FLATTEN for nested data in Snowflake - Commands & Configuration

Introduction

Sometimes data is stored inside other data, like a list inside a table. FLATTEN helps to open up this nested data so you can see and use each item separately.

When you have a column with a list of items and want to see each item as its own row.

When you want to analyze or filter elements inside nested JSON data stored in a table.

When you need to join nested data with other tables by breaking it into simple rows.

When you want to count or aggregate values inside nested arrays.

When you want to transform complex nested data into a flat table for easier reporting.

Commands

Create a table named orders with a nested JSON column called items, then insert sample data with arrays of products and quantities.

Terminal

CREATE OR REPLACE TABLE orders (id INT, items VARIANT);

INSERT INTO orders VALUES
(1, PARSE_JSON('[{"product":"apple","qty":2},{"product":"banana","qty":3}]')),
(2, PARSE_JSON('[{"product":"orange","qty":1}]'));

Expected Output

Query executed successfully.

2 rows inserted.

Use FLATTEN to open the nested items array into separate rows, extracting product name and quantity for each item.

Terminal

SELECT id, f.value:product AS product, f.value:qty AS quantity
FROM orders,
LATERAL FLATTEN(input => items) f;

Expected Output

ID  | PRODUCT | QUANTITY

----|---------|---------

1   | apple   | 2

1   | banana  | 3

2   | orange  | 1

→

LATERAL - Allows FLATTEN to work on each row of the orders table.

→

input => items - Specifies the nested array column to flatten.

Count how many items each order has by flattening the items array and grouping by order id.

Terminal

SELECT id, COUNT(*) AS item_count
FROM orders,
LATERAL FLATTEN(input => items) f
GROUP BY id;

Expected Output

ID  | ITEM_COUNT

----|-----------

1   | 2

2   | 1

Key Concept

If you remember nothing else from this pattern, remember: FLATTEN turns nested arrays into simple rows so you can work with each element easily.

Common Mistakes

Trying to select nested elements without using LATERAL FLATTEN.

The nested array stays as one value and does not split into rows, so you can't access individual items.

Always use LATERAL FLATTEN on the nested column to expand it before selecting nested fields.

Not aliasing the FLATTEN output and trying to access fields directly.

Without aliasing, Snowflake cannot identify the flattened data to extract fields from.

Use an alias like 'f' for FLATTEN and then access nested fields with f.value:<field>.

Summary

Create a table with nested JSON data using VARIANT type.

Use LATERAL FLATTEN to expand nested arrays into rows.

Select nested fields from the flattened output for easy querying.

Practice

1. What does the FLATTEN function do in Snowflake when working with nested data?

easy

A. It encrypts nested data for security.

B. It compresses data to save storage space.

C. It converts nested arrays or objects into simple rows.

D. It creates a backup of nested data.

Solution

1. Step 1: Understand the purpose of FLATTEN

   FLATTEN is designed to take nested arrays or objects and turn them into individual rows so they are easier to query.

2. Step 2: Compare options to FLATTEN's function

   Options A, B, and D describe encryption, compression, and backup, which are unrelated to FLATTEN's role.

3. Final Answer:

   It converts nested arrays or objects into simple rows. -> Option C

4. Quick Check:

   FLATTEN = convert nested data to rows [OK]

Hint: FLATTEN breaks nested data into rows for easy reading [OK]

Common Mistakes:

• Thinking FLATTEN compresses or encrypts data
• Confusing FLATTEN with backup or storage functions
• Assuming FLATTEN changes data format instead of structure

2. Which of the following is the correct syntax to use FLATTEN on a JSON column named data in Snowflake?

easy

A. SELECT FLATTEN(data) FROM table;

B. SELECT * FROM FLATTEN(input => data);

C. SELECT FLATTEN(input = data) FROM table;

D. SELECT * FROM table, LATERAL FLATTEN(input => data);

Solution

1. Step 1: Recall FLATTEN usage in FROM clause

   FLATTEN is used as a table function in the FROM clause with LATERAL to expand nested data.

2. Step 2: Analyze each option's syntax

   SELECT * FROM table, LATERAL FLATTEN(input => data); correctly uses FROM table, LATERAL FLATTEN(input => data). Options A and C misuse FLATTEN as a scalar function. SELECT * FROM FLATTEN(input => data); misses the table reference.

