INSERT with RETURNING clause in PostgreSQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What does this INSERT with RETURNING output?

Given the table users(id SERIAL PRIMARY KEY, name TEXT, age INT), what is the output of this query?

INSERT INTO users (name, age) VALUES ('Alice', 30) RETURNING id, name;

PostgreSQL

INSERT INTO users (name, age) VALUES ('Alice', 30) RETURNING id, name;

A [{"name": "Alice", "age": 30}]

B [{"id": 1, "name": "Alice"}]

C No output, just inserts the row

D Syntax error

💡 Hint

The RETURNING clause returns the specified columns of the inserted row.

The RETURNING clause outputs the columns requested from the newly inserted row. Here, it returns the generated id and the name 'Alice'.

📝 Syntax

intermediate

Which INSERT with RETURNING syntax is correct?

Choose the correct syntax to insert a row and return the generated id and age from table people(id SERIAL, age INT).

A INSERT INTO people (age) VALUES (25) RETURNING id, age;

B INSERT INTO people (age) VALUES (25) RETURN id, age;

C INSERT INTO people (age) VALUES (25) RETURNING (id, age);

D INSERT INTO people (age) VALUES (25) OUTPUT id, age;

💡 Hint

The keyword is RETURNING without parentheses.

PostgreSQL uses RETURNING without parentheses to specify columns to return after an INSERT.

❓ optimization

advanced

Why use INSERT with RETURNING instead of separate SELECT?

You want to insert a new product and get its generated id immediately. Which is the main advantage of using INSERT ... RETURNING id over inserting then running a separate SELECT to get the id?

A It returns all columns by default without specifying.

B It automatically commits the transaction.

C It locks the entire table to prevent other inserts.

D It reduces the number of queries and avoids race conditions.

💡 Hint

Think about efficiency and data consistency.

Using RETURNING returns the inserted row's data immediately, avoiding extra queries and potential conflicts.

🔧 Debug

advanced

What error does this INSERT with RETURNING cause?

Given table orders(order_id SERIAL PRIMARY KEY, amount INT NOT NULL), what error occurs with this query?

INSERT INTO orders (amount) VALUES (NULL) RETURNING order_id;

A Syntax error near RETURNING

B ERROR: duplicate key value violates unique constraint

C ERROR: null value in column "amount" violates not-null constraint

D No error, returns order_id

💡 Hint

Check the NOT NULL constraint on amount.

Inserting NULL into a NOT NULL column causes a constraint violation error.

🧠 Conceptual

expert

How does RETURNING behave with multi-row INSERT?

Consider this query inserting multiple rows:

INSERT INTO employees(name, salary) VALUES ('John', 5000), ('Jane', 6000) RETURNING id, name;

What will the RETURNING clause output?

A A result set with two rows, each containing the id and name of the inserted employees.

B An error because RETURNING cannot be used with multiple rows.

C No output because RETURNING works only for single-row inserts.

D Only the id and name of the first inserted row.

💡 Hint

RETURNING returns data for all inserted rows.

RETURNING outputs one row per inserted row, showing requested columns for each.

Practice

1. What does the RETURNING clause do in a PostgreSQL INSERT statement?

easy

A. It creates a backup of the inserted data.

B. It deletes rows after insertion.

C. It updates existing rows instead of inserting.

D. It returns specified columns of the newly inserted rows immediately.

Solution

1. Step 1: Understand the purpose of RETURNING

   The RETURNING clause in PostgreSQL is used to get back specified columns from the rows that were just inserted.

2. Step 2: Compare with other options

   The other options describe actions unrelated to RETURNING. RETURNING does not delete, update, or backup data.

3. Final Answer:

   It returns specified columns of the newly inserted rows immediately. -> Option D

4. Quick Check:

   RETURNING returns inserted data [OK]

Hint: RETURNING fetches inserted row data instantly [OK]

Common Mistakes:

• Thinking RETURNING deletes or updates data
• Confusing RETURNING with SELECT
• Assuming RETURNING creates backups

2. Which of the following is the correct syntax to insert a row and return the generated id in PostgreSQL?

easy

A. INSERT INTO users VALUES ('Alice') RETURNING id;

B. INSERT INTO users (name) RETURN id VALUES ('Alice');

C. INSERT INTO users (name) VALUES ('Alice') RETURNING id;

D. INSERT INTO users (name) VALUES ('Alice') RETURN id;

Solution

1. Step 1: Check correct RETURNING syntax

   The correct syntax places RETURNING after VALUES and specifies the column to return, like RETURNING id.

