How to write a conditional query with Elasticsearch?

You can achieve it using the following -

{
  "query": {
    "bool": {
      "must": [
        {
          "match": {
            "id": "1"
          }
        },
        {
          "bool": {
            "should": [
              {
                "missing": {
                  "field": "tags.name"
                }
              },
              {
                "match": {
                  "tags.name": "a1"
                }
              }
            ]
          }
        }
      ]
    }
  }
}