SELECT with PostgreSQL-specific features - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this query using PostgreSQL's DISTINCT ON?

Consider the table employees with columns department and salary. What rows does this query return?

SELECT DISTINCT ON (department) department, salary FROM employees ORDER BY department, salary DESC;

PostgreSQL

CREATE TABLE employees (department TEXT, salary INT);
INSERT INTO employees VALUES
('Sales', 5000),
('Sales', 6000),
('HR', 4500),
('HR', 4700),
('IT', 7000);

SELECT DISTINCT ON (department) department, salary FROM employees ORDER BY department, salary DESC;

A [{'Sales', 5000}, {'HR', 4500}, {'IT', 7000}]

B [{'Sales', 6000}, {'HR', 4700}, {'IT', 7000}]

C [{'Sales', 6000}, {'HR', 4500}, {'IT', 7000}]

D [{'Sales', 5000}, {'HR', 4700}, {'IT', 7000}]

💡 Hint

DISTINCT ON returns the first row of each group based on the ORDER BY clause.

DISTINCT ON (department) picks the first row per department ordered by salary descending, so highest salary per department is returned.

❓ query_result

intermediate

What does this query using PostgreSQL's RETURNING clause output?

Given the table products with columns id and price, what is the output of this query?

UPDATE products SET price = price * 1.1 WHERE id = 2 RETURNING id, price;

PostgreSQL

CREATE TABLE products (id INT PRIMARY KEY, price NUMERIC);
INSERT INTO products VALUES (1, 100), (2, 200);

UPDATE products SET price = price * 1.1 WHERE id = 2 RETURNING id, price;

A [{id: 2, price: 220}]

B [{id: 1, price: 100}, {id: 2, price: 220}]

C []

D [{id: 2, price: 200}]

💡 Hint

RETURNING shows the rows affected by the UPDATE.

Only the row with id=2 is updated, price multiplied by 1.1, so price becomes 220.

📝 Syntax

advanced

Which option correctly uses PostgreSQL's JSONB operators to extract a value?

Given a table data with a JSONB column info, which query correctly extracts the text value of the key name?

PostgreSQL

SELECT info->>'name' FROM data;

A SELECT info->>name FROM data;

B SELECT info->'name' FROM data;

C SELECT info->>'name' FROM data;

D SELECT info->name FROM data;

💡 Hint

Use ->> with quotes around the key to get text.

The operator ->> extracts the JSON value as text. The key must be a string literal in quotes.

❓ optimization

advanced

Which index type is best for optimizing full-text search in PostgreSQL?

You want to speed up searches on a text column using PostgreSQL's full-text search. Which index type should you create?

A CREATE INDEX idx ON table USING gin (to_tsvector('english', column));

B CREATE INDEX idx ON table USING btree (column);

C CREATE INDEX idx ON table USING hash (column);

D CREATE INDEX idx ON table USING gist (column);

💡 Hint

Full-text search uses tsvector and GIN indexes.

GIN indexes on to_tsvector expressions are optimized for full-text search queries.

🧠 Conceptual

expert

What is the effect of the PostgreSQL-specific clause FOR UPDATE SKIP LOCKED in a SELECT query?

Consider a SELECT query with FOR UPDATE SKIP LOCKED. What does this clause do?

A It causes the query to wait until all rows are unlocked before returning.

B It skips locking any rows and returns all rows immediately.

C It locks all rows in the table regardless of selection.

D It locks selected rows and skips rows already locked by other transactions.

💡 Hint

SKIP LOCKED avoids waiting on locked rows.

FOR UPDATE locks rows selected, but SKIP LOCKED skips rows locked by others, allowing non-blocking row selection.

Practice

1. What does the LIMIT clause do in a PostgreSQL SELECT query?

easy

A. It combines rows from two tables.

B. It restricts the number of rows returned by the query.

C. It removes duplicate rows from the result.

D. It sorts the rows in ascending order.

Solution

1. Step 1: Understand the purpose of LIMIT

   The LIMIT clause is used to specify the maximum number of rows to return from a query result.

2. Step 2: Compare with other options

   Sorting is done by ORDER BY, removing duplicates by DISTINCT, and combining tables by JOIN, so these are not correct.

