Composite types in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a composite type in PostgreSQL?

A composite type is a user-defined data type that groups multiple fields together, like a mini table row, allowing you to store related data as a single unit.

beginner

How do you create a composite type in PostgreSQL?

Use the CREATE TYPE statement with AS and list the fields with their data types inside parentheses. For example:
CREATE TYPE address AS (street text, city text, zip int);

intermediate

How can you use a composite type in a table?

You can define a column in a table to have the composite type, so each row stores a structured value. For example:
CREATE TABLE person (name text, home address); where address is a composite type.

intermediate

How do you access individual fields of a composite type column in a query?

Use the dot notation to get a field. For example, if home is a composite type column, SELECT home.city FROM person; returns the city field.

advanced

Can composite types be nested inside other composite types?

Yes, composite types can include fields that are themselves composite types, allowing complex structured data within a single column.

Which SQL command creates a composite type in PostgreSQL?

A CREATE TYPE ... AS (...)

B CREATE TABLE ... AS (...)

C CREATE DOMAIN ... AS (...)

D CREATE FUNCTION ... AS (...)

The correct syntax to create a composite type is using CREATE TYPE ... AS (...).

How do you select the 'zip' field from a composite type column named 'home'?

A SELECT home->zip FROM table;

B SELECT zip(home) FROM table;

C SELECT home.zip FROM table;

D SELECT zip FROM home;

Use dot notation: SELECT home.zip FROM table; to access the 'zip' field.

Can a composite type be used as a column type in a PostgreSQL table?

A Yes, it can store multiple related fields in one column.

B No, composite types are only for functions.

C No, composite types cannot be stored in tables.

D Yes, but only for temporary tables.

Composite types can be used as column types to store grouped fields.

What happens if you try to insert a row with missing fields for a composite type column?

A The insert fails due to missing data.

B PostgreSQL fills missing fields with NULL.

C PostgreSQL ignores the composite column.

D The row is inserted but fields have default values.

Missing fields in composite types default to NULL if not provided.

Is it possible to nest composite types inside other composite types?

A Composite types cannot be nested at all.

B No, composite types must only have simple data types.

C Only arrays can be nested, not composite types.

D Yes, composite types can contain other composite types.

Composite types can be nested to create complex structures.

Explain what a composite type is in PostgreSQL and how you create one.

Think of it like a small table structure inside a type.

Describe how to use a composite type as a column in a table and how to query its fields.

Remember you can treat the composite column like an object with properties.

Practice

1. What is a composite type in PostgreSQL?

easy

A. A special index type for faster queries

B. A built-in type for storing large text data

C. A custom type that groups multiple related fields together

D. A function that returns multiple rows

Solution

1. Step 1: Understand the definition of composite types

   Composite types are user-defined types that group several fields into one unit, like a record.

2. Step 2: Compare with other options

   Options A, B, and D describe unrelated PostgreSQL features, not composite types.

3. Final Answer:

   A custom type that groups multiple related fields together -> Option C

4. Quick Check:

   Composite type = grouped fields [OK]

Hint: Composite types group fields like a mini table row [OK]

Common Mistakes:

• Confusing composite types with arrays
• Thinking composite types are indexes
• Mixing composite types with functions

2. Which of the following is the correct syntax to define a composite type named person with fields name (text) and age (integer)?

easy

A. CREATE TYPE person AS (name text, age integer);

B. CREATE TABLE person (name text, age integer);

C. CREATE TYPE person (name text, age integer);

D. CREATE COMPOSITE person AS (name text, age integer);

Solution

1. Step 1: Recall the syntax for creating composite types

   The correct syntax uses CREATE TYPE with AS and parentheses listing fields and types.

2. Step 2: Check each option

   CREATE TYPE person AS (name text, age integer); matches the correct syntax. CREATE TABLE person (name text, age integer); creates a table, not a type. CREATE TYPE person (name text, age integer); misses AS keyword. CREATE COMPOSITE person AS (name text, age integer); uses invalid keyword COMPOSITE.

