Returning child rows formatted as JSON in SQL Server queries

In this post I will talk about one annoying thing – how to return a result set containing one to many relationships between tables (i.e. parent’child relationships such as company-products, person-address, sales order-items, etc.)?

Problem

I have relational structure with several one to many relationships (e.g. Person may have several phones, or several email addresses). In order to read Person and related information; In need to run several queries (e.g. one to return person info, another to return person phones, third to return person email addresses), or join tables and process one flat result set on the client side.

It would be nice if I could retrieve person information and all related information with a single SQL query.

Solution

Related information can be formatted as JSON arrays and returned as arrays in parent Person row. this way we need a single query to return all Person related information.

Scenario – Exporting complex relational data structures as JSON

In practice you will have one to many relationships in many cases, e.g. person can have many email addresses, many phones, etc. If you want to store these information in relational database, you would need to use something like a structure on the following figure:

 

Even for the simple structures such as phone numbers or emails that will be represented as arrays in C# or Java, you would need to create separate tables with foreign key relationships. Now if you try to join these tables you would need to use something like a following query:

SELECT Person.Person.BusinessEntityID, Person.Person.FirstName,

       Person.Person.LastName, Person.EmailAddress.EmailAddress,

       Person.PersonPhone.PhoneNumber, Person.PhoneNumberType.Name AS PhoneNumberType

FROM         Person.Person INNER JOIN

                      Person.EmailAddress ON Person.Person.BusinessEntityID = Person.EmailAddress.BusinessEntityID INNER JOIN

                      Person.PersonPhone ON Person.Person.BusinessEntityID = Person.PersonPhone.BusinessEntityID INNER JOIN

                      Person.PhoneNumberType ON Person.PersonPhone.PhoneNumberTypeID = Person.PhoneNumberType.PhoneNumberTypeID

WHERE Person.Person.BusinessEntityID = 274

 

If you run this query in AdventureWorks database, you will get information about person with email address and phone number:

BusinessEntityID

FirstName

LastName

EmailAddress

PhoneNumber

PhoneNumberType

274

Stephen

Jiang

stephen0@adventure-works.com

112-555-6207

Work

 

One row is returned because in AW database there is only one telephone number and email per person. Similar results will be returned if you query Sales.vSalesPerson view.

Now, since database schema allows us to add more than one address what would happen if you add another email address to Stephen?

INSERT INTO Person.EmailAddress(BusinessEntityID,EmailAddress)

       VALUES (274,’stephen.jiang@outlook.com’)

 

If you execute this query again, you will get the following result:

BusinessEntityID

FirstName

LastName

EmailAddress

PhoneNumber

PhoneNumberType

274

Stephen

Jiang

stephen0@adventure-works.com

112-555-6207

Work

274

Stephen

Jiang

stephen.jiang@outlook.com

112-555-6207

Work

 

Now let’s assume that someone adds Stephen’s home and cell phone numbers in the system. Query will now return the following results:

BusinessEntityID

FirstName

LastName

EmailAddress

PhoneNumber

PhoneNumberType

274

Stephen

Jiang

stephen0@adventure-works.com

112-555-6207

Work

274

Stephen

Jiang

stephen.jiang@outlook.com

112-555-6207

Work

274

Stephen

Jiang

stephen0@adventure-works.com

238-555-0197

Cell

274

Stephen

Jiang

stephen.jiang@outlook.com

238-555-0197

Cell

274

Stephen

Jiang

stephen0@adventure-works.com

817-555-1797

Home

274

Stephen

Jiang

stephen.jiang@outlook.com

817-555-1797

Home

 

This is a messy because now instead of a single row you are getting 6 rows because two email addressed are cross combined with three phone numbers.

Similar results will be returned if you query Sales.vSalesPerson view:

SELECT BusinessEntityID AS ID, FirstName, LastName, EmailAddress, PhoneNumber, PhoneNumberType

FROM Sales.vSalesPerson

WHERE BusinessEntityID = 274

 

Now you have two choices – handle these duplicates in client-side or try to return related data in single row. In this post I will show you how to use the second option with FOR JSON clause in SQL Server 2016.

Use Case 1: Formatting set of related rows as JSON array

Instead of joining related tables we can just attach related information as an array of records formatted as JSON array. We can select data from Person table, and add related email addresses as subquery formatted as JSON text:

SELECT     Person.Person.BusinessEntityID, Person.Person.FirstName, Person.Person.LastName,

                                                (SELECT Person.EmailAddress.EmailAddress

                                                                FROM Person.EmailAddress

                                                                WHERE Person.Person.BusinessEntityID = Person.EmailAddress.BusinessEntityID

                                                                FOR JSON PATH) AS Emails

FROM         Person.Person

WHERE Person.Person.BusinessEntityID = 274

Since FOR JSON clause returns a single text field you can put it in any column:

ID

FirstName

LastName

Emails

274

Stephen

Jiang

[{“EmailAddress”:”stephen0@adventure-works.com”},

{“EmailAddress”:”stephen.jiang@outlook.com”}]

In this case we are returning hybrid result set – relational data from Person table are returned as columns, while related information from Person.emailAddress table are returned as an array of JSON objects.

