To answer our final exam, we need to have knowledge about SQL Joins. Well, I used this cheat sheet to answer that exam. Hehe. Shhhh. :))))
Ø LEFT OUTER JOIN: produces a complete set of records from Table A, with the matching records (where available) in Table B. If there is no match, the right side will contain null.
Ø RIGHT OUTER JOIN: produces a complete set of records from Table B, with the matching records (where available) in Table A. If there is no match, the left side will contain null.
Ø FULL OUTER JOIN: produces the set of all records in Table A and Table B, with matching records from both sides where available. If there is no match, the missing side will contain null.
Ø INNER JOIN: produces only the set of records that match in both Table A and Table B.
I am no blog writer, heck I’m not even a writer at all. I’m not used to sharing my thoughts in writing but I have this personal mission that I should teach my beloved followers tech related stuff and maybe even anything under the sun. Please forgive my writing. I know it’s bad. This is the first time I’ll be maintaining a blog.
So here it goes…
I’m a second year Computer Science student currently taking up CMSC127 (File Processing & Database Systems). On the first day, we were given an assignment to research about ERDs or Entity Relationship Diagrams.
Let me share to you what I've found…
What is an Entity Relationship Diagram (ERD)?
Entity Relationship Diagram is a data modeling technique that shows relationships between information system’s entities though a graphical representation. It is a conceptual model used to represent the entity and database framework.
Why are ERDs important?
Useful communication tool since programmers, developers, and even clients without programming knowledge can easily comprehend the diagram.
It maps out the structure of the database so it can be used to transform into a physical working database
It confirms business rules, a statement/s that defines or constrains some aspects of the relationships between entities of the database.
What are the components of an ERD?
1. Entity is a person, place, object, event, or a concept that an organization wants to maintain data. Entities are represented as rectangles. These could be students, teachers, products, employees, vehicles, etc.
a. Entity Type - is a collection of entities that share common properties
b. Entity Instance - is an individual occurrence of an entity type
2. Attribute is a characteristic of an entity that is relevant to the organization. Attributes are represented as ovals, or it can be listed within entity rectangles. Examples are student’s names, student’s address, teacher’s teacher ID, etc.
a. Multivalued Attributes – attributes that may contain multiple values
b. Required Attributes – attributes that must contain a value for each instance
c. Optional Attributes – attributes that do not need a value for each instance
d. Composite Attributes – attributes that contains meaningful component parts
e. Derived Attributes – attributes whose values are computed from other data in the database
3. Relationships link the various components in an E-R diagram together. It indicates how entities interact with one another. Relationships are represented as diamond/rhombus. The three most common types of relationship degrees are:
a. Unary - between instances of one entity type
b. Binary - between instances of two entity types
c. Ternary - between three entity types
4. Cardinalities in a relationship identify the number of instances of an entity that can (or in some cases, must) associate with another entity. Minimum and maximum cardinality determine the status of relationships, which can be: (1) one-to-one, (2) one-to-many, and (3) many-to-many.
Whew! That nearly used up all my brain power! I know it’s too much to take in but with a little bit of practice, I know we can familiarize ourselves with the concepts pertaining to E-R Diagrams.
Honestly, I’m kinda tired now. I want to rest. But with this information I have shared with you, it will be enough for you to watch out for the next one. See you on the next post! :D
This is my first blog. I will be using this blog to mostly talk about tech related stuff. All my discoveries and learning will be posted here. I know we'll have fun having intellectual discussions about tech. :D
Utvecklare av Teknik @leetguiltyspark-blog - Tumblr Blog | Tumgag
