Exactly. Project is just an operator that takes a table and gives you a table with a different number of attributes. All the relational operations take and retu

Exactly. Project is just an operator that takes a table and gives you a table with a different number of attributes. All the relational operations take and return table values (aka relation values), which is what makes the algebra closed under operations. SQL select actually combines project and map (with select...as) by letting you define maps for the individual attributes of the table. That's fine since you can't break the table structure. You just can't map on tables or table rows.