Reprinted Source: https://blog.csdn.net/weixin_46218781/article/details/104698996
Connection of relational algebra is an important but easily confused knowledge, I summarize the knowledge associated with the connection point by referring to a lot of information, and found a relationship between them. Through theoretical knowledge to understand the meaning of important terms related to the connection, and then to understand the relationship between the idea of drawing connections and drawing them through.
Theoretical knowledge
definition:
A Cartesian product of
two, connected to [theta]
(a) equivalent connections
(ii) Non-equijoins
[theta] is not "=" operator is referred to as a non-connection equivalent connection.
Third, the natural connection
V. outer join
(a) left outer join (Left outer join / left join)
If only the left relation R to abandon tuple retention called a left outer join on the basis of natural connection.
(B) the right outer join (rightouter join / right join)
if only to give up the right to be in relationship S tuples connected on the basis of natural reservations called right outer joins.
(C) the full outer join (fullouter join / full join)
left and right tables do not limit, all the records have shown that less than two-table place filled with null.
Paint
Title
a Cartesian product of
two, connected to [theta]
(a) equivalent connections
(ii) Non-equijoins
three natural join
five external connection
