selection


Should this be selection or projection


I have am learning relational algebra. I have a table,
Staff(name,salary,position)
Now a question states,
List all the managers with salary >25,000
Now the correct answer is,
selection position = manager ^ salary >25000(staff)
But I was thinking that it was,
projection position = manager ^ salary >25000(staff)
As we are selecting a particular attribute in the table and not selecting all the attributes. Please guide. Thanks
You want to "list all the managers", that means "select a number of rows".
You are not "selecting a particular attribute", but you are filtering on a particular attribute (to select a row).
Projection would be "list all the managers' names and salaries".
In terms of SQL:
Selection is the WHERE clause
Projection is the list of columns in the SELECT clause

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