Projection operation in DBMS Theory - Step-by-Step Execution

Concept Flow - Projection operation

Start with a Table

↓

Select Columns to Keep

↓

Remove Other Columns

↓

Create New Table with Selected Columns

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Result: Table with Only Chosen Columns

Projection operation takes a table and creates a new table with only the selected columns, removing all others.

Execution Sample

DBMS Theory

Table: Students(ID, Name, Age, Grade)
Projection: Select columns Name, Grade
Result: Table with columns Name, Grade only

This operation extracts only the Name and Grade columns from the Students table.

Analysis Table

Step	Input Table Columns	Columns Selected	Action	Output Table Columns
1	ID, Name, Age, Grade	Name, Grade	Identify columns to keep	Name, Grade
2	ID, Name, Age, Grade	Name, Grade	Remove columns ID, Age	Name, Grade
3	N/A	N/A	Create new table with selected columns	Name, Grade

💡 Projection complete: only selected columns remain in the output table.

State Tracker

Variable	Start	After Step 1	After Step 2	Final
Input Table Columns	ID, Name, Age, Grade	ID, Name, Age, Grade	ID, Name, Age, Grade	N/A
Selected Columns	N/A	Name, Grade	Name, Grade	Name, Grade
Output Table Columns	N/A	N/A	N/A	Name, Grade

Key Insights - 2 Insights

Why do we remove columns instead of just copying the whole table?

Because projection only keeps the selected columns, removing others to focus on needed data, as shown in execution_table rows 2 and 3.

Does projection change the number of rows in the table?

No, projection keeps all rows but only changes which columns are shown, as seen in the output table columns staying the same in number of rows.

Visual Quiz - 3 Questions

Look at the execution table, what columns remain after Step 2?

A ID, Name, Age, Grade

B ID, Age

C Name, Grade

D No columns remain

💡 Hint

Check the 'Output Table Columns' column in row for Step 2 in execution_table.

At which step is the new table with only selected columns created?

A Step 3

B Step 2

C Step 1

D Projection does not create a new table

💡 Hint

Look at the 'Action' column in execution_table to find when the new table is created.

If we select columns ID and Age instead, what would be the output columns after Step 3?

A Name, Grade

B ID, Age

C ID, Name, Age, Grade

D No columns

💡 Hint

Refer to variable_tracker for how selected columns affect output columns.

Concept Snapshot

Projection operation:
- Select specific columns from a table
- Remove all other columns
- Keep all rows unchanged
- Result is a new table with only chosen columns
- Used to focus on needed data fields

Full Transcript

Projection operation in databases means selecting certain columns from a table and creating a new table with only those columns. The process starts with the original table, then identifies which columns to keep, removes the rest, and finally outputs a new table with only the selected columns. The number of rows stays the same, only columns change. This helps focus on relevant data without altering the original table structure.

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