JSONB modification functions in PostgreSQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this JSONB update?

Given the table data with a JSONB column info containing {"name": "Alice", "age": 30}, what is the result of this query?

SELECT info || '{"age": 31, "city": "NY"}'::jsonb FROM data;

A {"name": "Alice", "age": 31}

B {"name": "Alice", "age": 30, "city": "NY"}

C {"name": "Alice", "age": 31, "city": "NY"}

D {"name": "Alice", "age": 30}

💡 Hint

The || operator merges JSONB objects, replacing existing keys.

The || operator merges two JSONB objects. If keys overlap, the right side's value replaces the left's. So 'age' changes from 30 to 31, and 'city' is added.

❓ query_result

intermediate

What does jsonb_set do in this query?

Consider a JSONB column info with value {"user": {"name": "Bob", "age": 25}}. What is the output of:

SELECT jsonb_set(info, '{user,age}', '26', false) FROM data;

A {"user": {"name": "Bob"}}

B {"user": {"name": "Bob", "age": "26"}}

C {"user": {"name": "Bob", "age": 25}}

D {"user": {"name": "Bob", "age": 26}}

💡 Hint

jsonb_set replaces the value at the specified path with the new value.

jsonb_set replaces the value at the path '{user,age}' with the new JSONB value "26" (string). The fourth argument false means it won't create missing keys.

📝 Syntax

advanced

Which option correctly removes a key from JSONB?

You want to remove the key city from a JSONB column info. Which query is valid and works as expected?

A SELECT info - 'city' FROM data;

B SELECT jsonb_remove(info, 'city') FROM data;

C SELECT info \ 'city' FROM data;

D SELECT jsonb_delete(info, 'city') FROM data;

💡 Hint

The - operator removes a key from JSONB.

The - operator removes a key from a JSONB object. jsonb_remove and jsonb_delete do not exist. The \ operator is invalid syntax.

❓ optimization

advanced

Best way to update nested JSONB key efficiently?

You have a large JSONB column data with nested keys. You want to update settings.notifications.email to false for many rows. Which approach is most efficient?

A Delete the entire settings key and insert a new JSONB object with updated values.

B Extract JSONB to text, replace with string functions, then cast back to JSONB.

C Use jsonb_insert to add the key email with value false.

D Use jsonb_set with path '{settings,notifications,email}' and update only changed rows.

💡 Hint

jsonb_set updates nested keys without rewriting entire JSONB.

jsonb_set efficiently updates nested keys by specifying the path. Extracting to text and replacing is slow and error-prone. Deleting entire keys loses data. jsonb_insert only inserts new keys, not update existing.

🧠 Conceptual

expert

What happens if jsonb_set tries to update a missing path with create_missing = false?

Given a JSONB column doc with value {"a": {"b": 1}}, what is the result of:

SELECT jsonb_set(doc, '{a,c}', '2', false) FROM data;

A {"a": {"b": 1}}

B {"a": {"b": 1, "c": 2}}

C NULL

D Error: path does not exist

💡 Hint

create_missing = false means do not create keys if path is missing.

When create_missing is false and the path does not exist, jsonb_set returns the original JSONB unchanged without error.

Practice

1. What does the PostgreSQL function jsonb_set do when used on a JSONB column?

easy

A. It creates a new JSONB column in the table.

B. It deletes the entire JSONB column from the table.

C. It converts JSONB data into plain text format.

D. It updates or adds a value at a specified path inside the JSONB data.

Solution

1. Step 1: Understand the purpose of jsonb_set

   The function jsonb_set is designed to update or add a value at a specific location inside JSONB data using a path.

2. Step 2: Compare with other options

   The other options describe actions unrelated to jsonb_set's purpose, such as creating or deleting columns or converting data types.

3. Final Answer:

   It updates or adds a value at a specified path inside the JSONB data. -> Option D

4. Quick Check:

   jsonb_set updates JSONB data [OK]

Hint: jsonb_set changes or adds JSONB values at paths [OK]

Common Mistakes:

• Thinking jsonb_set deletes data
• Confusing jsonb_set with data type conversion
• Assuming jsonb_set creates new columns

2. Which of the following is the correct syntax to remove a key named "age" from a JSONB column data in PostgreSQL?

easy

A. UPDATE table SET data = data - 'age';

B. UPDATE table SET data = jsonb_set(data, '{age}', NULL);

C. UPDATE table SET data = data + 'age';

D. UPDATE table SET data = jsonb_remove(data, 'age');

Solution

1. Step 1: Identify the correct operator for key removal

   In PostgreSQL, the minus operator (-) removes a key from JSONB data, so data - 'age' removes the key "age".

