[Last Call] Learn about multicloud storage options and how to improve your company's cloud strategy. Register Now

x
?
Solved

relation between two tables/views

Posted on 2012-04-12
10
Medium Priority
?
491 Views
Last Modified: 2012-04-20
hi guys


In my ER diagram i see

table1  arrow  table2

what does the arrow mean ?

thanks
0
Comment
Question by:royjayd
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
  • 3
  • 2
  • 2
  • +2
10 Comments
 
LVL 74

Accepted Solution

by:
sdstuber earned 548 total points
ID: 37837578
assuming the arrow is from table1 to table2

table1 has a many to one relationship with table2

so table2 is likely the parent of table1
0
 

Assisted Solution

by:gnivkor
gnivkor earned 100 total points
ID: 37837626
that there is a relationship to that table through some sort of constraint, maybe primary / foreign or unique
0
 
LVL 16

Assisted Solution

by:Milleniumaire
Milleniumaire earned 252 total points
ID: 37838045
The fact that the line is dotted means the relationship is optional.  The small vertical line through the dotted line implies that the key of table1 (table on the left) is part of the key of table2 (on the right).

I've never seen an ER diagram that uses arrows on the end of the lines and I can only guess that this is suggesting the direction of the relationship, as described by sdstuber.
0
What is SQL Server and how does it work?

The purpose of this paper is to provide you background on SQL Server. It’s your self-study guide for learning fundamentals. It includes both the history of SQL and its technical basics. Concepts and definitions will form the solid foundation of your future DBA expertise.

 
LVL 23

Expert Comment

by:David
ID: 37838123
Partial correction to gnivkor above, the relationship is one or more columns in column, commonly called a parent to child relationship.  IOW for any given parent table row, there MAY be one or more child table rows.  Constraints are required to enforce relational integrity -- but they are not required.  

royjayd, here's a reference to explain the different relationships shown by crow's-foot notation:  w2.cs.uregina.ca/~bernatja/crowsfoot.html

HTH,
dvz
0
 

Author Comment

by:royjayd
ID: 37838284
My inititial understanding in laymans terms: The table which has primary key (one)  --> maps to table in which that primary key is used as a foreign key (many).
0
 
LVL 23

Assisted Solution

by:David
David earned 500 total points
ID: 37838316
Nods, the parent shares a family name with her/his child(ren).  In this example, there would be a primary (unique) key constraint on the parent, and that value is also stored in each child.  For RI you define a foreign key in the child table, referencing the parent key value.
0
 
LVL 16

Expert Comment

by:Milleniumaire
ID: 37838323
Yes, that is correct in terms of primary and foreign key references.  In terms of the ERD, the many side of the relationship is shown with a "crows foot" and the one side with a single line.
0
 

Author Comment

by:royjayd
ID: 37856989
is Many to One relationship just flipping One to Many relationship ? or is there anything extra to it? For example the above arrow can be used to represent Many-to-One and
One-to-Many relationship aswell, right?

thanks
0
 
LVL 74

Assisted Solution

by:sdstuber
sdstuber earned 548 total points
ID: 37857004
yes

I used the phrase "many to one" based on the direction of the arrow.

but had it been pointing the other way I'd have said "one to many"
0
 
LVL 23

Assisted Solution

by:David
David earned 500 total points
ID: 37857294
Different notations are used for other business rules beyond 1-M, roy.  In your example the bisecting line shows one-or-more.  Replace the "1" line with "0" and (no surprise) it represents zero or more.  The order in which the entities/tables are described may be flipped (sbstuber above, 37867004).

See if this example helps:  a business must have one and only one tax ID number, and a tax ID number is unique to one business.  However, a business must have at least one address; that business MAY have multiple addresses; and an address MAY be used by one or more businesses.
0

Featured Post

Windows Server 2016: All you need to know

Learn about Hyper-V features that increase functionality and usability of Microsoft Windows Server 2016. Also, throughout this eBook, you’ll find some basic PowerShell examples that will help you leverage the scripts in your environments!

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Cursors in Oracle: A cursor is used to process individual rows returned by database system for a query. In oracle every SQL statement executed by the oracle server has a private area. This area contains information about the SQL statement and the…
Using SQL Scripts we can save all the SQL queries as files that we use very frequently on our database later point of time. This is one of the feature present under SQL Workshop in Oracle Application Express.
This video shows, step by step, how to configure Oracle Heterogeneous Services via the Generic Gateway Agent in order to make a connection from an Oracle session and access a remote SQL Server database table.
Video by: Steve
Using examples as well as descriptions, step through each of the common simple join types, explaining differences in syntax, differences in expected outputs and showing how the queries run along with the actual outputs based upon a simple set of dem…

650 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question