The simplest multifield query to deal with is the one where we can map search terms to specific fields. If we know that War and Peace is the title, and Leo Tolstoy is the author, it is easy to write each of these conditions as a match clause and to combine them with a bool query:

GET /_search
{
  "query": {
    "bool": {
      "should": [
        { "match": { "title":  "War and Peace" }},
        { "match": { "author": "Leo Tolstoy"   }}
      ]
    }
  }
}

The bool query takes a more-matches-is-better approach, so the score from each match clause will be added together to provide the final _score for each document. Documents that match both clauses will score higher than documents that match just one clause.

Of course, you’re not restricted to using just match clauses: the bool query can wrap any other query type, including other bool queries. We could add a clause to specify that we prefer to see versions of the book that have been translated by specific translators:

GET /_search
{
  "query": {
    "bool": {
      "should": [
        { "match": { "title":  "War and Peace" }},
        { "match": { "author": "Leo Tolstoy"   }},
        { "bool":  {
          "should": [
            { "match": { "translator": "Constance Garnett" }},
            { "match": { "translator": "Louise Maude"      }}
          ]
        }}
      ]
    }
  }
}

Why did we put the translator clauses inside a separate bool query? All four match queries are should clauses, so why didn’t we just put the translator clauses at the same level as the title and author clauses?

The answer lies in how the score is calculated. The bool query runs each match query, adds their scores together, then multiplies by the number of matching clauses, and divides by the total number of clauses. Each clause at the same level has the same weight. In the preceding query, the bool query containing the translator clauses counts for one-third of the total score. If we had put the translator clauses at the same level as title and author, they would have reduced the contribution of the title and author clauses to one-quarter each.

GET /_search
{
  "query": {
    "bool": {
      "should": [
        { "match": { 
            "title":  {
              "query": "War and Peace",
              "boost": 2
        }}},
        { "match": { 
            "author":  {
              "query": "Leo Tolstoy",
              "boost": 2
        }}},
        { "bool":  { 
            "should": [
              { "match": { "translator": "Constance Garnett" }},
              { "match": { "translator": "Louise Maude"      }}
            ]
        }}
      ]
    }
  }
}