Functions returning TABLE in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does a PostgreSQL function returning TABLE do?

It returns a set of rows with defined columns, like a table, allowing you to get multiple rows and columns from a single function call.

beginner

How do you define the columns returned by a function returning TABLE in PostgreSQL?

You specify the column names and their data types in the RETURNS TABLE clause of the function definition.

intermediate

What SQL statement inside a function returning TABLE provides the rows to return?

A RETURN QUERY statement is used to return the rows from a SELECT or other query inside the function.

intermediate

Can a function returning TABLE be used in a FROM clause like a regular table?

Yes, you can call the function in the FROM clause and treat its output like a table to join or filter results.

advanced

What is the benefit of using functions returning TABLE over returning SETOF RECORD?

Functions returning TABLE have named columns and types defined upfront, making them easier to use and safer because the output structure is fixed.

Which keyword defines the columns and types a PostgreSQL function will return as a table?

A RETURNS TABLE

B RETURNS SETOF

C RETURNS RECORD

D RETURNS ROW

RETURNS TABLE specifies the column names and types the function will return as a table.

Inside a function returning TABLE, which statement returns the query result rows?

A RETURN ROW

B RETURN NEXT

C RETURN SET

D RETURN QUERY

RETURN QUERY returns the result of a query as the function output rows.

How can you use a function returning TABLE in a SQL query?

A In the FROM clause like a table

B Only in the WHERE clause

C Only in the SELECT list

D Only in the ORDER BY clause

Functions returning TABLE can be called in the FROM clause and treated like tables.

What is a key advantage of RETURNS TABLE over RETURNS SETOF RECORD?

A RETURNS TABLE is faster to execute

B RETURNS TABLE defines output columns explicitly

C RETURNS TABLE can return scalar values only

D RETURNS TABLE does not support multiple rows

RETURNS TABLE explicitly defines output columns and types, making usage clearer and safer.

Which of these is a valid way to define a function returning TABLE in PostgreSQL?

A CREATE FUNCTION get_users() RETURNS VOID AS $$ BEGIN END; $$ LANGUAGE plpgsql;

B CREATE FUNCTION get_users() RETURNS INT AS $$ SELECT id FROM users; $$ LANGUAGE sql;

C CREATE FUNCTION get_users() RETURNS TABLE(id INT, name TEXT) AS $$ BEGIN RETURN QUERY SELECT id, name FROM users; END; $$ LANGUAGE plpgsql;

D CREATE FUNCTION get_users() RETURNS RECORD AS $$ SELECT id, name FROM users; $$ LANGUAGE sql;

Option C correctly defines a function returning TABLE with columns and uses RETURN QUERY.

Explain how to create a PostgreSQL function that returns multiple rows and columns using RETURNS TABLE.

Think about how to define output columns and how to return query results inside the function.

Describe the difference between RETURNS TABLE and RETURNS SETOF RECORD in PostgreSQL functions.

Consider how output structure is handled in both cases.

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