Difference between Selection and Projection in DBMS
Last Updated :
12 Jun, 2020
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 |
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