3. Final Answer:

   It restricts the number of rows returned by the query. -> Option B

4. Quick Check:

   LIMIT = restrict rows [OK]

Hint: LIMIT caps rows returned, like a page size [OK]

Common Mistakes:

• Confusing LIMIT with ORDER BY
• Thinking LIMIT removes duplicates
• Mixing LIMIT with JOIN functionality

2. Which of the following is the correct syntax to select distinct city names from a table called customers?

easy

A. SELECT city FROM DISTINCT customers;

B. SELECT city DISTINCT FROM customers;

C. SELECT DISTINCT city FROM customers;

D. SELECT city FROM customers DISTINCT;

Solution

1. Step 1: Recall correct DISTINCT syntax

   In PostgreSQL, DISTINCT comes immediately after SELECT and before the column list.

2. Step 2: Validate each option

   SELECT DISTINCT city FROM customers; correctly places DISTINCT after SELECT. Options A, B, and D misuse DISTINCT placement causing syntax errors.

3. Final Answer:

   SELECT DISTINCT city FROM customers; -> Option C

4. Quick Check:

   SELECT DISTINCT = correct syntax [OK]

Hint: DISTINCT always follows SELECT keyword [OK]

Common Mistakes:

• Placing DISTINCT after column names
• Using DISTINCT after FROM clause
• Misordering keywords causing syntax errors

3. Given the table products with columns id, name, and price, what will this query return?

SELECT name FROM products ORDER BY price DESC LIMIT 2;

medium

A. The two products with the lowest prices.

B. An error because LIMIT cannot be used with ORDER BY.

C. All products sorted by price descending.

D. The two products with the highest prices.

Solution

1. Step 1: Understand ORDER BY with DESC

   ORDER BY price DESC sorts products from highest to lowest price.

2. Step 2: Apply LIMIT 2

   LIMIT 2 restricts the output to only the first two rows after sorting, so the two most expensive products.

3. Final Answer:

   The two products with the highest prices. -> Option D

4. Quick Check:

   ORDER BY DESC + LIMIT = top rows [OK]

Hint: ORDER BY DESC + LIMIT picks top items [OK]

Common Mistakes:

• Thinking LIMIT returns lowest prices
• Believing LIMIT causes syntax error with ORDER BY
• Ignoring DESC means descending order

4. Identify the error in this PostgreSQL query:

SELECT DISTINCT name, price FROM products LIMIT;

medium

A. LIMIT requires a number to specify how many rows to return.

B. DISTINCT cannot be used with multiple columns.

C. SELECT must include WHERE clause when using LIMIT.

D. The table name is missing.

Solution

1. Step 1: Check LIMIT usage

   LIMIT must be followed by a number indicating how many rows to return; here it is missing.

2. Step 2: Validate other parts

   DISTINCT can be used with multiple columns, WHERE is optional, and table name is present as products.

3. Final Answer:

   LIMIT requires a number to specify how many rows to return. -> Option A

4. Quick Check:

   LIMIT needs a number [OK]

Hint: Always put a number after LIMIT [OK]

Common Mistakes:

• Omitting number after LIMIT
• Thinking DISTINCT can't use multiple columns
• Assuming WHERE is mandatory with LIMIT

5. You want to get the third and fourth highest salaries from the employees table with column salary. Which query correctly achieves this in PostgreSQL?

hard

A. SELECT salary FROM employees ORDER BY salary DESC LIMIT 2 OFFSET 2;

B. SELECT salary FROM employees ORDER BY salary ASC LIMIT 2 OFFSET 3;

C. SELECT DISTINCT salary FROM employees LIMIT 2 OFFSET 3 ORDER BY salary DESC;

D. SELECT salary FROM employees LIMIT 2 OFFSET 2 ORDER BY salary DESC;

Solution

1. Step 1: Understand OFFSET and LIMIT

   OFFSET 2 skips the first two highest salaries, LIMIT 2 returns the next two rows.

2. Step 2: Check ORDER BY direction

   ORDER BY salary DESC sorts salaries from highest to lowest, so OFFSET 2 skips top two highest.

3. Step 3: Validate options

   SELECT salary FROM employees ORDER BY salary DESC LIMIT 2 OFFSET 2; correctly orders DESC, uses LIMIT 2 and OFFSET 2. SELECT salary FROM employees ORDER BY salary ASC LIMIT 2 OFFSET 3; orders ASC (wrong direction), C misplaces ORDER BY, D places ORDER BY after LIMIT (syntax error).

4. Final Answer:

   SELECT salary FROM employees ORDER BY salary DESC LIMIT 2 OFFSET 2; -> Option A

5. Quick Check:

   OFFSET skips rows, LIMIT picks next [OK]

Hint: Use OFFSET to skip, LIMIT to pick rows [OK]

Common Mistakes:

• Using ASC instead of DESC for highest values
• Placing ORDER BY after LIMIT
• Confusing OFFSET number with row count