Difference Between Nested and Repeated Fields in BigQuery

BigQuery is a powerful data warehousing solution that offers unique capabilities for handling complex data structures. Two of the important features in BigQuery for managing these structures are nested and repeated fields. Understanding the differences between these two can help you design your BigQuery schemas more effectively.

Nested Fields

A nested field in BigQuery is a field that contains a record (struct) as its datatype. This means a single field can hold multiple attributes grouped together. Nested fields allow you to represent hierarchical data in a structured and organized manner.

Example:

{
  "name": "John Doe",
  "address": {
    "street": "123 Main St",
    "city": "Anytown",
    "state": "CA"
  }
}

In the above JSON, `address` is a nested field containing `street`, `city`, and `state`.

Repeated Fields

A repeated field in BigQuery is a field that can contain an array of values, meaning the field can hold multiple instances of a value. This is useful for storing lists of items such as tags, categories, or multiple records within a single field.

Example:

{
  "name": "Jane Smith",
  "phone_numbers": [
    "123-456-7890",
    "987-654-3210"
  ]
}

In the above JSON, `phone_numbers` is a repeated field containing an array of phone number strings.

Key Differences

Feature Nested Fields Repeated Fields
Definition A field containing a record (struct) A field containing an array of values
Usage To represent hierarchical data To represent lists or multiple instances of the same type of value
Schema Definition Defined as a RECORD type with nested sub-fields Defined as a REPEATED mode for a field
Example { “address”: { “street”: “123 Main St”, “city”: “Anytown” } } { “phone_numbers”: [“123-456-7890”, “987-654-3210”] }
Querying Access sub-fields using dot notation (e.g., address.city) Use UNNEST to flatten arrays for querying (e.g., UNNEST(phone_numbers))
Storage More efficient for hierarchical data storage More efficient for storing and querying lists of simple values
Nested Repeated Fields Can contain repeated fields within a nested record Repeated fields can themselves be records containing nested fields

Example in BigQuery Schema

Here’s an example of a BigQuery schema that includes both nested and repeated fields:

[
  {
    "name": "name",
    "type": "STRING"
  },
  {
    "name": "address",
    "type": "RECORD",
    "fields": [
      {
        "name": "street",
        "type": "STRING"
      },
      {
        "name": "city",
        "type": "STRING"
      },
      {
        "name": "state",
        "type": "STRING"
      }
    ]
  },
  {
    "name": "phone_numbers",
    "type": "STRING",
    "mode": "REPEATED"
  }
]

Query Examples

Nested Field Query:

SELECT name, address.city
FROM my_table

Repeated Field Query:

SELECT name, phone_number
FROM my_table, UNNEST(phone_numbers) AS phone_number

Summary

Both nested and repeated fields in BigQuery provide flexibility in designing your data schema, enabling you to store complex and hierarchical data efficiently. Nested fields are suitable for hierarchical data structures, while repeated fields are ideal for arrays and lists of values. Understanding these differences helps in optimizing both data storage and query performance in BigQuery.

Difference Between Nested and Repeated Fields in BigQuery

Published on 15-Jun-2024 0:08:41

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