先决条件 – 关系代数
1. 选择：
此操作从满足选择语法中提到的给定条件的关系中选择元组的子集。
符号 –
σc (R)这里，’c’ 是选择条件，’σ (sigma)’ 用于表示Select Operator 。
2. 投影：
此操作选择某些必需的属性，同时丢弃其他属性。
符号 –
πA (R)其中’A’是属性列表，它是来自relation(R)属性的期望属性集，
符号 ‘π(pi)’ 用于表示项目运算符，
R 通常是一个关系代数表达式，它导致一个关系。
DBMS中选择和投影的区别
| 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 |