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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, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (729) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00009

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	unit_name
729	<p><span style="color: var(--redactor2-text-color);"></span></p><p><strong><span style="color: rgb(192, 80, 77);">By choosing this prosthesis, you are opting for the production of a transparent retention, whitening, bruxism splint, of French origin, with a manufacturing time of 3 days.</span></strong></p><p> <strong><span style="color: rgb(155, 187, 89);">This range of prostheses is entirely designed in France by qualified dental prosthetists, with more than ten years of experience.</span></strong></p><p><br></p>