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

Concept Flow - Projection operation

Start with a Table

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Select Columns to Keep

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Remove Other Columns

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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.