What are projection and selection?

sql database relational-algebra
prabhahar picture prabhahar · Jun 23, 2009 · Viewed 128.6k times · Source

What is the difference between projection and selection? Is it:

  • Projection --> for selecting the columns of table; and
  • Selection ---> to select the rows of table?

So are projection and selection vertical and horizontal slicing respectively?

Answer

Erich Kitzmueller picture Erich Kitzmueller · Jun 23, 2009

Exactly.

Projection means choosing which columns (or expressions) the query shall return.

Selection means which rows are to be returned.

if the query is 

select a, b, c from foobar where x=3;

then "a, b, c" is the projection part, "where x=3" the selection part.

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