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Selection is an operation in DBMS that is used to filter rows (records) from a table based on a specific condition.
It is represented by the symbol σ (sigma) in relational algebra.
Suppose you have a table named Students:
| ID | Name | City |
|---|---|---|
| 1 | Ravi | Mumbai |
| 2 | Anita | Pune |
| 3 | Rahul | Mumbai |
If you want only students from Mumbai, selection is used.
SQL Query:
SELECT * FROM Students WHERE City = 'Mumbai';
Output:
| ID | Name | City |
|---|---|---|
| 1 | Ravi | Mumbai |
| 3 | Rahul | Mumbai |
Selection works on rows, not columns.
Projection is an operation used to select specific columns from a table, ignoring the rest.
It is represented by the symbol π (pi) in relational algebra.
Using the same Students table:
If you only want student names:
SQL Query:
SELECT Name FROM Students;
Output:
| Name |
|---|
| Ravi |
| Anita |
| Rahul |
Projection works on columns, not rows.
| Feature | Selection | Projection |
|---|---|---|
| Operation On | Rows (records) | Columns (attributes) |
| Purpose | Filter data | Choose specific fields |
| Symbol | σ (Sigma) | π (Pi) |
| SQL Equivalent | WHERE clause | SELECT column names |
| Data Reduction | Reduces number of rows | Reduces number of columns |
Both operations are fundamental in relational algebra and are widely used in SQL queries.
If you want to deeply understand DBMS concepts like selection, projection, joins, normalization, and SQL queries, practical training is essential.
You can explore:
This training focuses on real-world database usage, query writing, and hands-on projects to make you job-ready.
Selection and projection are basic yet powerful operations in DBMS. While selection filters rows based on conditions, projection helps you choose only the required columns.
Understanding both is essential for writing efficient SQL queries and managing databases effectively.
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