"self-join" meaning in English

{
  "etymology_text": ": self-join < → δείτε τις λέξεις self και join",
  "forms": [
    {
      "form": "self-join"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "Βάσεις δεδομένων (αγγλικά)",
          "parents": [],
          "source": "w"
        }
      ],
      "examples": [
        {
          "text": "You often use self-join to query parents/child relationship stored in a table or to obtain running totals."
        },
        {
          "text": "Άλλη γραφή: self join"
        }
      ],
      "glosses": [
        "αυτοσυνένωση"
      ],
      "id": "el-self-join-en-noun--aiICGpO",
      "raw_tags": [
        "βάσεις δεδομένων"
      ]
    }
  ],
  "word": "self-join"
}

{
  "etymology_text": ": self-join < → δείτε τις λέξεις self και join",
  "forms": [
    {
      "form": "self-join"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        "Βάσεις δεδομένων (αγγλικά)"
      ],
      "examples": [
        {
          "text": "You often use self-join to query parents/child relationship stored in a table or to obtain running totals."
        },
        {
          "text": "Άλλη γραφή: self join"
        }
      ],
      "glosses": [
        "αυτοσυνένωση"
      ],
      "raw_tags": [
        "βάσεις δεδομένων"
      ]
    }
  ],
  "word": "self-join"
}