Functions returning TABLE in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a function that returns a table.

PostgreSQL

CREATE FUNCTION get_users() RETURNS [1] AS $$ BEGIN RETURN QUERY SELECT id, name FROM users; END; $$ LANGUAGE plpgsql;

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AINT

BSETOF users

CTEXT

DTABLE(id INT, name TEXT)

Attempts:

3 left

💡 Hint

Common Mistakes

Using RETURNS INT or RETURNS TEXT instead of RETURNS TABLE.

Using SETOF users when the function specifies columns explicitly.

The function returns a table with columns id and name. So RETURNS TABLE(id INT, name TEXT) is correct.

2fill in blank

medium

Complete the code to return rows from the function.

PostgreSQL

CREATE FUNCTION get_active_users() RETURNS TABLE(id INT, name TEXT) AS $$ BEGIN RETURN [1] SELECT id, name FROM users WHERE active = true; END; $$ LANGUAGE plpgsql;

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ARETURN NEXT

BRETURN QUERY

CRETURN ROW

DRETURN TABLE

Attempts:

3 left

💡 Hint

Common Mistakes

Using RETURN NEXT without a loop to return multiple rows.

Using RETURN ROW which is for single row returns.

In PL/pgSQL, RETURN QUERY is used to return the result of a query from a function returning TABLE.

3fill in blank

hard

Fix the error in the function that returns a table.

PostgreSQL

CREATE FUNCTION get_scores() RETURNS TABLE(player TEXT, score INT) AS $$ BEGIN [1] SELECT player, score FROM scores; END; $$ LANGUAGE plpgsql;

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ARETURN QUERY

BRETURN

CRETURN NEXT

DRETURN ROW

Attempts:

3 left

💡 Hint

Common Mistakes

Using RETURN without QUERY causes syntax error.

Using RETURN NEXT without looping over rows.

The function must use RETURN QUERY to return multiple rows from the SELECT statement.

4fill in blank

hard

Fill both blanks to define a function returning a table with two columns and returning rows.

PostgreSQL

CREATE FUNCTION get_products() RETURNS [1] AS $$ BEGIN [2] SELECT id, name FROM products; END; $$ LANGUAGE plpgsql;

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ATABLE(id INT, name TEXT)

BRETURN QUERY

CSETOF products

DRETURN NEXT

Attempts:

3 left

💡 Hint

Common Mistakes

Using SETOF products with RETURNS TABLE syntax.

Using RETURN NEXT without a loop.

The function returns a table with columns, so use RETURNS TABLE(id INT, name TEXT). To return rows, use RETURN QUERY.

5fill in blank

hard

Fill all three blanks to create a function returning a table with columns, returning rows, and specifying language.

PostgreSQL

CREATE FUNCTION get_orders() RETURNS [1] AS $$ BEGIN [2] SELECT order_id, total FROM orders; END; $$ LANGUAGE [3];

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ATABLE(order_id INT, total NUMERIC)

BRETURN QUERY

Cplpgsql

DSETOF orders

Attempts:

3 left

💡 Hint

Common Mistakes

Using SETOF orders with RETURNS TABLE syntax.

Forgetting to specify LANGUAGE plpgsql.

The function returns a table with columns, so use RETURNS TABLE(order_id INT, total NUMERIC). Use RETURN QUERY to return rows. The language is plpgsql.

Practice

1. What does a PostgreSQL function declared with RETURNS TABLE do?

easy

A. Returns no result, only performs actions

B. Returns only a single scalar value

C. Returns multiple rows and columns as a table result

D. Returns a JSON object

Solution

1. Step 1: Understand the purpose of RETURNS TABLE

   The RETURNS TABLE clause defines that the function will return a set of rows with specified columns, like a table.

2. Step 2: Compare with other return types

   Unlike scalar returns or void, RETURNS TABLE returns multiple rows and columns.

3. Final Answer:

   Returns multiple rows and columns as a table result -> Option C

4. Quick Check:

   RETURNS TABLE means multiple rows/columns [OK]

Hint: RETURNS TABLE means function outputs rows and columns [OK]

Common Mistakes:

• Thinking RETURNS TABLE returns a single value
• Confusing RETURNS TABLE with RETURNS VOID
• Assuming it returns JSON automatically

2. Which of the following is the correct syntax to declare a PostgreSQL function returning a table with columns id INT and name TEXT?

easy

A. CREATE FUNCTION f() RETURNS TABLE(id TEXT, name INT) AS $$ BEGIN RETURN QUERY SELECT 1, 'a'; END; $$ LANGUAGE plpgsql;

B. CREATE FUNCTION f() RETURNS SETOF RECORD AS $$ BEGIN RETURN QUERY SELECT 1, 'a'; END; $$ LANGUAGE plpgsql;

C. CREATE FUNCTION f() RETURNS INT AS $$ BEGIN RETURN 1; END; $$ LANGUAGE plpgsql;

D. CREATE FUNCTION f() RETURNS TABLE(id INT, name TEXT) AS $$ BEGIN RETURN QUERY SELECT 1, 'a'; END; $$ LANGUAGE plpgsql;

Solution

1. Step 1: Check RETURNS TABLE syntax

   CREATE FUNCTION f() RETURNS TABLE(id INT, name TEXT) AS $$ BEGIN RETURN QUERY SELECT 1, 'a'; END; $$ LANGUAGE plpgsql; correctly declares RETURNS TABLE(id INT, name TEXT) matching the column names and types.

2. Step 2: Verify RETURN QUERY usage

   CREATE FUNCTION f() RETURNS TABLE(id INT, name TEXT) AS $$ BEGIN RETURN QUERY SELECT 1, 'a'; END; $$ LANGUAGE plpgsql; uses RETURN QUERY SELECT 1, 'a'; which returns rows matching the table structure.

