JSONB containment (@>) operator in PostgreSQL - Mini Project: Build & Apply

Using JSONB Containment (@>) Operator in PostgreSQL

📖 Scenario: You are managing a PostgreSQL database for a small online bookstore. Each book's details are stored in a table with a JSONB column that holds various attributes like genre, author, and publication year.

🎯 Goal: Learn how to use the JSONB containment operator @> to find books that contain specific JSON attributes.

📋 What You'll Learn

Create a table called books with an id column and a details column of type JSONB.

Insert sample book data with JSONB details including genre, author, and year.

Write a query using the @> operator to find books with a specific genre.

Write a query using the @> operator to find books by a specific author.

💡 Why This Matters

🌍 Real World

Many modern applications store flexible data in JSONB columns in PostgreSQL. Using the @> operator helps quickly find records matching specific JSON criteria.

💼 Career

Understanding JSONB queries is valuable for backend developers, data engineers, and database administrators working with PostgreSQL and semi-structured data.

1

Create the books table with JSONB column

Create a table called books with two columns: id as an integer primary key and details as a JSONB type.

PostgreSQL

-- Create the books table with id and details columns
-- Your code here

Hint

Use CREATE TABLE with id SERIAL PRIMARY KEY and details JSONB.

2

Insert sample book data with JSONB details

Insert three rows into the books table with details JSONB values exactly as follows: {"genre": "fiction", "author": "Alice", "year": 2020}, {"genre": "non-fiction", "author": "Bob", "year": 2018}, and {"genre": "fiction", "author": "Charlie", "year": 2021}.

PostgreSQL

CREATE TABLE books (
    id SERIAL PRIMARY KEY,
    details JSONB
);
-- Insert three rows with specified JSONB details
-- Your code here

Hint

Use INSERT INTO books (details) VALUES with JSON strings in single quotes.

3

Query books with genre 'fiction' using @> operator

Write a SELECT query to get all columns from books where the details JSONB column contains the key-value pair {"genre": "fiction"} using the @> operator.

PostgreSQL

CREATE TABLE books (
    id SERIAL PRIMARY KEY,
    details JSONB
);

INSERT INTO books (details) VALUES
    ('{"genre": "fiction", "author": "Alice", "year": 2020}'),
    ('{"genre": "non-fiction", "author": "Bob", "year": 2018}'),
    ('{"genre": "fiction", "author": "Charlie", "year": 2021}');

-- Select books where details contain {"genre": "fiction"}
-- Your code here

Hint

Use WHERE details @> '{"genre": "fiction"}' to filter JSONB rows.

4

Query books by author 'Bob' using @> operator

Write a SELECT query to get all columns from books where the details JSONB column contains the key-value pair {"author": "Bob"} using the @> operator.

PostgreSQL

CREATE TABLE books (
    id SERIAL PRIMARY KEY,
    details JSONB
);

INSERT INTO books (details) VALUES
    ('{"genre": "fiction", "author": "Alice", "year": 2020}'),
    ('{"genre": "non-fiction", "author": "Bob", "year": 2018}'),
    ('{"genre": "fiction", "author": "Charlie", "year": 2021}');

SELECT * FROM books WHERE details @> '{"genre": "fiction"}';

-- Select books where details contain {"author": "Bob"}
-- Your code here

Hint

Use WHERE details @> '{"author": "Bob"}' to filter JSONB rows by author.

Practice

1.

What does the PostgreSQL @> operator do when used with JSONB data?

easy

A. Checks if the right JSONB contains the left JSONB

B. Deletes the right JSONB from the left JSONB

C. Merges two JSONB objects into one

D. Checks if the left JSONB contains the right JSONB

Solution

1. Step 1: Understand the operator's purpose

   The @> operator in PostgreSQL is used to check containment between JSONB values.

2. Step 2: Identify the direction of containment

   It checks if the JSONB on the left side contains the JSONB on the right side.

3. Final Answer:

   Checks if the left JSONB contains the right JSONB -> Option D

4. Quick Check:

   JSONB containment = left contains right [OK]

Hint: Remember: left @> right means left contains right [OK]

Common Mistakes:

• Confusing which side contains which
• Thinking it merges JSONB objects
• Assuming it deletes data

2.

