DB2 JSON Query Example: Unnesting Nested Data

When working with JSON data, it's common to encounter nested arrays that require flattening into individual rows. In this article, we'll explore how to track the hierarchy when unnesting nested data using DB2's JSON_TABLE function.

Imagine having multiple arrays of comments and likes, each representing different 'things' related to a posting. For instance, in Facebook's Graph API, you might have an array of likes and another array of comments for a single posting. To process this data effectively, it's essential to understand how to unnest these nested arrays.

Let's start with a simple example. Suppose we have the following JSON data:

```json

{

'postings': '

{

'id': 1,

'comments': ''Comment 1', 'Comment 2'',

'likes': ''Like 1', 'Like 2''

{

'id': 2,

'comments': ''Comment 3', 'Comment 4'',

'likes': ''Like 3', 'Like 4''

}

```

To unnest this data, we can use the JSON_TABLE function in DB2. The basic syntax is as follows:

```sql

SELECT *

FROM (

SELECT JSON_TABLE(

postings,

'$.*' COLUMNS (id INTEGER PATH '$.id',

comments VARCHAR(255) PATH '$.comments'*'',

likes VARCHAR(255) PATH '$.likes'*''

)

) AS posting_rows

) AS result;

```

In this example, we're using the JSON_TABLE function to extract the 'postings' array and then unnesting it into individual rows using the `COLUMNS` clause.

By default, DB2 will repeat the values for the outer columns (in this case, 'id') for each item in the inner array. This is because the nested arrays produce rows that are repeated for the outer columns. To avoid this repetition, we can use the `NESTED PATH` clause to specify a new `COLUMNS` clause for the JSON_TABLE function.

For instance, let's say we want to unnest only the 'likes' array and repeat the 'id' column for each like:

```sql

SELECT *

FROM (

SELECT JSON_TABLE(

postings,

'$.*' COLUMNS (id INTEGER PATH '$.id',

likes VARCHAR(255) PATH '$.likes'*'' COLUMNS (like_id INTEGER DEFAULT id)

)

) AS posting_rows

) AS result;

```

In this modified example, we're using the `COLUMNS` clause to specify a new column 'like_id' that defaults to the value of the 'id' column. This way, each like will have its own unique 'id' value.

When dealing with nested arrays, it's essential to keep track of the hierarchy by using the correct syntax and specifying the nested path accurately. By mastering this technique, you'll be able to efficiently process complex JSON data in your DB2 database.

In conclusion, DB2's JSON_TABLE function provides an effective way to unnest nested arrays and extract valuable insights from JSON data. With practice and a solid understanding of the syntax, you can master the art of working with JSON in DB2 and unlock new possibilities for your applications.

Db2 json query example