Understanding joins

Hi all

I want to understand something about the join of two tables with a single column.

When you join emp and dept of scott/tiger and write it

Select * from EMP, dept

where emp.deptno = dept.deptno;

What does this mean? say I have the engine to do? , can I tell the motor to "select all the data from the two tables when you see a record in the deptno from emp table" is equivalent to "a record in the deptno in dept table?

This is how it is done?

and if so, how the engine to know if it has values equal or not, should be the first value in the emp.deptno and make a loop on the dept.deptno to know, or how do you?

I want to know what I'm saying the engine 'in English' to understand, and how this is done?

Thank you

Hello

ORA-01400 wrote:

... let's talk about what's going on.

I think I am saying when the deptno in the main table is equal to a record in the secondary table, he gets the master record and the detail record? Yes or no?

You don't need to have tables 'master' and 'detail '. Whenever there is a line in the emp table, he will join every line in the dept table that has the same deptno. If there are 2 dept lines that have the same deptno as a line in emp, so that the rank of emp will join these 2 rows of Dept.

If there are 3 rows in dept with the same deptno as a row in the EMP, so that the rank of emp will join the 3 lines of Dept.

...

If there are N lines in dept with the same deptno as a row in the EMP, so that the rank of emp will join lines N of Dept.

In a join internal (like the one you posted) that goes for N = 0, so if 0 rows in dept have the same deptno like a row of EMP, emp line will appear 0 times in the result set (that is, the row of PGE will be not in the game at all.)

Tags: Database

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Understand the joins with more than one column?

Hi all
I want to know if I understand.
When you join two tables, and the join condition has four columns (two for each table) as follows:
where d.deptno = e.deptno
and d.name = e.name
I think it means the engine must choose "the selection list" when he sees a number of d.deptno (for example 10) equal number in e.deptno, and
the d.name from the same folder (10) must be equal to the value of e.name.
I understand?
Another example, I want to understand, when we say:
where d.deptno = e.deptno
and d.deptno = s.deptno
This means that the number (10) should for example equal to one (10) in the table (e), and the same 10 must be equal to a value in the table (s)?
Am I wrong?
Thank you

Hello

Yes, what you say is right; but don't take my word for it. Try it yourself. Create a couple of small tables. Insert some data that matches (and will appear in the results of joints) and others that does not (and should not be in the results) and see if the results are what you expected.

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