Selection operation implementation in DBMS Theory - Step-by-Step Execution

Concept Flow - Selection operation implementation

Start with a Relation (Table)

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Apply Selection Condition

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Check each Tuple

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Include Tuple

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Form Result Relation

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End

The selection operation scans each row of a table and keeps only those rows that satisfy a given condition, forming a new table as the result.

Execution Sample

DBMS Theory

SELECT * FROM Employees WHERE Age > 30;

This query selects all rows from the Employees table where the Age column is greater than 30.

Analysis Table

Step	Tuple (Employee)	Condition (Age > 30)	Result Action
1	{ID: 1, Name: 'Alice', Age: 28}	28 > 30? False	Discard
2	{ID: 2, Name: 'Bob', Age: 35}	35 > 30? True	Include
3	{ID: 3, Name: 'Charlie', Age: 30}	30 > 30? False	Discard
4	{ID: 4, Name: 'Diana', Age: 40}	40 > 30? True	Include
5	{ID: 5, Name: 'Evan', Age: 25}	25 > 30? False	Discard
6	All tuples checked	-	Selection complete

💡 All tuples have been checked; selection condition applied to each tuple.

State Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
Current Tuple	None	{ID:1, Age:28}	{ID:2, Age:35}	{ID:3, Age:30}	{ID:4, Age:40}	{ID:5, Age:25}	None
Result Relation	Empty	Empty	[{ID:2, Age:35}]	[{ID:2, Age:35}]	[{ID:2, Age:35}, {ID:4, Age:40}]	[{ID:2, Age:35}, {ID:4, Age:40}]	[{ID:2, Age:35}, {ID:4, Age:40}]

Key Insights - 3 Insights

Why is the tuple with Age 30 not included even though it equals 30?

Because the condition is Age > 30, which means strictly greater than 30. The tuple with Age 30 does not satisfy this, as shown in step 3 of the execution_table.

What happens to tuples that do not meet the condition?

They are discarded and not included in the result relation, as seen in steps 1, 3, and 5 where the action is 'Discard'.

When does the selection operation stop checking tuples?

It stops after all tuples in the original relation have been checked, indicated by step 6 in the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the Result Action for the tuple with Age 40?

A Include

B Discard

C Not checked

D Error

💡 Hint

Refer to step 4 in the execution_table where Age is 40.

At which step does the condition Age > 30 become false for the first time?

A Step 2

B Step 3

C Step 1

D Step 5

💡 Hint

Check the Condition column in execution_table starting from step 1.

If the condition changed to Age >= 30, which tuple's Result Action would change?

A Tuple with Age 28

B Tuple with Age 30

C Tuple with Age 25

D Tuple with Age 40

💡 Hint

Look at step 3 in execution_table where Age is exactly 30.

Concept Snapshot

Selection Operation:
- Filters rows from a table based on a condition.
- Syntax example: SELECT * FROM Table WHERE condition;
- Checks each tuple; includes only those satisfying the condition.
- Result is a new relation with selected tuples.
- Stops after all tuples are checked.

Full Transcript

The selection operation in a database scans each row of a table and applies a condition to decide if the row should be included in the result. For example, selecting employees older than 30 means checking each employee's age and including only those with age greater than 30. The process continues until all rows are checked, forming a new table with only the matching rows. Tuples not meeting the condition are discarded. This operation is fundamental for filtering data in queries.