2. Step 2: Check other options for correctness

   UPDATE table SET data = jsonb_set(data, '{age}', NULL); tries to set the key to NULL but does not remove it. UPDATE table SET data = data + 'age'; adds a key, which is incorrect. UPDATE table SET data = jsonb_remove(data, 'age'); uses a non-existent function jsonb_remove.

3. Final Answer:

   UPDATE table SET data = data - 'age'; -> Option A

4. Quick Check:

   Minus operator removes JSONB key [OK]

Hint: Use minus (-) operator to remove JSONB keys [OK]

Common Mistakes:

• Using jsonb_set with NULL to remove keys
• Trying to add keys to remove them
• Using non-existent functions like jsonb_remove

3. Given the JSONB column data with value '{"name": "Alice", "details": {"age": 30}}', what will be the result of this query?

SELECT jsonb_set(data, '{details,age}', '31', false) FROM table;

medium

A. {"name": "Alice", "details": {"age": 31}}

B. {"name": "Alice", "details": {"age": "31"}}

C. {"name": "Alice", "details": {"age": 30}}

D. {"name": "Alice", "details": {"age": null}}

Solution

1. Step 1: Understand jsonb_set parameters

   The function jsonb_set updates the value at path '{details,age}' to '31' (a JSON string) because the new value is given as text, not as a number.

2. Step 2: Check the effect of the 'create_missing' flag

   The last parameter is false, so it updates existing key without creating new ones. The age key is updated to the string '31', not the number 31.

3. Final Answer:

   {"name": "Alice", "details": {"age": "31"}} -> Option B

4. Quick Check:

   jsonb_set updates value as text unless cast [OK]

Hint: jsonb_set inserts text values unless cast to JSONB [OK]

Common Mistakes:

• Assuming '31' is numeric without casting
• Expecting no change when create_missing is false
• Confusing JSON string and number types

4. You want to update a nested key in a JSONB column but your query

UPDATE table SET data = jsonb_set(data, '{info,score}', 100);

throws an error. What is the likely cause?

medium

A. The new value 100 is not cast to JSONB, causing a type error.

B. The path '{info,score}' is invalid syntax and must be a string.

C. jsonb_set cannot update nested keys, only top-level keys.

D. The data column is not JSONB type.

Solution

1. Step 1: Check the new value type in jsonb_set

   The third argument must be JSONB type. Passing 100 as an integer without casting causes a type error.

2. Step 2: Verify other options

   The path '{info,score}' is invalid syntax and must be a string. is incorrect because the path syntax is valid. jsonb_set cannot update nested keys, only top-level keys. is false; jsonb_set supports nested keys. The data column is not JSONB type. might cause error but question implies the error is from the query syntax.

3. Final Answer:

   The new value 100 is not cast to JSONB, causing a type error. -> Option A

4. Quick Check:

   jsonb_set value must be JSONB type [OK]

Hint: Cast new value to JSONB to avoid type errors [OK]

Common Mistakes:

• Passing plain integers without casting
• Misunderstanding path syntax
• Assuming jsonb_set can't update nested keys

5. You have a JSONB column data storing user info. You want to increment the "visits" count inside "stats" by 1, but visits might be missing. Which query correctly updates data to add or increment visits?

hard

A. UPDATE table SET data = data || '{"stats":{"visits":1}}';

B. UPDATE table SET data = jsonb_set(data, '{stats,visits}', '1');

C. UPDATE table SET data = jsonb_set(data, '{stats,visits}', to_jsonb((COALESCE((data->'stats'->>'visits')::int, 0) + 1)));

D. UPDATE table SET data = jsonb_set(data, '{visits}', to_jsonb(1));

Solution

1. Step 1: Understand the need to increment or add visits

   The query must get the current visits count, convert it to integer, handle missing key with COALESCE to 0, add 1, then convert back to JSONB.

2. Step 2: Analyze each option

   UPDATE table SET data = jsonb_set(data, '{stats,visits}', to_jsonb((COALESCE((data->'stats'->>'visits')::int, 0) + 1))); correctly uses COALESCE and to_jsonb to update visits inside stats. UPDATE table SET data = jsonb_set(data, '{stats,visits}', '1'); sets visits to string '1' always. UPDATE table SET data = data || '{"stats":{"visits":1}}'; overwrites stats with visits=1, losing other keys. UPDATE table SET data = jsonb_set(data, '{visits}', to_jsonb(1)); updates visits at top level, not inside stats.

3. Final Answer:

   UPDATE table SET data = jsonb_set(data, '{stats,visits}', to_jsonb((COALESCE((data->'stats'->>'visits')::int, 0) + 1))); -> Option C

4. Quick Check:

   Use COALESCE and to_jsonb for safe increment [OK]

Hint: Use COALESCE and cast to increment JSONB numbers safely [OK]

Common Mistakes:

• Overwriting nested objects accidentally
• Not casting string to int before increment
• Updating wrong JSONB path