3. Final Answer:

   SELECT * FROM table, LATERAL FLATTEN(input => data); -> Option D

4. Quick Check:

   FLATTEN used with LATERAL in FROM clause [OK]

Hint: Use FLATTEN with LATERAL in FROM clause for nested data [OK]

Common Mistakes:

• Using FLATTEN as a scalar function in SELECT
• Omitting LATERAL keyword
• Not specifying input parameter correctly

3. Given the JSON column data with value '{"items": ["apple", "banana", "cherry"]}', what will the query below return?

SELECT f.value FROM table, LATERAL FLATTEN(input => data:items) f;

medium

A. Rows with values: apple, banana, cherry

B. A single row with the entire array as a string

C. An error because data:items is invalid syntax

D. Rows with keys and values of the JSON object

Solution

1. Step 1: Understand FLATTEN on JSON array

   FLATTEN(input => data:items) expands the array under 'items' into multiple rows, each with one element.

2. Step 2: Analyze the query output

   The query selects f.value, which will be each element: 'apple', 'banana', 'cherry' as separate rows.

3. Final Answer:

   Rows with values: apple, banana, cherry -> Option A

4. Quick Check:

   FLATTEN on array returns each element as a row [OK]

Hint: FLATTEN on JSON array returns each element as a separate row [OK]

Common Mistakes:

• Expecting a single row with the whole array
• Confusing keys with values in FLATTEN output
• Misreading JSON path syntax

4. You wrote this query to flatten nested JSON data:

SELECT f.value FROM table, FLATTEN(input => data:items) f;

But it returns an error. What is the likely cause?

medium

A. Missing LATERAL keyword before FLATTEN

B. Incorrect JSON path syntax in input

C. FLATTEN cannot be used on JSON arrays

D. SELECT statement missing WHERE clause

Solution

1. Step 1: Identify FLATTEN usage requirements

   FLATTEN is a table function that requires LATERAL when used with another table to expand nested data.

2. Step 2: Check query syntax

   The query misses the LATERAL keyword before FLATTEN, causing a syntax error.

3. Final Answer:

   Missing LATERAL keyword before FLATTEN -> Option A

4. Quick Check:

   FLATTEN needs LATERAL in FROM clause [OK]

Hint: Always add LATERAL before FLATTEN in FROM clause [OK]

Common Mistakes:

• Forgetting LATERAL keyword
• Assuming FLATTEN works without LATERAL
• Blaming JSON path syntax instead of syntax structure

5. You have a table with a column orders storing nested JSON arrays of items per order. You want to list each item with its order ID. Which query correctly uses FLATTEN to achieve this?

hard

A. SELECT order_id, FLATTEN(orders) FROM orders_table;

B. SELECT order_id, f.value FROM orders_table, LATERAL FLATTEN(input => orders) f;

C. SELECT order_id, f.value FROM orders_table JOIN FLATTEN(input => orders) f ON TRUE;

D. SELECT order_id, f.value FROM orders_table, FLATTEN(orders) f;

Solution

1. Step 1: Understand how to join FLATTEN with table

   FLATTEN must be used with LATERAL in the FROM clause to expand nested arrays per row.

2. Step 2: Evaluate each option's correctness

   SELECT order_id, f.value FROM orders_table, LATERAL FLATTEN(input => orders) f; correctly uses FROM orders_table, LATERAL FLATTEN(input => orders) f, selecting order_id and each item value. Options A and D misuse FLATTEN syntax. SELECT order_id, f.value FROM orders_table JOIN FLATTEN(input => orders) f ON TRUE; uses JOIN incorrectly without LATERAL.

3. Final Answer:

   SELECT order_id, f.value FROM orders_table, LATERAL FLATTEN(input => orders) f; -> Option B

4. Quick Check:

   Use LATERAL FLATTEN with table to list nested items [OK]

Hint: Use LATERAL FLATTEN(input => column) to expand nested arrays per row [OK]

Common Mistakes:

• Using FLATTEN without LATERAL
• Trying to JOIN FLATTEN without LATERAL
• Incorrect FLATTEN syntax in SELECT clause