How do you merge JSON data in PostgreSQL (psql)?

In PostgreSQL, you can merge JSON data using several built-in functions, primarily focusing on combining JSON objects or arrays. The method you choose will depend on the specific structure of your data and how you want to merge it.

Here are some common techniques:

1. Merging JSON Objects with jsonb_set or || Operator

- Using jsonb_set: This function allows you to update or add values to a specific key in a JSONB object. If a key already exists, the value is updated; otherwise, a new key-value pair is added. It's useful for merging specific key-value pairs.

- Example:

SELECT jsonb_set('{"a": 1, "b": 2}'::jsonb, '{c}', '3'::jsonb); -- Adds "c": 3 resulting in {"a": 1, "b": 2, "c": 3}
SELECT jsonb_set('{"a": 1, "b": 2}'::jsonb, '{b}', '3'::jsonb); -- Updates "b" resulting in {"a": 1, "b": 3}


- Using the || operator: This operator concatenates two JSON objects. If there are any duplicate keys, the key in the second object will override the key in the first object. This is convenient for merging objects where you want to overwrite keys in the original JSON.

- Example:

SELECT '{"a": 1, "b": 2}'::jsonb || '{"c": 3, "b": 4}'::jsonb; -- Result: {"a": 1, "b": 4, "c": 3}


2. Merging JSON Arrays with || Operator

- The || operator can also be used to concatenate JSON arrays, producing a new array containing all elements of the input arrays.

- Example:

SELECT '[1, 2]'::jsonb || '[3, 4]'::jsonb; -- Result: [1, 2, 3, 4]


3. Merging JSON Arrays or objects using custom SQL functions

-If you require more complex logic for merging arrays or nested structures, consider writing your own SQL functions, and using JSON functions like jsonb_array_elements, jsonb_object_keys.

4. Important notes:

- Make sure to use jsonb datatype when handling JSON. json is more efficient, less verbose and supports indexing.

- When merging JSON objects, keys from the second object override keys of the same name in the first object.

- When concatenating JSON arrays, you're appending the elements of the second array to the end of the first array. This will create duplicate entries in the merged JSON array

In a PostgreSQL database, merging operations can be included as part of a select query, an update statement, or inside a function. Here is an example of merging in an update:

UPDATE my_table
SET my_json_column = my_json_column || '{"d": 5}'::jsonb
WHERE id = 1;


By using these techniques, you can effectively merge JSON data in PostgreSQL, handling both basic concatenations and more complex key-value manipulation.

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