17. Pizica Degli Ucci<br /> 18. Chrystal/Drive Daisy/End Title<br /> 19. George Trip/End Montage/End Title<br /> 20. Big Drum Tribe <br /> 21. Regret</p>