Which of the following is the correct syntax to check if a JSONB column data contains the JSON {"key": "value"}?

SELECT * FROM table WHERE data ___ '{"key": "value"}'::jsonb;

easy

A. @>

B. <@

C. ->

D. ->>

Solution

1. Step 1: Identify the containment operator

   The @> operator checks if the left JSONB contains the right JSONB.

2. Step 2: Apply correct syntax

   To check if data contains the JSON, use data @> '{"key": "value"}'::jsonb.

3. Final Answer:

   @> -> Option A

4. Quick Check:

   Containment operator = @> [OK]

Hint: Use @> with right side cast to jsonb [OK]

Common Mistakes:

• Using <@ which reverses containment
• Using -> or ->> which access keys, not contain
• Forgetting to cast string to jsonb

3.

Given the table items with a JSONB column attributes, what will this query return?

SELECT id FROM items WHERE attributes @> '{"color": "red"}'::jsonb;

Assuming items contains:

id | attributes
---+-------------------------
1  | {"color": "red", "size": "M"}
2  | {"color": "blue", "size": "L"}
3  | {"color": "red"}
4  | {"size": "S"}

medium

A. [4]

B. [2, 4]

C. [1, 3]

D. [1, 2, 3]

Solution

1. Step 1: Understand the query condition

   The query selects rows where attributes contain the JSON {"color": "red"}.

2. Step 2: Check each row's attributes

   Rows 1 and 3 have "color": "red"; rows 2 and 4 do not.

3. Final Answer:

   [1, 3] -> Option C

4. Quick Check:

   Only rows with color red = [1, 3] [OK]

Hint: Look for rows where JSON contains the key-value pair exactly [OK]

Common Mistakes:

• Including rows without the key-value pair
• Confusing @> with key existence operator
• Ignoring the JSONB cast requirement

4.

Identify the error in this query that tries to find rows where info JSONB column contains {"active": true}:

SELECT * FROM users WHERE info @> '{active: true}';

medium

A. Wrong operator used, should be <@

B. JSON keys must be in double quotes

C. Missing cast to jsonb for the right side

D. Boolean true must be in quotes

Solution

1. Step 1: Check JSON syntax correctness

   JSON keys must be enclosed in double quotes, so active should be "active".

2. Step 2: Verify casting and operator

   The query is missing ::jsonb cast, but the main syntax error is the missing quotes around the key.

3. Final Answer:

   JSON keys must be in double quotes -> Option B

4. Quick Check:

   JSON keys need double quotes [OK]

Hint: Always quote JSON keys and cast string to jsonb [OK]

Common Mistakes:

• Forgetting double quotes around keys
• Not casting string to jsonb
• Using wrong containment operator

5.

You want to find all rows in products where the details JSONB column contains {"features": {"waterproof": true}} and also the price is less than 100. Which query is correct?

hard

A. SELECT * FROM products WHERE details @> '{"features": {"waterproof": true}}'::jsonb AND price < 100;

B. SELECT * FROM products WHERE details <@ '{"features": {"waterproof": true}}'::jsonb AND price < 100;

C. SELECT * FROM products WHERE details @> '{features: {waterproof: true}}' AND price < 100;

D. SELECT * FROM products WHERE details -> 'features' @> '{"waterproof": true}' AND price < 100;

Solution

1. Step 1: Use correct containment operator and JSON syntax

   The @> operator checks if details contains the given JSON. The JSON must have keys in double quotes and be cast to jsonb.

2. Step 2: Combine with price condition

   Use AND price < 100 to filter by price.

3. Final Answer:

   SELECT * FROM products WHERE details @> '{"features": {"waterproof": true}}'::jsonb AND price < 100; -> Option A

4. Quick Check:

   Correct operator, JSON syntax, and price filter [OK]

Hint: Use @> with correct JSON and add AND for price filter [OK]

Common Mistakes:

• Using <@ instead of @>
• Missing quotes in JSON keys
• Incorrect JSONB casting
• Trying to use -> operator for containment