3. Final Answer:

   CREATE TYPE person AS (name text, age integer); -> Option A

4. Quick Check:

   CREATE TYPE ... AS (...) is correct [OK]

Hint: Use CREATE TYPE name AS (fields) for composite types [OK]

Common Mistakes:

• Using CREATE TABLE instead of CREATE TYPE
• Omitting AS keyword
• Using non-existent COMPOSITE keyword

3. Given the composite type and table:
CREATE TYPE address AS (city text, zip integer);
CREATE TABLE users (id serial, home address);
What will the query return?
SELECT (home).city FROM users WHERE id = 1;

medium

A. An error because you cannot access composite fields like this

B. The zip code of the home address for user with id 1

C. The entire home composite value as text

D. The city name stored in the home column for user with id 1

Solution

1. Step 1: Understand composite field access syntax

   Using (home).city extracts the city field from the composite column home.

2. Step 2: Analyze the query result

   The query selects city from home for user with id 1, so it returns that city name.

3. Final Answer:

   The city name stored in the home column for user with id 1 -> Option D

4. Quick Check:

   (column).field extracts field from composite [OK]

Hint: Use (column).field to get composite field value [OK]

Common Mistakes:

• Trying to access composite fields without parentheses
• Expecting entire composite instead of single field
• Confusing city with zip field

4. Identify the error in this code snippet:
CREATE TYPE product_info AS (name text, price numeric);
CREATE TABLE products (id serial, info product_info);
INSERT INTO products (info) VALUES ('Laptop', 999.99);

medium

A. The serial type is invalid for id column

B. The INSERT statement syntax is incorrect for composite type

C. The table products should not use composite types

D. The CREATE TYPE statement is missing a semicolon

Solution

1. Step 1: Check INSERT syntax for composite types

   When inserting a composite type, values must be wrapped in parentheses or ROW(), not as separate values.

2. Step 2: Identify the error in the INSERT statement

   The statement tries to insert two separate values without wrapping them as a composite, causing syntax error.

3. Final Answer:

   The INSERT statement syntax is incorrect for composite type -> Option B

4. Quick Check:

   Insert composite as (value1, value2) or ROW(...) [OK]

Hint: Wrap composite values in parentheses or use ROW() [OK]

Common Mistakes:

• Inserting composite fields as separate values
• Forgetting parentheses around composite values
• Misunderstanding serial type usage

5. You have a composite type location with fields latitude and longitude. You want to create a function that returns this composite type and use it in a query. Which of the following is the correct way to define the function?

hard

A. CREATE FUNCTION get_location() RETURNS location AS $$ BEGIN RETURN ROW(40.7128, -74.0060); END; $$ LANGUAGE plpgsql;

B. CREATE FUNCTION get_location() RETURNS location AS $$ BEGIN RETURN 40.7128, -74.0060; END; $$ LANGUAGE plpgsql;

C. CREATE FUNCTION get_location() RETURNS TABLE(latitude float, longitude float) AS $$ BEGIN RETURN (40.7128, -74.0060); END; $$ LANGUAGE plpgsql;

D. CREATE FUNCTION get_location() RETURNS location AS $$ BEGIN RETURN '40.7128, -74.0060'; END; $$ LANGUAGE plpgsql;

Solution

1. Step 1: Understand how to return composite types from functions

   Functions returning composite types must return a row value, typically using ROW() constructor.

2. Step 2: Analyze each option

   CREATE FUNCTION get_location() RETURNS location AS $$ BEGIN RETURN ROW(40.7128, -74.0060); END; $$ LANGUAGE plpgsql; correctly uses RETURN ROW(...) to return the composite type. CREATE FUNCTION get_location() RETURNS location AS $$ BEGIN RETURN 40.7128, -74.0060; END; $$ LANGUAGE plpgsql; returns multiple values without row constructor (parentheses or ROW()), causing syntax error. CREATE FUNCTION get_location() RETURNS TABLE(latitude float, longitude float) AS $$ BEGIN RETURN (40.7128, -74.0060); END; $$ LANGUAGE plpgsql; returns a table, not the composite type. CREATE FUNCTION get_location() RETURNS location AS $$ BEGIN RETURN '40.7128, -74.0060'; END; $$ LANGUAGE plpgsql; returns a string, not composite.

3. Final Answer:

   CREATE FUNCTION get_location() RETURNS location AS $$ BEGIN RETURN ROW(40.7128, -74.0060); END; $$ LANGUAGE plpgsql; -> Option A

4. Quick Check:

   Use RETURN ROW(...) for composite return [OK]

Hint: Use RETURN ROW(...) to return composite types in functions [OK]

Common Mistakes:

• Returning values without row constructor
• Returning string instead of composite
• Confusing RETURNS TABLE with RETURNS composite