You might notice that FOR JSON returns an array of key:value pairs even if we might like to have something simpler e.g. plain array of values instead of array of objects. In this case we can write simple T-SQL user defined function that removes keys from the array and return plain array:

SELECT     Person.Person.BusinessEntityID AS ID, Person.Person.FirstName, Person.Person.LastName,

                dbo.ufnToRawJsonArray((SELECT Person.EmailAddress.EmailAddress

                                                                FROM Person.EmailAddress

                                                                WHERE Person.Person.BusinessEntityID = Person.EmailAddress.BusinessEntityID

                                                                FOR JSON PATH), ‘EmailAddress’) AS Emails

FROM         Person.Person

WHERE Person.Person.BusinessEntityID = 274

Result of this query might look like:

ID

FirstName

LastName

Emails

274

Stephen

Jiang

[“stephen0@adventure-works.com”,”stephen.jiang@outlook.com”]

 

This is more readable format that array of key value pairs.

Use Case 2: Formatting related information from multiple child tables as JSON arrays

In the previous use case, I have returned content of one related child table as JSON array. We can also return related information both from emails and phone tables. Second set of related information would be returned as column level JSON expression:

SELECT     Person.Person.BusinessEntityID AS ID, Person.Person.FirstName, Person.Person.LastName,

                                                (SELECT Person.EmailAddress.EmailAddress

                                                                FROM Person.EmailAddress

                                                                WHERE Person.Person.BusinessEntityID = Person.EmailAddress.BusinessEntityID

                                                                FOR JSON PATH) AS Emails,

                                                (SELECT Person.PersonPhone.PhoneNumber, Person.PhoneNumberType.Name AS PhoneNumberType

                                                                FROM  Person.PersonPhone INNER JOIN

                      Person.PhoneNumberType ON Person.PersonPhone.PhoneNumberTypeID = Person.PhoneNumberType.PhoneNumberTypeID

                                                                WHERE Person.Person.BusinessEntityID = Person.PersonPhone.BusinessEntityID

                                                                FOR JSON PATH) AS Phones

FROM         Person.Person

WHERE Person.Person.BusinessEntityID = 274

In this case, we have returned a single row for a particular person and all related information are returned as an array of JSON object in the single line.

ID

FirstName

LastName

Emails

Phones

274

Stephen

Jiang

[

{“EmailAddress”:”stephen0@adventure-works.com”},

{“EmailAddress”:”stephen.jiang@outlook.com”}

]

[

{“PhoneNumber”:”112-555-6207″,”PhoneNumberType”:”Work”},

{“PhoneNumber”:”238-555-0197″,”PhoneNumberType”:”Cell”},

{“PhoneNumber”:”817-555-1797″,”PhoneNumberType”:”Home”}

]

 

You can easily unpack these arrays on the client side using some JSON deserializer such as JSON.NET. You might notice that we still have one array. The fact that we are adding new child items do not affects number of returned results.

Use Case 3: Formatting parent and child rows as JSON

Instead of hybrid result set, we can format all results as JSON (i.e. both parent Person columns and child rows). If you want to return all results as JSON there is even simpler syntax. FOR JSON AUTO will automatically indent related email addresses as it is shown in the following script:

SELECT     Person.Person.BusinessEntityID, Person.Person.FirstName, Person.Person.LastName, Person.EmailAddress.EmailAddress

FROM         Person.Person INNER JOIN

                      Person.EmailAddress ON Person.Person.BusinessEntityID = Person.EmailAddress.BusinessEntityID

WHERE Person.Person.BusinessEntityID = 274

FOR JSON AUTO

Results might look like:

[

 { “ID”:274,”FirstName”:”Stephen”,”LastName”:”Jiang”,

   “Person.EmailAddress”:[

       {“EmailAddress”:”stephen0@adventure-works.com”},

       {“EmailAddress”:”stephen.jiang@outlook.com”}

    ]

 }

]

 This format is pure JSON and it can be used as a response of JSON web service.

Use Case 4: Returning complex JSON hierarchies

Finally we can format entire result-set as JSON by using FOR JSON cause on the main query:

SELECT     Person.Person.BusinessEntityID AS ID, Person.Person.FirstName, Person.Person.LastName,

                                                (SELECT Person.EmailAddress.EmailAddress

                                                                FROM Person.EmailAddress

                                                                WHERE Person.Person.BusinessEntityID = Person.EmailAddress.BusinessEntityID

                                                                FOR JSON PATH) AS Emails,

                                                (SELECT Person.PersonPhone.PhoneNumber, Person.PhoneNumberType.Name AS PhoneNumberType

                                                                FROM  Person.PersonPhone INNER JOIN

                      Person.PhoneNumberType ON Person.PersonPhone.PhoneNumberTypeID = Person.PhoneNumberType.PhoneNumberTypeID

                                                                WHERE Person.Person.BusinessEntityID = Person.PersonPhone.BusinessEntityID

                                                                FOR JSON PATH) AS Phones

FROM         Person.Person

WHERE Person.Person.BusinessEntityID = 274

FOR JSON PATH

Results will look like:

[

 {

   “ID”:274,”FirstName”:”Stephen”,”LastName”:”Jiang”,

   “Emails”:[

              {“EmailAddress”:”stephen0@adventure-works.com”},

              {“EmailAddress”:”stephen.jiang@outlook.com”}],

   “Phones”:[

             {“PhoneNumber”:”112-555-6207″,”PhoneNumberType”:”Work”},

             {“PhoneNumber”:”238-555-0197″,”PhoneNumberType”:”Cell”},

             {“PhoneNumber”:”817-555-1797″,”PhoneNumberType”:”Home”}

    ]

 }

]

 

FOR JSON clause enables you to format complex results and return them in more convenient format to the client. Note that you will need SQL Server 2016 CTP2 or higher for these code examples.