Prerequisite – Relational Algebra
1. Selection :
This operation chooses the subset of tuples from the relation that satisfies the given condition mentioned in the syntax of selection.
Notation –
σc (R)
Here, ‘c’ is selection condition and ‘σ (sigma)’ is used to denote Select Operator.
2. Projection :
This operation selects certain required attributes, while discarding other attributes.
Notation –
πA (R)
where ‘A’ is the attribute list, it is the desired set of attributes from the attributes of relation(R),
symbol ‘π(pi)’ is used to denote the Project operator,
R is generally a relational algebra expression, which results in a relation.
Difference between Selection and Projection in DBMS
S. No. | Category | Selection | Projection |
---|---|---|---|
1. | Other Names | The selection operation is also known as horizontal partitioning. | The Project operation is also known as vertical partitioning. |
2. | Use | It is used to choose the subset of tuples from the relation that satisfies the given condition mentioned in the syntax of selection. | It is used to select certain required attributes, while discarding other attributes. |
3. | Partitioning | It partitions the table horizontally. | It partitions the table vertically. |
4. | Which used first | The selection operation is performed before projection (if they are to be used together). | The projection operation is performed after selection (if they are to be used together). |
5. | Operator Used | Select operator is used in Selection Operation. | Project operator is used in Projection Operation. |
6. | Operator Symbol | Select operator is denoted by Sigma symbol. | Project operator is denoted by Pi symbol. |
7. | Commutative | Selection is commutative. | Projection is not commutative. |
8. | Column Selection | Select is used to select all columns of a specific tuple. | Project is used to select specific columns. |
9. | SQL Statements used | SELECT, FROM, WHERE | SELECT, FROM |