问题
The image down below shows a small database,
I got a question
Give a relational algebra expression to find the name of the judge who presides over the trial for defendant known as “Mike” starting on 1.12.2012.
And the answer given is
But I think it should Join JUDGE table as well something like this :
Please leave some comment if I' wrong about this
回答1:
Nope, you don't need to join the JUDGE table to get the judge's name.
Notice how Jname is the primary key of JUDGEs, that is to say, a judge is uniquely identified by her name; and the TRIAL table will actually contain Jname denoting which judge presided over a given trial:
Judge | Defendant | Number | Sdate
Dredd | 1 | 1 | 1.12.2012
Dredd | 2 | 2 | 2.12.2012
Rico | 1 | 3 | 1.12.2012
You can see how selecting tuples yields us the judge's name without joining anything in. (In case you're wondering, this is a very bad design that you should never employ in the real world.)
In contrast, we do have to join DEFENDANT with TRIAL: a DEFENDANT's primary key is his ID, so only that ID appears in the TRIAL table, so if we need to select a defendant by name, we have to employ a join.
As for your question title: yes, you can of course join more than three tables. If you find it hard to imagine, recall that R ⨝ S ⨝ T = (R ⨝ S) ⨝ T. Note that natural join is nicely commutative and associative as well.
来源：https://stackoverflow.com/questions/19740069/can-i-join-3-table-using-natural-join