When you first connect to a MongoDB server, the driver automatically generates a normalized view of your data. You can also use the Schema Tool to normalize your native MongoDB data. When your native data is normalized, each MongoDB collection is decompounded into a set of parent-child tables. Fields containing simple types are mapped as columns in a parent table, while complex types, such as subdocuments and arrays, are mapped as child tables. Child tables share a foreign key relationship with their parent table.
Note: Child tables are only extracted from arrays if all the sampled rows in the column have the native Array data type. See ConfigOptions for information on setting the sample size.

For example, the collection residents contains the array vehicles and subdocuments in the address document (or object). The collection's JSON structure can be rendered as follows:

{"_id": "ajx363",
 "name": "Sydney Smith",
 "address": {"street": "101 Main Street", "city": "Raleigh", "state": "NC"},
 "county": "Wake",
 "vehicles: ["car", "boat", "bicycle"]
}

{"_id": "tzn525",
 "name": "Cora Welch",
 "address": {"street": "191 First Street", "city": "Chapel Hill", "state": "NC"},
 "county": "Orange",
 "vehicles": ["scooter", "truck", "bicycle"]
}

The collection has been decompounded into separate but related tables. The normalization of the residents collection yields one parent table and two child tables. In the parent table, the _id field is mapped as a primary key column, and fields with other simple types are mapped as relational columns. The parent table adopts the name RESIDENTS and takes the following form:

_ID (PK) NAME COUNTY
ajx363 Sydney Smith Wake
tzn525 Cora Welch Orange

The information in the vehicles array maps to a RESIDENTS_VEHICLES child table. In this table, a column which refers back to the parent table is mapped as a concatenation of the parent table and the _id field: RESIDENTS_ID. (This column establishes a foreign key relationship to the parent table based on the _id field.) The vehicles array is mapped to the column VEHICLES. Each value in the VEHICLES column corresponds to an element in the vehicles array. In addition, the driver generates a third column to establish a primary key for the child table: VEHICLES_GENERATED_ID. (This automatically generated column enables the driver to normalize nested arrays to any depth. The format of the unique keys are internal to the driver.) This child table would take the following form:

RESIDENTS_ID (FK) VEHICLES VEHICLES_GENERATED_ID (PK)
ajx363 car unique key
ajx363 boat unique key
ajx363 bicycle unique key
tzn525 bicycle unique key
tzn525 scooter unique key
tzn525 truck unique key

The information in the address object maps to a RESIDENTS_ADDRESS child table. In this table, a column which refers back to the parent table is mapped as a concatenation of the parent name and the _id field: RESIDENTS_ID. (This column establishes a foreign key relationship to the parent table and functions as a primary key within the child table.) Each subdocument found in the address object is mapped as a column. This table would take the following form:

RESIDENTS_ID (PK & FK) STREET CITY STATE
ajx363 101 Main Street Raleigh NC
tzn525 191 First Street Chapel Hill NC