In optics, projection refers to projecting the shape of an object onto a plane with a set of rays, which is called "projection". The image obtained on this plane is also called "projection". Projection can be divided into orthographic projection and oblique projection. Orthographic projection means that the center line of the projection line is perpendicular to the projection plane, and its projection center line is not perpendicular to the projection plane, which is called oblique projection.
In the computer: In SQL, the operation of selecting some columns from a table is called projection.
R is the k-ary relation, ai 1, ai2, ..., and the target is i 1, I2, ..., R (I, J ≤ K, J = 1, ..., m).
Ti 1, Ti2, ..., Tim are i 1, I2, ..., IM attribute values, then the relation R is projected to the attribute sequence AI 1, AI2, ..., aiming.
Shadow is an m-element relation, and its attribute set is {Ai 1, Ai2, …, Aim}.
The projection operation selects some attributes from the set of attributes (fields) of the specified relationship to form a new similar relationship. Duplicate tuples due to attribute reduction are automatically deleted. Projection operations are for attributes.
In SQL, the operation of selecting some columns from a table is called projection.