JSONB containment (@>) operator in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the JSONB containment operator @> do in PostgreSQL?

It checks if the left JSONB value contains the right JSONB value. In other words, it returns true if the right JSONB is a subset of the left JSONB.

beginner

Example: What will this query return?
SELECT '{"a":1, "b":2}'::jsonb @> '{"a":1}'::jsonb;

It will return true because the left JSONB contains the key-value pair {"a":1} from the right JSONB.

intermediate

Can the @> operator be used to check if a JSONB array contains a specific element?

Yes, it can check if a JSONB array contains a specific element by using the element inside an array on the right side. For example, '[1,2,3]'::jsonb @> '[2]'::jsonb returns true.

beginner

What will this query return?
SELECT '{"a":1, "b":2}'::jsonb @> '{"b":3}'::jsonb;

It will return false because the value for key "b" in the left JSONB is 2, which does not match the value 3 in the right JSONB.

beginner

Why is the JSONB containment operator useful in real life?

It helps quickly find rows where JSONB columns contain specific data without extracting or parsing the whole JSON. This is useful for filtering data stored in flexible JSON formats.

What does jsonb_col @> '{"key":"value"}' check?

A If jsonb_col contains the key-value pair {"key":"value"}

B If jsonb_col equals {"key":"value"}

C If jsonb_col is contained in {"key":"value"}

D If jsonb_col is an array

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

Which of these returns true?
SELECT '[1,2,3]'::jsonb @> '[2]'::jsonb;

A null

B false

C error

D true

The array on the left contains the element 2, so the containment check returns true.

What will this return?
SELECT '{"a":1}'::jsonb @> '{"a":1, "b":2}'::jsonb;

A true

B error

C false

D null

The left JSONB does not contain the key "b", so it does not contain the right JSONB.

Can @> operator be used to check nested JSON objects?

A Yes, it checks nested objects as well

B No, only top-level keys

C Only arrays, not objects

D Only strings

The operator checks containment recursively, including nested objects.

What type of data does the @> operator work with?

A Text data type

B JSONB data type

C Integer data type

D Date data type

The @> operator is specific to the JSONB data type in PostgreSQL.

Explain how the JSONB containment operator @> works in PostgreSQL.

Think about subset and containment in JSON data.

Give an example query using @> to find rows where a JSONB column contains a specific key-value pair.

Use a simple JSON object on the right side.

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