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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 (176) 
  AND cscart_product_descriptions.lang_code = 'ja'

Query time 0.00034

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
176		<p style="font-size: 12px;f;"> <span style="font-size:16px;"><strong><span id="_ctl0_MainContent_lblTitle">Quartet® Euro™ Magnetic Porcelain Total Erase® プレゼンテーションボード</span><span id="_ctl0_MainContent_lblFeatures"> </span></strong></span></p> <ul style="color: #000000; font-family: Verdana, Arial, Helvetica, sans-serif; font-size: 11px; text-align: left; background-color: #ffffff;"> <li class="body12" style="color: #333333; font-size: 12px; text-align: left; text-decoration: none;"> 磁器製の DuraMax® 採用により傷・へこみがつきにくく、裏写りも少ないプレゼンテーションボード</li> <li class="body12" style="color: #333333; font-size: 12px; text-align: left; text-decoration: none;"> 幅68cm x 高さ104cm</li> <li class="body12" style="color: #333333; font-size: 12px; text-align: left; text-decoration: none;"> マグネットつきで掲示板としても利用可能</li> <li class="body12" style="color: #333333; font-size: 12px; text-align: left; text-decoration: none;"> Quartet® 洗浄液でお手入れかんたん</li> <li class="body12" style="color: #333333; font-size: 12px; text-align: left; text-decoration: none;"> スター型に配置された5個のキャスターでスムーズに移動</li> <li class="body12" style="color: #333333; font-size: 12px; text-align: left; text-decoration: none;"> スプリング式の高さ調節機能つき</li> <li class="body12" style="color: #333333; font-size: 12px; text-align: left; text-decoration: none;"> Includes spacious folding accessory tray and 4 assorted Quartet® whiteboard markers</li> <li class="body12" style="color: #333333; font-size: 12px; text-align: left; text-decoration: none;"> スタイルと耐久性を両立させたマットなアルミ仕上げ</li> <li class="body12" style="color: #333333; font-size: 12px; text-align: left; text-decoration: none;"> 100%満足保証</li> </ul> <p>  </p>