3. Final Answer:

   Correct RETURNS TABLE syntax and return statement -> Option D

4. Quick Check:

   RETURNS TABLE with matching columns and RETURN QUERY [OK]

Hint: RETURNS TABLE needs column names/types and RETURN QUERY [OK]

Common Mistakes:

• Using RETURNS SETOF RECORD without column definition
• Swapping column types in RETURNS TABLE
• Returning scalar instead of query

3. Given this function:

CREATE FUNCTION get_numbers() RETURNS TABLE(num INT) AS $$ BEGIN RETURN QUERY SELECT generate_series(1,3); END; $$ LANGUAGE plpgsql;

What will be the output of SELECT * FROM get_numbers();?

medium

A. 3 rows with values 1, 2, 3

B. 1 row with value 3

C. Error: function does not return a table

D. Empty result set

Solution

1. Step 1: Understand generate_series(1,3)

   This function generates rows with values 1, 2, and 3.

2. Step 2: RETURN QUERY returns all rows

   The function returns all rows from generate_series(1,3) as a table with column num.

3. Final Answer:

   3 rows with values 1, 2, 3 -> Option A

4. Quick Check:

   generate_series(1,3) returns 3 rows [OK]

Hint: generate_series returns multiple rows, so function returns them all [OK]

Common Mistakes:

• Thinking it returns a single row
• Confusing RETURNS TABLE with scalar return
• Expecting an error due to missing RETURNS SETOF

4. Identify the error in this function:

CREATE FUNCTION get_data() RETURNS TABLE(id INT, val TEXT) AS $$ BEGIN RETURN SELECT 1, 'a'; END; $$ LANGUAGE plpgsql;

medium

A. Missing RETURN QUERY before SELECT

B. Wrong column types in RETURNS TABLE

C. Function must return VOID

D. Missing LANGUAGE declaration

Solution

1. Step 1: Check RETURN statement in RETURNS TABLE function

   In PL/pgSQL, to return rows from a query, use RETURN QUERY, not just RETURN.

2. Step 2: Identify missing RETURN QUERY

   The function uses RETURN SELECT which is invalid syntax; it should be RETURN QUERY SELECT.

3. Final Answer:

   Missing RETURN QUERY before SELECT -> Option A

4. Quick Check:

   Use RETURN QUERY to return rows in RETURNS TABLE [OK]

Hint: Use RETURN QUERY, not RETURN, to return table rows [OK]

Common Mistakes:

• Using RETURN instead of RETURN QUERY for table results
• Omitting LANGUAGE plpgsql
• Incorrect RETURNS TABLE column types

5. You want to create a function that returns a table of user IDs and their uppercase names from a users table with columns id INT and name TEXT. Which function definition correctly achieves this?

hard

A. CREATE FUNCTION get_upper_users() RETURNS TABLE(id INT, name TEXT) AS $$ BEGIN RETURN QUERY SELECT id, LOWER(name) FROM users; END; $$ LANGUAGE plpgsql;

B. CREATE FUNCTION get_upper_users() RETURNS TABLE(id INT, name TEXT) AS $$ BEGIN RETURN QUERY SELECT id, UPPER(name) FROM users; END; $$ LANGUAGE plpgsql;

C. CREATE FUNCTION get_upper_users() RETURNS SETOF record AS $$ BEGIN RETURN QUERY SELECT id, UPPER(name) FROM users; END; $$ LANGUAGE plpgsql;

D. CREATE FUNCTION get_upper_users() RETURNS TABLE(id TEXT, name TEXT) AS $$ BEGIN RETURN QUERY SELECT id, UPPER(name) FROM users; END; $$ LANGUAGE plpgsql;

Solution

1. Step 1: Match RETURNS TABLE columns with SELECT output

   The function returns id INT and name TEXT, so the SELECT must output these types. CREATE FUNCTION get_upper_users() RETURNS TABLE(id INT, name TEXT) AS $$ BEGIN RETURN QUERY SELECT id, UPPER(name) FROM users; END; $$ LANGUAGE plpgsql; matches this.

2. Step 2: Check the transformation applied

   CREATE FUNCTION get_upper_users() RETURNS TABLE(id INT, name TEXT) AS $$ BEGIN RETURN QUERY SELECT id, UPPER(name) FROM users; END; $$ LANGUAGE plpgsql; uses UPPER(name) to convert names to uppercase as required.

3. Step 3: Verify other options

   CREATE FUNCTION get_upper_users() RETURNS TABLE(id INT, name TEXT) AS $$ BEGIN RETURN QUERY SELECT id, LOWER(name) FROM users; END; $$ LANGUAGE plpgsql; uses LOWER instead of UPPER, CREATE FUNCTION get_upper_users() RETURNS SETOF record AS $$ BEGIN RETURN QUERY SELECT id, UPPER(name) FROM users; END; $$ LANGUAGE plpgsql; returns SETOF record (no column definition), CREATE FUNCTION get_upper_users() RETURNS TABLE(id TEXT, name TEXT) AS $$ BEGIN RETURN QUERY SELECT id, UPPER(name) FROM users; END; $$ LANGUAGE plpgsql; mismatches id type (TEXT instead of INT).

4. Final Answer:

   Correctly returns id and uppercase name as a table -> Option B

5. Quick Check:

   RETURNS TABLE with matching columns and UPPER(name) [OK]

Hint: Match RETURNS TABLE columns and use RETURN QUERY with correct SELECT [OK]

Common Mistakes:

• Using LOWER instead of UPPER
• Mismatching column types in RETURNS TABLE
• Using SETOF record instead of RETURNS TABLE