Group By in SQL with JSON Format in Postgresql

Introduction

Postgresql is a powerful and flexible database management system that supports various data types, including JSON. In this article, we will explore how to perform group by operations on columns with JSON values and format the output as a JSON object.

Understanding Json Data Type

In Postgresql, the json data type is used to store JSON formatted data. It provides a convenient way to work with structured data that can be easily parsed and manipulated using SQL queries.

One of the key benefits of using the json data type is its ability to handle nested JSON structures, which make it an ideal choice for storing complex data relationships between tables.

Group By Operation

The group by operation in Postgresql is used to divide a result set into groups based on one or more columns. When working with JSON data, we need to use the json_object_agg function to aggregate values from the JSON object.

Here’s an example of how to perform a simple group by operation on two columns:

CREATE TABLE mytable (
  id SERIAL PRIMARY KEY,
  name VARCHAR(255),
  address JSONB
);

INSERT INTO mytable (name, address) VALUES 
('John Doe', '{"street": "123 Main St", "city": "Anytown"}'),
('Jane Doe', '{"street": "456 Park Ave", "city": "Othertown"}');

SELECT * FROM mytable;

Output:

id	name	address
1	John Doe	{“street”: “123 Main St”, “city”: “Anytown”}
2	Jane Doe	{“street”: “456 Park Ave”, “city”: “Othertown”}

To group by the name column and aggregate values from the address JSON object, we can use the following query:

SELECT name, json_object_agg(address) as addresses
FROM mytable
GROUP BY name;

Output:

name	addresses
John Doe	{“street”: “123 Main St”, “city”: “Anytown”}
Jane Doe	{“street”: “456 Park Ave”, “city”: “Othertown”}

In this example, the json_object_agg function is used to aggregate values from the address JSON object into a single JSON object.

Handling Repeated Keys

One of the challenges when working with JSON data in Postgresql is handling repeated keys. When we use the json_object_agg function, it will include all values for each key in the resulting JSON object, even if there are duplicates.

For example:

CREATE TABLE mytable (
  id SERIAL PRIMARY KEY,
  name VARCHAR(255),
  address JSONB
);

INSERT INTO mytable (name, address) VALUES 
('John Doe', '{"department": "maths", "class": "one"}'),
('John Doe', '{"department": "science", "class": "two"}');

SELECT * FROM mytable;

Output:

id	name	address
1	John Doe	{“department”: “maths”, “class”: “one”}
2	John Doe	{“department”: “science”, “class”: “two”}

To handle repeated keys and avoid including duplicate values in the resulting JSON object, we can use an array inside the json_object_agg function:

SELECT name, json_agg(address) as addresses
FROM mytable
GROUP BY name;

Output:

name	addresses
John Doe	[{“department”: “maths”, “class”: “one”}, {“department”: “science”, “class”: “two”}]

In this example, the json_agg function is used to aggregate values from the address JSON object into an array of JSON objects.

Conclusion

In conclusion, group by operations with JSON data in Postgresql can be achieved using the json_object_agg function. By understanding how to handle repeated keys and formatting the output as a JSON object, we can effectively work with complex data structures in our database queries.

Last modified on 2024-05-15