Projection operation in DBMS Theory - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the SQL query to select only the 'name' column from the 'students' table.

DBMS Theory

SELECT [1] FROM students;

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

AWHERE

B*

Cstudents

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Using '*' selects all columns, not just 'name'.

Using table name instead of column name after SELECT.

The projection operation in SQL is done using the SELECT statement with the column names you want to retrieve. Here, selecting 'name' returns only that column.

2fill in blank

medium

Complete the SQL query to select the 'id' and 'email' columns from the 'users' table.

DBMS Theory

SELECT id, [1] FROM users;

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

Apassword

Bemail

Caddress

Dphone

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting unrelated columns like 'password' or 'phone'.

Forgetting to separate columns with commas.

To project multiple columns, list them separated by commas after SELECT. Here, 'email' is the second column to select.

3fill in blank

hard

Fix the error in the SQL query to select the 'title' column from the 'books' table.

DBMS Theory

SELECT [1] FROM books

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

Atitle;

Btitle

C*

Dbooks

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting the semicolon at the end of the query.

Using table name instead of column name after SELECT.

SQL statements must end with a semicolon. Adding ';' after 'title' fixes the syntax error.

4fill in blank

hard

Fill both blanks to select the 'first_name' and 'last_name' columns from the 'employees' table.

DBMS Theory

SELECT [1], [2] FROM employees;

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

Afirst_name

Bsalary

Clast_name

Ddepartment

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting columns unrelated to names.

Forgetting the comma between column names.

To project multiple columns, list their names separated by commas after SELECT. Here, 'first_name' and 'last_name' are the correct columns.

5fill in blank

hard

Fill all three blanks to select the uppercase 'city' name, the 'population', and only cities with population greater than 100000 from the 'cities' table.

DBMS Theory

SELECT [1](city) AS city_upper, [2] FROM cities WHERE population [3] 100000;

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

AUPPER

Bpopulation

C>

Dcity

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase function names or missing parentheses.

Selecting wrong columns or using wrong comparison operators.

The UPPER function converts city names to uppercase. We select 'population' as the second column. The WHERE clause filters cities with population greater than 100000 using '>'.

Practice

1. What does the projection operation in a database primarily do?

easy

A. Select specific columns from a table

B. Combine rows from two tables

C. Delete rows from a table

D. Sort rows in ascending order

Solution

1. Step 1: Understand the purpose of projection

   Projection is used to choose certain columns from a table, not rows or sorting.

2. Step 2: Compare options with definition

   Only Select specific columns from a table matches the definition of projection as selecting columns.

3. Final Answer:

   Select specific columns from a table -> Option A

4. Quick Check:

   Projection = Selecting columns [OK]

Hint: Projection means picking columns only, not rows or sorting [OK]

Common Mistakes:

• Confusing projection with selection (which filters rows)
• Thinking projection changes row order
• Mixing projection with join operations

2. Which of the following is the correct SQL syntax for projection to get columns name and age from a table students?

easy

A. SELECT * FROM students WHERE name, age;

B. SELECT name age FROM students;

C. SELECT name AND age FROM students;

D. SELECT name, age FROM students;

Solution

1. Step 1: Recall correct SELECT syntax

   The correct syntax uses SELECT followed by column names separated by commas, then FROM table.

2. Step 2: Check each option

   SELECT name, age FROM students; matches the correct syntax: SELECT name, age FROM students;

3. Final Answer:

   SELECT name, age FROM students; -> Option D

4. Quick Check:

   Correct SELECT syntax = SELECT name, age FROM students; [OK]

Hint: Use commas between columns after SELECT, then FROM table [OK]

Common Mistakes:

• Using * instead of column names for projection
• Using AND instead of comma between columns
• Omitting commas between column names

3. Given a table employees with columns id, name, and salary, what will the result of SELECT name FROM employees; be?

medium

A. All rows with only the name column values

B. All rows with id and name columns

C. All rows with all columns

D. Only the first row with name column

Solution

1. Step 1: Understand the SELECT statement

   SELECT name FROM employees returns all rows but only the 'name' column values.

2. Step 2: Compare options with expected output

   All rows with only the name column values correctly describes the output as all rows with only the 'name' column.

3. Final Answer:

   All rows with only the name column values -> Option A

4. Quick Check:

   Projection selects columns, so only 'name' column shown [OK]

Hint: SELECT column returns all rows but only that column [OK]

Common Mistakes:

• Thinking it returns only one row
• Expecting multiple columns without listing them
• Confusing projection with filtering rows

4. Identify the error in this SQL query for projection: SELECT name age FROM employees;

medium

A. SELECT keyword is misspelled

B. Table name is incorrect

C. Missing comma between column names

D. No error, query is correct

Solution

1. Step 1: Analyze the column list syntax

   Column names must be separated by commas in SELECT statements.

2. Step 2: Check the given query

   The query has 'name age' without a comma, which is a syntax error.

3. Final Answer:

   Missing comma between column names -> Option C

4. Quick Check:

   Columns need commas between them [OK]

Hint: Always separate columns with commas in SELECT [OK]

Common Mistakes:

• Forgetting commas between columns
• Assuming space works as separator
• Ignoring syntax errors in column list

5. You have a table orders with columns order_id, customer_id, product, and quantity. You want to create a new table order_summary that only stores order_id and quantity. Which SQL statement correctly uses projection to do this?

hard

A. CREATE TABLE order_summary SELECT * FROM orders;

B. CREATE TABLE order_summary AS SELECT order_id, quantity FROM orders;

C. INSERT INTO order_summary SELECT customer_id, product FROM orders;

D. CREATE order_summary AS SELECT order_id, quantity FROM orders;

Solution

1. Step 1: Understand creating a new table with selected columns

   To create a new table with specific columns, use CREATE TABLE ... AS SELECT with projection.

2. Step 2: Evaluate each option

   CREATE TABLE order_summary AS SELECT order_id, quantity FROM orders; correctly creates the table with only order_id and quantity columns from orders.

3. Final Answer:

   CREATE TABLE order_summary AS SELECT order_id, quantity FROM orders; -> Option B

4. Quick Check:

   Projection + CREATE TABLE AS SELECT = CREATE TABLE order_summary AS SELECT order_id, quantity FROM orders; [OK]

Hint: Use CREATE TABLE ... AS SELECT columns for projection [OK]

Common Mistakes:

• Using SELECT * instead of specific columns
• Omitting CREATE TABLE keyword
• Selecting wrong columns for the new table