Selectionselectionσ condition (r): the tuples of r for which the condition is true keeps the tuples of a relation that make a condition true:

σ dept_name = 'Physics' (instructor)

In SQL, the condition goes in WHERE:

SELECT * FROM instructor WHERE dept_name = 'Physics';

Conditions

A condition compares attributes with values or with other attributes, and joins comparisons with logical connectives:

What selection does not do

• It never removes attributes. The result has the same schema as the input.
• It never adds tuples. The result has at most as many tuples as the input.
• Two selections in a row are one selection with ∧:

σ a (σ b (r))  =  σ a ∧ b (r)