psql command-line tool basics in PostgreSQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this psql command?

You run the command \dt inside the psql tool connected to a database with three tables: users, orders, and products. What will be displayed?

PostgreSQL

\dt

A A list of all databases available on the server

B An error saying '\dt' is not recognized

C A list showing the three tables: users, orders, products with their schema and type

D The content of the users table

💡 Hint

The \dt command lists tables in the current database.

The \dt command in psql shows all tables in the current database schema. It does not show databases or table contents.

📝 Syntax

intermediate

Which psql command correctly connects to a database named 'shopdb'?

You want to connect to the database named 'shopdb' using psql. Which command will do this?

A psql -d shopdb

B psql connect shopdb

C psql --database=shopdb

D psql shopdb

💡 Hint

psql accepts the database name as the first argument or with -d option.

The simplest ways to connect to a database are 'psql shopdb' or 'psql -d shopdb'. Option C is invalid syntax; option B is invalid.

🔧 Debug

advanced

Why does this psql command fail?

You run the command SELECT * FROM users\; inside psql and get a syntax error. Why?

PostgreSQL

SELECT * FROM users\;

A The semicolon is missing at the end of the statement

B psql commands cannot be run inside SQL statements

C The table 'users' does not exist

D The backslash before the semicolon is invalid syntax in SQL

💡 Hint

Backslash is used for psql commands, not inside SQL statements.

The backslash before the semicolon is not valid SQL syntax. The semicolon alone ends the statement. Backslash commands are separate from SQL.

🧠 Conceptual

advanced

What does the psql command \x do?

You type \x in the psql prompt. What is the effect?

A It toggles expanded output mode, showing query results vertically

B It exits the psql tool

C It shows the current database connection info

D It lists all indexes in the current database

💡 Hint

Expanded mode changes how query results are displayed.

The \x command toggles expanded output mode, which shows each column on its own line for easier reading of wide tables.

❓ optimization

expert

Which psql command efficiently imports a large SQL file named 'backup.sql' into the current database?

You want to load a large SQL dump file 'backup.sql' into your current database using psql. Which command is best?

A \i backup.sql

B \load backup.sql

C psql -f backup.sql

D \copy backup.sql

💡 Hint

The \i command runs SQL commands from a file inside psql.

The \i command inside psql reads and executes SQL commands from a file. Option A runs psql from shell, not inside psql. \copy is for data import/export, not SQL scripts. \load is for loading shared libraries.

Practice

1. What is the primary purpose of the psql command-line tool in PostgreSQL?

easy

A. To connect to a PostgreSQL database and run SQL commands interactively

B. To create new PostgreSQL database clusters

C. To backup PostgreSQL databases automatically

D. To monitor server hardware performance

Solution

1. Step 1: Understand the role of psql

   psql is a command-line interface tool designed to connect to PostgreSQL databases and allow users to run SQL commands interactively.

2. Step 2: Differentiate from other tools

   Other options like creating clusters or backups are done by different tools or commands, not psql.

3. Final Answer:

   To connect to a PostgreSQL database and run SQL commands interactively -> Option A

4. Quick Check:

   psql = interactive SQL client [OK]

Hint: Remember: psql = PostgreSQL interactive shell [OK]

Common Mistakes:

• Confusing psql with backup tools
• Thinking psql manages hardware
• Assuming psql creates database clusters

2. Which of the following is the correct way to start psql and connect to a database named mydb as user alice?

easy

A. psql start mydb alice

B. psql --connect mydb alice

C. psql connect mydb user alice

D. psql -d mydb -U alice

Solution

1. Step 1: Recall psql connection syntax

   The correct syntax to connect is psql -d database -U user.

2. Step 2: Match options to syntax

   psql -d mydb -U alice matches this syntax exactly, while others use invalid or unsupported flags.

3. Final Answer:

   psql -d mydb -U alice -> Option D

4. Quick Check:

   psql -d db -U user = connect [OK]

Hint: Use -d for database and -U for user [OK]

Common Mistakes:

• Using unsupported flags like --connect
• Writing commands without dashes
• Mixing command words incorrectly

3. Inside psql, what will be the result of running the command \dt?

medium

A. It lists all tables in the current database

B. It deletes the current table

C. It displays the current database connection info

D. It shows the last query execution plan

Solution

1. Step 1: Understand psql meta-commands

   Commands starting with a backslash (\) are special psql commands. \dt lists tables.

2. Step 2: Identify what \dt does

   \dt shows all tables in the current database schema, not deleting or showing connection info.

3. Final Answer:

   It lists all tables in the current database -> Option A

4. Quick Check:

   \dt = list tables [OK]

Hint: Backslash commands show info; \dt lists tables [OK]

Common Mistakes:

• Thinking \dt deletes tables
• Confusing \dt with connection info commands
• Expecting \dt to show query plans

4. You try to run psql -d mydb -U alice but get an error: psql: FATAL: password authentication failed for user "alice". What is the most likely cause?

medium

A. The database mydb does not exist

B. The psql command is misspelled

C. The password entered for user alice is incorrect or missing

D. The PostgreSQL server is not running

Solution

1. Step 1: Analyze the error message

   The error says "password authentication failed for user alice," indicating a login problem.

2. Step 2: Identify the cause

   This usually means the password is wrong or not provided, not that the database or command is missing.

3. Final Answer:

   The password entered for user alice is incorrect or missing -> Option C

4. Quick Check:

   Password error = authentication failed [OK]

Hint: Password errors mean wrong or missing password [OK]

Common Mistakes:

• Assuming database doesn't exist from this error
• Thinking psql command is wrong
• Ignoring server status

5. You want to list all databases available on your PostgreSQL server using psql. Which command should you run inside the psql prompt?

hard

A. SHOW DATABASES;

B. \l

C. \listdb

D. SELECT * FROM pg_databases;

Solution

1. Step 1: Recall psql meta-commands for listing databases

   The command \l (or \list) lists all databases in psql.

2. Step 2: Evaluate other options

   SHOW DATABASES; is MySQL syntax, \listdb is invalid, and SELECT * FROM pg_databases; is incorrect because the catalog table is pg_database, not pg_databases.

3. Final Answer:

   \l -> Option B

4. Quick Check:

   \l = list databases [OK]

Hint: Use \l inside psql to list databases [OK]

Common Mistakes:

• Using MySQL syntax SHOW DATABASES;
• Typing wrong meta-command \listdb
• Querying wrong system table name