Public schema vs custom schemas in PostgreSQL - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all tables from the default schema in PostgreSQL.

PostgreSQL

SELECT tablename FROM pg_catalog.pg_tables WHERE schemaname = '[1]';

Drag options to blanks, or click blank then click option'

Apublic

Bcustom

Cdefault

Dmain

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'default' or 'main' instead of 'public' as the schema name.

Confusing schema with database name.

The default schema in PostgreSQL is named public. This query lists all tables in that schema.

2fill in blank

medium

Complete the code to create a new schema named 'sales'.

PostgreSQL

CREATE SCHEMA [1];

Drag options to blanks, or click blank then click option'

Apublic

Bdefault

Csales

Dmain

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to create a schema named 'public' which already exists.

Using keywords like 'default' or 'main' which are not schema names.

To create a custom schema named sales, use CREATE SCHEMA sales;.

3fill in blank

hard

Fix the error in the code to select from a table named 'orders' in the 'sales' schema.

PostgreSQL

SELECT * FROM [1].orders;

Drag options to blanks, or click blank then click option'

Apublic

Bmain

Cdefault

Dsales

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'public' when the table is in 'sales' schema.

Omitting the schema name causing an error if search_path doesn't include it.

To select from a table in a custom schema, prefix the table name with the schema name, like sales.orders.

4fill in blank

hard

Fill both blanks to set the search path to use the 'sales' schema first, then the default schema.

PostgreSQL

SET search_path TO [1], [2];

Drag options to blanks, or click blank then click option'

Asales

Bpublic

Cdefault

Dmain

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'default' or 'main' which are not valid schema names.

Reversing the order and putting 'public' first.

The search_path controls which schemas PostgreSQL looks in first. Setting it to sales, public means it looks in sales first, then public.

5fill in blank

hard

Fill all three blanks to grant usage on the 'sales' schema to user 'alice' and allow her to create tables.

PostgreSQL

GRANT [1], [2] ON SCHEMA [3] TO alice;

Drag options to blanks, or click blank then click option'

AUSAGE

BCREATE

Csales

DSELECT

Attempts:

3 left

💡 Hint

Common Mistakes

Granting 'SELECT' which is for tables, not schemas.

Using the wrong schema name like 'public' instead of 'sales'.

Granting USAGE allows access to the schema, and CREATE allows creating tables inside it. The schema name is sales.

Practice

1. What is the role of the public schema in PostgreSQL?

easy

A. It is a special schema that cannot contain tables or other objects.

B. It is the default schema where database objects are created if no schema is specified.

C. It is used only for system tables and cannot be modified by users.

D. It automatically encrypts all data stored within it.

Solution

1. Step 1: Understand the default schema concept

   In PostgreSQL, the public schema is created by default in every new database.

2. Step 2: Identify the role of the public schema

   When you create tables or other objects without specifying a schema, they go into the public schema automatically.

3. Final Answer:

   It is the default schema where database objects are created if no schema is specified. -> Option B

4. Quick Check:

   Default schema = public [OK]

Hint: Remember: public schema is default for new objects [OK]

Common Mistakes:

• Thinking public schema is read-only
• Assuming public schema is for system tables only
• Believing public schema encrypts data automatically

2. Which of the following is the correct way to create a new schema named sales in PostgreSQL?

easy

A. CREATE SCHEMA sales;

B. CREATE DATABASE sales;

C. NEW SCHEMA sales;

D. MAKE SCHEMA sales;

Solution

1. Step 1: Recall the SQL syntax for creating schemas

   The correct SQL command to create a schema is CREATE SCHEMA schema_name;.

2. Step 2: Check each option

   CREATE SCHEMA sales; uses the correct syntax. MAKE SCHEMA sales;, CREATE DATABASE sales;, and NEW SCHEMA sales; are invalid SQL commands for creating schemas.

3. Final Answer:

   CREATE SCHEMA sales; -> Option A

4. Quick Check:

   CREATE SCHEMA = create new schema [OK]

Hint: Use CREATE SCHEMA to make new schemas [OK]

Common Mistakes:

• Using CREATE DATABASE instead of CREATE SCHEMA
• Using non-existent commands like NEW SCHEMA or MAKE SCHEMA
• Forgetting the semicolon at the end

3. Given the following commands:

CREATE SCHEMA marketing;
CREATE TABLE marketing.campaigns (id SERIAL PRIMARY KEY, name TEXT);
SET search_path TO marketing, public;
SELECT * FROM campaigns;

What will be the result of the SELECT query?

medium

A. It will return rows from a campaigns table in the public schema if it exists.

B. It will cause an error because the table name is not schema-qualified.

C. It will return all rows from the marketing.campaigns table.

D. It will return no rows because search_path is set incorrectly.

Solution

1. Step 1: Understand search_path usage

   The search_path is set to look first in marketing schema, then public.

2. Step 2: Identify which table is found

   The table campaigns exists in marketing schema, so the query finds and returns data from marketing.campaigns.

3. Final Answer:

   It will return all rows from the marketing.campaigns table. -> Option C

4. Quick Check:

   search_path order finds marketing first [OK]

Hint: search_path order decides which schema is used first [OK]

Common Mistakes:

• Assuming table must be schema-qualified always
• Thinking query errors without schema prefix
• Ignoring search_path order

4. You run this command:

SELECT * FROM sales.orders;

But get the error: ERROR: schema "sales" does not exist.
What is the most likely cause?

medium

A. The sales schema has not been created yet.

B. The orders table does not exist in the public schema.

C. You forgot to set the search_path to include sales.

D. You need to use double quotes around sales.

Solution

1. Step 1: Analyze the error message

   The error says the schema "sales" does not exist, meaning PostgreSQL cannot find that schema.

2. Step 2: Identify the cause

   This usually means the schema was never created or was dropped, so the database has no knowledge of it.

3. Final Answer:

   The sales schema has not been created yet. -> Option A

4. Quick Check:

   Schema missing causes this error [OK]

Hint: Check if schema exists before querying it [OK]

Common Mistakes:

• Assuming search_path affects schema existence errors
• Thinking table absence causes schema not found error
• Trying to fix with quotes unnecessarily

5. You want to organize your database by creating two schemas: hr and finance. You create tables with the same name employees in both schemas. How can you write a query to select all employees from the finance schema without changing the search_path?

hard

A. SELECT * FROM public.employees WHERE schema = 'finance';

B. SET search_path TO finance; SELECT * FROM employees;

C. SELECT * FROM employees WHERE schema = 'finance';

D. SELECT * FROM finance.employees;

Solution

1. Step 1: Understand schema-qualified table names

   When tables have the same name in different schemas, you can specify the schema explicitly before the table name.

2. Step 2: Identify the correct query

   SELECT * FROM finance.employees; uses finance.employees which directly accesses the table in the finance schema without changing search_path.

3. Final Answer:

   SELECT * FROM finance.employees; -> Option D

4. Quick Check:

   Schema-qualified names access tables directly [OK]

Hint: Use schema.table to access specific schema tables [OK]

Common Mistakes:

• Trying to filter by schema column which doesn't exist
• Changing search_path unnecessarily
• Assuming public schema contains all tables