UNNEST clause

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    The UNNEST clause creates an input object by flattening an array in the parent document.

    Purpose

    The UNNEST clause is used within the FROM clause. If a document or object contains a nested array, UNNEST conceptually performs a join of the nested array with its parent object. Each resulting joined object becomes an output of the query. Unnests can be chained.

    Syntax

    unnest-clause ::= unnest-type? ( 'UNNEST' | 'FLATTEN' ) expr ( 'AS'? alias )?
    Syntax diagram
    unnest-type

    Unnest Type

    expr

    Unnest Path

    alias

    AS Alias

    Left-Hand Side

    The UNNEST clause cannot be the first term within the FROM clause; it must be preceded by another FROM term. The term immediately preceding the UNNEST clause represents the left-hand side of the UNNEST clause.

    You can chain the UNNEST clause with any of the other permitted FROM terms, including another UNNEST clause. For more information, see the page on the FROM clause.

    There are restrictions on what types of FROM terms may be chained and in what order — see the descriptions on this page for more details.

    The types of FROM term that may be used as the left-hand side of the UNNEST clause are summarized in the following table.

    Type Example

    keyspace identifier

    `travel-sample`.inventory.hotel

    generic expression

    20+10 AS Total

    subquery

    SELECT ARRAY_AGG(t1.city) AS cities,
      SUM(t1.city_cnt) AS apnum
    FROM (
      SELECT city, city_cnt, country,
        ARRAY_AGG(airportname) AS apnames
      FROM `travel-sample`.inventory.airport
      GROUP BY city, country
      LETTING city_cnt = COUNT(city)
    ) AS t1
    WHERE t1.city_cnt > 5;

    previous join, nest, or unnest

    SELECT *
    FROM `travel-sample`.inventory.route AS rte
    JOIN `travel-sample`.inventory.airport AS apt
      ON rte.destinationairport = apt.faa
    NEST `travel-sample`.inventory.landmark AS lmk
      ON apt.city = lmk.city
    LIMIT 5;

    Unnest Type

    unnest-type ::= 'INNER' | ( 'LEFT' 'OUTER'? )
    Syntax diagram

    This clause represents the type of unnest.

    INNER

    For each result object produced, the array object in the left-hand side keyspace must be non-empty.

    LEFT [OUTER]

    [Query Service interprets LEFT as LEFT OUTER]

    A left-outer unnest is performed, and at least one result object is produced for each left source object.

    This clause is optional. If omitted, the default is INNER.

    Unnest Path

    expr

    The path to the nested array.

    The path expression in each UNNEST clause must reference some preceding path.

    AS Alias

    Assigns another name to the right-hand side of the unnest. For details, see AS Clause.

    Assigning an alias to the path is optional. If you assign an alias to the path, the AS keyword may be omitted.

    In Couchbase Server 6.5 and later, if you want to use an ARRAY index for the UNNEST query, you can use any arbitrary alias for the right side of the UNNEST — the alias does not have to be the same as the ARRAY index variable name in order to use that index.

    Limitations

    You may chain UNNEST clauses with comma-separated joins; however, the comma-separated joins must come after any JOIN, NEST, or UNNEST clauses.

    Examples

    Example 1. UNNEST an array to select an item

    In the route keyspace, flatten the schedule array to get details of the flights on Monday (1).

    SELECT route.sourceairport, route.destinationairport, sched.flight, sched.utc
    FROM `travel-sample`.inventory.route
    UNNEST schedule sched
    WHERE  sched.day = 1
    LIMIT 3;
    Results
    [
      {
        "destinationairport": "MRS",
        "flight": "AF356",
        "sourceairport": "TLV",
        "utc": "12:40:00"
      },
      {
        "destinationairport": "MRS",
        "flight": "AF480",
        "sourceairport": "TLV",
        "utc": "08:58:00"
      },
      {
        "destinationairport": "MRS",
        "flight": "AF250",
        "sourceairport": "TLV",
        "utc": "12:59:00"
      }
    ]

    Another way to get similar results is by using a quantified expression to find array items that meet our criteria:

    SELECT route.sourceairport, route.destinationairport,
    ARRAY item FOR item IN schedule WHEN item.day = 1 END AS Monday_flights
    FROM `travel-sample`.inventory.route
    WHERE ANY item IN schedule SATISFIES item.day = 1 END
    LIMIT 3;

    However, without the UNNEST clause, the unflattened list results in 3 sets of flights instead of only 3 individual flights:

    [
      {
        "Monday_flights": [
          {
            "day": 1,
            "flight": "AF356",
            "utc": "12:40:00"
          },
          {
            "day": 1,
            "flight": "AF480",
            "utc": "08:58:00"
          },
          {
            "day": 1,
            "flight": "AF250",
            "utc": "12:59:00"
          },
          {
            "day": 1,
            "flight": "AF130",
            "utc": "04:45:00"
          }
        ],
        "destinationairport": "MRS",
        "sourceairport": "TLV"
      },
      {
        "Monday_flights": [
          {
            "day": 1,
            "flight": "AF517",
            "utc": "13:36:00"
          },
          {
            "day": 1,
            "flight": "AF279",
            "utc": "21:35:00"
          },
          {
            "day": 1,
            "flight": "AF753",
            "utc": "00:54:00"
          },
          {
            "day": 1,
            "flight": "AF079",
            "utc": "15:29:00"
          },
          {
            "day": 1,
            "flight": "AF756",
            "utc": "06:16:00"
          }
        ],
        "destinationairport": "NCE",
        "sourceairport": "TLV"
      },
      {
        "Monday_flights": [
          {
            "day": 1,
            "flight": "AF975",
            "utc": "11:23:00"
          },
          {
            "day": 1,
            "flight": "AF225",
            "utc": "16:05:00"
          }
        ],
        "destinationairport": "CDG",
        "sourceairport": "TNR"
      }
    ]
    Example 2. Use UNNEST to collect items from one array to use in another query

    In this example, the UNNEST clause iterates over the reviews array and collects the author names of the reviewers who rated the rooms less than a 2 to be contacted for ways to improve. r is an element of the array generated by the UNNEST operation.

    SELECT RAW r.author
    FROM `travel-sample`.inventory.hotel
    UNNEST reviews AS r
    WHERE r.ratings.Rooms < 2
    LIMIT 4;
    Results
    [
      "Kayli Cronin",
      "Shanelle Streich",
      "Catharine Funk",
      "Tyson Beatty"
    ]