PostgreSQL 9.4 introduced very powerful data type called JSON data type. It also introduced a variety of new operators and functions related to JSON data type.

This is one of the interesting topics for us and I have recently published a series of articles on PostgreSQL JSON Data type.

You can visit related articles here:

PostgreSQL 9.4: The JSON data type is Awesome (Part 1/3).
PostgreSQL 9.4: Introduced JSON Functions and Operators (Part 2/3).
PostgreSQL 9.4: Indexing on jsonb Data Type (Part 3/3).

In this post, I am also going share one of the important query to convert PostgreSQL tabular data into JSON formatted data.

Generally, We are storing JSON formatted data into PostgreSQL and access base on the different filters.

Here, I am sharing one type of utility script to convert PostgreSQL table data into JSON formatted data. Sometimes it requires to populate JSON formatted data for a web service purpose.

I am doing this using, json_agg and row_to_json function.
json_agg(expression) : aggregates values as a JSON array.
row_to_json(record [, pretty_bool]) : Returns the row as JSON.

Below is a full demonstration of this:

First, create table with sample data:

CREATE TABLE tbl_EmployeeDetails
(
	EmpID INTEGER
	,DepartmenetName VARCHAR(50)
	,EmpFirstName VARCHAR(50)
	,EmpLastName VARCHAR(50)
);

INSERT INTO tbl_EmployeeDetails VALUES
(1,'Sales','Anvesh','Patel')
,(2,'Sales','Neevan','Patel')
,(3,'Order','Roy','Loother')
,(4,'Marketing','Martin','Farook')
,(5,'Marketing','Jenny','Pandya')
,(6,'Marketing','Mahi','Patel');

Script to convert data into JSON format without column name for key:

SELECT 
	DepartmenetName
	,json_agg(row_to_json((EmpFirstName, EmpLastName))) AS JsonData
FROM tbl_EmployeeDetails 
GROUP BY DepartmenetName;

The Result:

Script to convert data into JSON format with column name for key:

SELECT 
	DepartmenetName
	,json_agg(row_to_json
	(
	  (SELECT ColumnName FROM (SELECT EmpFirstName, EmpLastName) AS ColumnName (EmpFirstName, EmpLastName))
	)) AS JsonData
FROM tbl_EmployeeDetails
GROUP BY DepartmenetName;

The Result:

Recently, PostgreSQL introduced JSON data type which is very powerful to manipulate JSON formatted data.

I published a couple of articles on JSON Data Type.
You can start from here.

In this post, I am showing aggregation of table data into a JSON formatted array using json_agg().

Our requirement is to aggregate non json formatted data into JSON array and even also required to aggregate JSON formatted data.

I prepared a small demonstration using json_agg().

Create a table with sample JSON formatted data:

CREATE TABLE tbl_TestJSON
(
	ID INTEGER PRIMARY KEY
	,DepartName VARCHAR(250)
	,EmployeeDetails JSONB
);

INSERT INTO tbl_TestJSON
VALUES 
(
	1
	,'Sales'
	,'
	{"employees":[
	    {"firstName":"Anvesh", "lastName":"Patel"}	  
	]}'
)
,(
	2
	,'Production'
	,'
	{"employees":[	    
	    {"firstName":"Nancy", "lastName":"Shah"}
	]}'
);

Aggregates JSON formatted data:

SELECT json_agg(employeedetails) AS JsonArray FROM tbl_TestJSON;

JsonArray
------------------------
[{"employees": [{"lastName": "Patel", "firstName": "Anvesh"}]}, {"employees": [{"lastName": "Shah", "firstName": "Nancy"}]}]

Aggregates non JSON formatted data:

SELECT json_agg(DepartName) AS JsonArray FROM tbl_TestJSON;

JsonArray
------------------------
["Sales", "Production"]