2. Step 2: Validate each option

   INSERT INTO users (name) VALUES ('Alice') RETURNING id; matches the correct syntax. Placing RETURNING before VALUES or using RETURN instead of RETURNING is incorrect. INSERT INTO users VALUES ('Alice') RETURNING id; misses column list before VALUES.

3. Final Answer:

   INSERT INTO users (name) VALUES ('Alice') RETURNING id; -> Option C

4. Quick Check:

   RETURNING follows VALUES with column list [OK]

Hint: RETURNING goes right after VALUES clause [OK]

Common Mistakes:

• Placing RETURNING before VALUES
• Omitting column list in INSERT
• Using RETURN instead of RETURNING

3. Given the table products(id SERIAL PRIMARY KEY, name TEXT, price NUMERIC), what will this query return?

INSERT INTO products (name, price) VALUES ('Pen', 1.5) RETURNING id, price;

medium

A. The new product's id and price, e.g., (5, 1.5)

B. Only the id of the new product

C. An error because RETURNING can't return multiple columns

D. Nothing, INSERT does not return rows

Solution

1. Step 1: Understand RETURNING with multiple columns

   RETURNING can return one or more columns from the inserted row(s). Here, id and price are requested.

2. Step 2: Predict output

   The query inserts a new product and returns its auto-generated id and the price 1.5 as a row, e.g., (5, 1.5).

3. Final Answer:

   The new product's id and price, e.g., (5, 1.5) -> Option A

4. Quick Check:

   RETURNING multiple columns returns all requested [OK]

Hint: RETURNING can list multiple columns separated by commas [OK]

Common Mistakes:

• Assuming RETURNING returns only one column
• Expecting no output from INSERT
• Thinking RETURNING causes syntax error with multiple columns

4. What is wrong with this query?

INSERT INTO orders (product_id, quantity) VALUES (10, 3) RETURNING;

medium

A. VALUES clause is incorrectly formatted.

B. RETURNING clause is missing column names to return.

C. INSERT statement cannot have RETURNING clause.

D. product_id and quantity columns do not exist.

Solution

1. Step 1: Check RETURNING syntax

   RETURNING must specify at least one column or * to return. Here, it is empty, which is invalid syntax.

2. Step 2: Verify other parts

   VALUES clause and columns look correct. INSERT supports RETURNING. Assuming columns exist, no error there.

3. Final Answer:

   RETURNING clause is missing column names to return. -> Option B

4. Quick Check:

   RETURNING needs columns or * [OK]

Hint: RETURNING must specify columns or * [OK]

Common Mistakes:

• Leaving RETURNING empty
• Confusing RETURNING with WHERE
• Assuming RETURNING is optional but empty

5. You want to insert multiple rows into employees(name, department) and get back their generated id values. Which query correctly does this and returns all new ids?

hard

A. INSERT INTO employees (name, department) VALUES ('John', 'HR'), ('Jane', 'IT') RETURNING id;

B. INSERT INTO employees (name, department) RETURNING id VALUES ('John', 'HR'), ('Jane', 'IT');

C. INSERT INTO employees (name, department) VALUES ('John', 'HR'); RETURNING id; INSERT INTO employees (name, department) VALUES ('Jane', 'IT'); RETURNING id;

D. INSERT INTO employees (name, department) VALUES ('John', 'HR'), ('Jane', 'IT') RETURN id;

Solution

1. Step 1: Understand multi-row INSERT with RETURNING

   PostgreSQL allows inserting multiple rows in one statement and RETURNING returns all inserted rows' specified columns.

2. Step 2: Analyze each option

   INSERT INTO employees (name, department) VALUES ('John', 'HR'), ('Jane', 'IT') RETURNING id; correctly inserts two rows and returns their ids. INSERT INTO employees (name, department) RETURNING id VALUES ('John', 'HR'), ('Jane', 'IT'); misplaces RETURNING before VALUES. INSERT INTO employees (name, department) VALUES ('John', 'HR'); RETURNING id; INSERT INTO employees (name, department) VALUES ('Jane', 'IT'); RETURNING id; splits inserts and RETURNING incorrectly. INSERT INTO employees (name, department) VALUES ('John', 'HR'), ('Jane', 'IT') RETURN id; uses invalid RETURN keyword.

3. Final Answer:

   INSERT INTO employees (name, department) VALUES ('John', 'HR'), ('Jane', 'IT') RETURNING id; -> Option A

4. Quick Check:

   Multi-row INSERT with RETURNING returns all rows [OK]

Hint: Use RETURNING after multi-row VALUES to get all ids [OK]

Common Mistakes:

• Placing RETURNING before VALUES
• Splitting inserts instead of multi-row insert
• Using RETURN instead of RETURNING