Creating databases and connecting in PostgreSQL - Quick Revision & Summary

Recall & Review

beginner

What is the SQL command to create a new database in PostgreSQL?

The command is CREATE DATABASE database_name;. It creates a new empty database with the given name.

beginner

How do you connect to a specific database using the psql command-line tool?

Use psql -d database_name or psql database_name to connect to that database.

beginner

What is the default database you connect to when you start psql without specifying a database?

By default, psql connects to a database with the same name as your operating system user.

intermediate

Can you create a database while connected to another database in PostgreSQL?

Yes, you can create a new database while connected to any existing database using the CREATE DATABASE command.

beginner

What command do you use to list all databases in PostgreSQL using psql?

Use the meta-command \l or \list inside psql to see all databases.

Which command creates a new database named 'shop' in PostgreSQL?

A CREATE DB shop;

B NEW DATABASE shop;

C MAKE DATABASE shop;

D CREATE DATABASE shop;

The correct syntax to create a database is CREATE DATABASE database_name;.

How do you connect to a database named 'sales' using psql?

A psql -d sales

B connect sales

C psql sales_db

D open database sales

Use psql -d sales or psql sales to connect to the 'sales' database.

What does the command \l do inside psql?

A Lists all tables

B Logs out from psql

C Lists all databases

D Loads a SQL file

The \l command lists all databases available in the PostgreSQL server.

If you start psql without specifying a database, which database do you connect to?

A A database with your OS username

B The 'postgres' database

C The 'template1' database

D No database, connection fails

psql tries to connect to a database named after your operating system username by default.

Can you create a new database while connected to another database in PostgreSQL?

A No, you must disconnect first

B Yes, using CREATE DATABASE command

C Only if you have superuser rights

D Only from the 'postgres' database

You can create a new database from any existing database connection using CREATE DATABASE.

Explain how to create a new database and connect to it using PostgreSQL command line.

Think about the commands you type in the terminal.

Describe how to list all databases in PostgreSQL and why this might be useful.

It's a meta-command inside psql.

Practice

1. What does the CREATE DATABASE command do in PostgreSQL?

easy

A. It deletes a database permanently.

B. It connects to an existing database.

C. It creates a new database to store data.

D. It lists all databases available.

Solution

1. Step 1: Understand the purpose of CREATE DATABASE

   This command is used to create a new database in PostgreSQL where data can be stored.

2. Step 2: Differentiate from other commands

   Connecting to a database uses \c, deleting uses DROP DATABASE, and listing uses \l.

3. Final Answer:

   It creates a new database to store data. -> Option C

4. Quick Check:

   CREATE DATABASE = create new database [OK]

Hint: Remember: CREATE DATABASE makes a new database [OK]

Common Mistakes:

• Confusing CREATE DATABASE with connecting commands
• Thinking it deletes or lists databases
• Using wrong command for creating database

2. Which of the following is the correct syntax to connect to a database named shopdb in PostgreSQL?

easy

A. \c shopdb

B. \\c shopdb

C. CONNECT TO shopdb;

D. USE shopdb;

Solution

1. Step 1: Identify the correct psql command for connecting

   In PostgreSQL's psql tool, the command to connect to a database is \c database_name.

2. Step 2: Check syntax correctness

   \c shopdb uses the correct backslash and no extra backslashes or SQL keywords.

3. Final Answer:

   \c shopdb -> Option A

4. Quick Check:

   Connect command = \c [OK]

Hint: Use single backslash \c to connect in psql [OK]

Common Mistakes:

• Using double backslashes instead of one
• Using SQL commands like USE which is not PostgreSQL
• Adding semicolons to psql meta-commands

3. What will be the result of running the following commands in psql?

CREATE DATABASE testdb;
\c testdb
SELECT current_database();

medium

A. An error because \c cannot be used after CREATE DATABASE

B. It connects to testdb and shows 'testdb' as current database

C. It shows the default database name instead of testdb

D. It creates testdb but does not connect, so current_database() shows nothing

Solution

1. Step 1: Understand command sequence

   The first command creates a new database named testdb. The second command connects to it. The third command queries the current database name.

2. Step 2: Predict output of current_database()

   Since connection switched to testdb, current_database() returns 'testdb'.

3. Final Answer:

   It connects to testdb and shows 'testdb' as current database -> Option B

4. Quick Check:

   CREATE + \c + current_database() = 'testdb' [OK]

Hint: After CREATE DATABASE, use \c to switch and check with current_database() [OK]

Common Mistakes:

• Thinking \c cannot be used after CREATE DATABASE
• Assuming current_database() shows default database
• Believing connection does not change automatically

4. You run the command CREATE DATABASE mydb but get an error. Which of these is the most likely cause?

medium

A. You do not have permission to create a database.

B. You forgot the semicolon at the end of the command.

C. You are already connected to the database you want to create.

D. You used the wrong command; CREATE DATABASE is not valid in PostgreSQL.

Solution

1. Step 1: Check command syntax

   The command CREATE DATABASE mydb is valid and semicolon is optional in some clients but recommended.

2. Step 2: Identify common permission issues

   Most errors creating databases happen because the user lacks the CREATE DATABASE privilege.

3. Final Answer:

   You do not have permission to create a database. -> Option A

4. Quick Check:

   Permission error = CREATE DATABASE fails [OK]

Hint: Check user permissions if CREATE DATABASE fails [OK]

Common Mistakes:

• Assuming missing semicolon always causes error
• Thinking you cannot create database while connected to it
• Believing CREATE DATABASE is invalid in PostgreSQL

5. You want to create a new database named salesdb and immediately connect to it using psql commands. Which sequence is correct?

hard

A. CREATE DATABASE salesdb; CONNECT salesdb;

B. \c salesdb; CREATE DATABASE salesdb;

C. CREATE DATABASE salesdb \c salesdb;

D. CREATE DATABASE salesdb; \c salesdb

Solution

1. Step 1: Understand command order

   You must first create the database, then connect to it. So CREATE DATABASE salesdb; comes before \c salesdb.

2. Step 2: Verify command syntax

   CREATE DATABASE salesdb; \c salesdb uses correct SQL and psql meta-command syntax. \c salesdb; CREATE DATABASE salesdb; tries to connect before creation. CREATE DATABASE salesdb \c salesdb; combines commands incorrectly. CREATE DATABASE salesdb; CONNECT salesdb; uses invalid CONNECT command.

3. Final Answer:

   CREATE DATABASE salesdb; \c salesdb -> Option D

4. Quick Check:

   Create then connect = correct sequence [OK]

Hint: Create first, then connect with \c [OK]

Common Mistakes:

• Trying to connect before database exists
• Using invalid commands like CONNECT
• Combining commands on one line incorrectly