Query parameters for filtering in Rest API - Step-by-Step Execution

Concept Flow - Query parameters for filtering

Client sends request with query parameters

↓

Server receives request

↓

Server extracts query parameters

↓

Server applies filters based on parameters

↓

Server fetches filtered data

↓

Server sends filtered data response

↓

Client receives filtered data

The client sends a request with query parameters; the server reads these parameters, filters data accordingly, and returns the filtered results.

Execution Sample

Rest API

GET /products?category=books&price_lt=20

# Server extracts 'category' and 'price_lt'
# Filters products where category='books' and price < 20
# Returns filtered product list

This example shows a client requesting products filtered by category and price using query parameters.

Execution Table

Step	Action	Query Parameters Extracted	Filter Applied	Result
1	Receive request	category=books, price_lt=20	None yet	Waiting to process
2	Extract parameters	category=books, price_lt=20	Prepare filters: category='books', price < 20	Filters ready
3	Apply filters	category=books, price_lt=20	Filter products by category='books' and price < 20	Filtered product list created
4	Send response	category=books, price_lt=20	Filtered data sent	Client receives filtered products
5	End	No new parameters	No further filtering	Request complete

💡 All query parameters processed; filtered data sent back to client.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
query_params	None	{category: 'books', price_lt: '20'}	{category: 'books', price_lt: '20'}	Used for filtering
filters	None	category='books', price < 20	Applied filters	Filters applied
filtered_data	None	None	List of products matching filters	Sent to client

Key Moments - 2 Insights

How does the server know which filters to apply from the query parameters?

The server extracts key-value pairs from the query parameters (see execution_table step 2) and maps them to filter conditions it understands.

What happens if a query parameter is missing or unknown?

If a parameter is missing, the server simply does not apply that filter (no restriction). Unknown parameters are ignored or cause an error depending on server design (not shown here).

Visual Quiz - 3 Questions

Look at the execution_table, what filters are prepared at step 2?

A No filters prepared yet

B category='electronics' and price > 50

C category='books' and price < 20

D All products without filtering

💡 Hint

Check the 'Filter Applied' column at step 2 in execution_table.

At which step does the server send the filtered data back to the client?

A Step 4

B Step 3

C Step 1

D Step 5

💡 Hint

Look for 'Send response' action in the execution_table.

If the client removes 'price_lt=20' from the query, how does the filtering change?

A Filters by price only

B Filters by category only

C No filters applied

D Filters by category and price

💡 Hint

Refer to variable_tracker for 'filters' and how query_params affect them.

Concept Snapshot

Query parameters are added to API URLs to filter data.
Server reads these parameters, applies filters to data.
Example: /items?color=red&size=small filters items by color and size.
Missing parameters mean no filter on that attribute.
Server returns only data matching all filters.

Full Transcript

When a client wants specific data from a server, it can add query parameters to the URL. These parameters are key-value pairs like 'category=books' or 'price_lt=20'. The server receives the request and extracts these parameters. Then it uses them to filter the data it holds, for example, selecting only products in the 'books' category priced less than 20. After filtering, the server sends back only the matching data. If some parameters are missing, the server does not filter by those attributes. This process helps clients get exactly the data they want without extra information.

Practice

1.

What is the main purpose of query parameters in a REST API URL?

easy

A. To update data on the server

B. To change the server settings

C. To filter or specify which data to return

D. To delete data from the server

Solution

1. Step 1: Understand query parameters role

   Query parameters are added after a question mark in a URL to specify or filter data.

2. Step 2: Identify their purpose in REST API

   They help clients ask for specific data subsets, making responses smaller and faster.

3. Final Answer:

   To filter or specify which data to return -> Option C

4. Quick Check:

   Query parameters = filter data [OK]

Hint: Query parameters filter data after '?' in URL [OK]

Common Mistakes:

• Thinking query parameters change server settings
• Confusing query parameters with request body
• Assuming query parameters update or delete data

2.

Which of the following is the correct way to add two query parameters type=fruit and color=red to a REST API URL?

easy

A. api/items?type=fruit,color=red

B. api/items&type=fruit?color=red

C. api/items?type=fruit;color=red

D. api/items?type=fruit&color=red

Solution

1. Step 1: Recall query parameter syntax

   Query parameters start with '?' and multiple parameters are joined with '&'.

2. Step 2: Check each option's format

   api/items?type=fruit&color=red uses '?' then '&' correctly; others misuse separators or order.

3. Final Answer:

   api/items?type=fruit&color=red -> Option D

4. Quick Check:

   Use '?' then '&' between parameters [OK]

Hint: Start with '?' then join parameters with '&' [OK]

Common Mistakes:

• Using '&' before '?'
• Separating parameters with commas or semicolons
• Mixing order of '?' and '&'

3.

Given the URL api/products?category=books&price_lt=20, what does the query parameter price_lt=20 most likely mean?

medium

A. Return products with price less than 20

B. Return products with price equal to 20

C. Return products with price greater than 20

D. Return products with price not equal to 20

Solution

1. Step 1: Interpret the query parameter name

   The suffix '_lt' usually means 'less than' in filtering.

2. Step 2: Apply meaning to the value

   price_lt=20 means filter products priced less than 20 units.

3. Final Answer:

   Return products with price less than 20 -> Option A

4. Quick Check:

   _lt means less than [OK]

Hint: _lt means less than, _gt means greater than [OK]

Common Mistakes:

• Confusing _lt with equal or greater than
• Ignoring suffix meaning in parameter names
• Assuming price_lt filters prices equal to 20

4.

Identify the error in this URL used to filter users by age and city:
api/users?age=30&city;=NewYork

medium

A. The semicolon ';' is incorrectly used instead of '='

B. The '&' should be replaced with '?' between parameters

C. The city value should be URL encoded as 'New%20York'

D. The age parameter should be 'age_gt=30' to filter correctly

Solution

1. Step 1: Check parameter separators and assignments

   Parameters are separated by '&' and assigned with '='. Here, 'city;=NewYork' uses ';' wrongly.

2. Step 2: Confirm correct syntax

   The semicolon should be '=' to assign the value to 'city'.

3. Final Answer:

   The semicolon ';' is incorrectly used instead of '=' -> Option A

4. Quick Check:

   Use '=' to assign values, not ';' [OK]

Hint: Use '=' for assignment, '&' to separate parameters [OK]

Common Mistakes:

• Using semicolon instead of equal sign
• Replacing '&' with '?' between parameters
• Not encoding spaces in values

5.

You want to filter a list of events to show only those happening in 2024 and located in either 'New York' or 'Los Angeles'. Which query parameter format correctly expresses this filter?

hard

A. api/events?year=2024&city=New York,Los Angeles

B. api/events?year=2024&city=New%20York&city=Los%20Angeles

C. api/events?year=2024&city=New York&city=Los Angeles

D. api/events?year=2024&city=New%20York|Los%20Angeles

Solution

1. Step 1: Understand filtering multiple values for one parameter

   To filter by multiple cities, repeat the parameter with URL-encoded values.

2. Step 2: Check URL encoding and parameter repetition

   api/events?year=2024&city=New%20York&city=Los%20Angeles repeats 'city' with URL-encoded spaces, which is standard practice.

3. Final Answer:

   api/events?year=2024&city=New%20York&city=Los%20Angeles -> Option B

4. Quick Check:

   Repeat parameters with URL encoding for multiple values [OK]

Hint: Repeat parameter with encoded values for multiple filters [OK]

Common Mistakes:

• Using commas without encoding or server support
• Not encoding spaces in city names
• Using '|' which is not standard in URLs