The basic operation of the relational algebra
Relational algebra operations are divided into a set of operations and pure relational operations:
and compatibility
Some relational algebra operations, and such difference, cross, etc. must meet "and compatibility."
There is a certain correspondence relationship between the two involved in the operation and their associated attributes, associations or meaning comparability.
R & lt (A1, A2, ..., An), S (Bl, B2, ..., Bm)
R & lt meet and compatibility and S: n = m and Domain (Ai) = Domain (Bi ).
"And" Operation
R ∪S and S ∪R same.
"Poor" Operation
R-S and S-R are different.
"Cartesian product" operation
number of tuples of a relation R 3, is 3 degrees; Relationship between the number of tuples S is 4, the degree is 3; X is the number of tuples S is R 12, is 6 degrees.
Two relations R and S, the number of attributes thereof, respectively n and m (n is the degree of relation R, S m is the degree of relationship), the number of tuples which are x and y (x cardinality relation R, S base y),
then the Cartesian product of the number of properties R x S = n-m + , the number of tuples = X × Y .
R and X are all tuples S S R are concatenated to each tuple.
S and X R are all tuples in R S is concatenated to each tuple. The result is the same.
"Select" operation
priorities operator attention, from high to low priority order is:
"projection" operation
duplicate tuples if the projector, should be removed.
(Projection operation singling columns from a new relationship in the relationship given, the selecting operation from some lines are selected in a given relationship to form new relations).
The basic relational algebra operations の Summary
Extension of relational algebra operations
"Pay" operation
"Connect" operation
"Equivalence - connection" operation (a special case of the above operation)
"Natural connection" operation
NATURAL equivalent connection is a special connection.
R, same or S, must be equal to the value of the connection, i.e. R.B1 = S.B1 and R.B2 = S.B2 ... and R.Bn = S.Bn to connect.
To remove duplicate attribute columns in results (because the results are always equal S.Bi R.Bi it is possible to keep only one)
Extension of relational algebra operations の Summary
Complex relational algebra of extended operation
"In addition to" Operation
Mathematical description:
"external connection" operation
When two relations R and S are connected, if the relationship of R (or S) of the tuples in S (or R), found matching tuple, the tuple is in order to avoid loss of information, so that the element group S (or R) is assumed to exist tuples formed are all connected to null, the result is placed in the relationship, this connection is called an outer connector (outer Join).
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(Pictures, order from China University of MOOC)
