What are the differences between Projection and Selection?

In the context of databases and query languages, "projection" and "selection" are operations that allow you to retrieve specific information from a database table. Here are the key differences between projection and selection:

Definition:

• Projection: It involves selecting specific columns from a table while excluding others. It is the process of creating a subset of the original table with only the columns of interest.
  
• Selection: It involves selecting specific rows from a table based on a certain condition. It is the process of creating a subset of the original table with only the rows that satisfy a specified condition.

Operation Type:

• Projection: It is a horizontal operation because it involves selecting columns, which are the horizontal elements of a table.
• Selection: It is a vertical operation because it involves selecting rows, which are the vertical elements of a table.

Result Content:

• Projection: The result of a projection operation includes all rows but only the specified columns.
• Selection: The result of a selection operation includes all columns but only the specified rows.

Syntax:

• Projection: In SQL, the projection operation is typically expressed using the SELECT statement followed by the list of columns to be retrieved. SELECT column1, column2 FROM table_name;
• Selection: In SQL, the selection operation is expressed using the WHERE clause to specify the condition for filtering rows. SELECT * FROM table_name WHERE condition;

Purpose:

• Projection: Used when you want to focus on specific attributes (columns) of the data and ignore others.
• Selection: Used when you want to filter the data based on certain criteria and retrieve only the relevant rows.

Example:

• Projection: If you have a table with columns "Name," "Age," and "City," projecting on "Name" and "City" would give you a table with only these two columns.
• Selection: If you have a table with a "Salary" column, selecting rows where "Salary" is greater than 50000 would give you a table with only those rows.

In summary, projection involves selecting specific columns, and selection involves selecting specific rows based on a condition. Both operations are fundamental in querying databases to extract the desired information.

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