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Database Basics
BCIS 3680 Enterprise Programming
Database Concepts
A database usually consists of a number of tables.
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It’s often file(s) on a hard drive or other data storage devices.
A database management system (DBMS) is the software
application that is used to manage databases.
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MySQL is a DBMS.
Table
A structured list of data of a specific type.
It describes a certain entity with various attributes.
A table must have a unique name.
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There is debate regarding whether the name should be in
singular or plural form.
Forta uses plural nouns.
My convention is to use singular nouns because it make more
sense when it comes to draw the Entity Relationship Diagram
(ERD) for the database.
Elements in a Table
Each record in a database is stored as a row in one of the
tables, e.g., a new customer is entered as a new row in
the Customer table.
Vertically, a table is made up of columns. A column is also
called a “field.”
Each column defines a certain attribute of a record, e.g.,
name, address, phone number, etc.
All rows in the table have the same number of columns.
For each record, you may enter its attributes as defined
by the various columns.
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Schema
Schema stores the “meta-data” about the table.
It defines the datatype for the information to be stored in
a column – text, numbers, currency values, even large
blobs of binary information.
See Appendix D for a list of MySQL datatypes.
Table and column names cannot be one of the SQL
keywords (Appendix E).
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Example of Schema
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null
A special “value” in database. It means there is no entry
in a particular field.
Regardless of the datatype for the field, null doesn’t
mean –
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0
0.0
Empty string
Space
It just means, there is nothing there.
If you want to force the entry in a field, you set “no
null” for the field in your schema.
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Primary Key
Each row in a table must have a unique identifier so that
it can be stored and retrieved correctly.
This is done by defining a field (or a combination of fields)
as the primary key.
For each record, the value stored in the primary key field
serves as its unique ID.
No two rows in the same table may have the same value
for primary key.
A primary key field cannot be null.
No all unique values are good candidate for PK. For
example, changeable values like email addresses are bad
PKs.
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“Sequel” to What?
Structured Query Language (SQL) is a language designed
to communicate with databases.
There are minor syntax differences in implementations
across different vendors (Oracle, Microsoft, IBM, MySQL
AB, etc.) and even within the same vendor (Microsoft
SQL Server vs. Access).
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E.g., the semicolon is not always mandatory.
Syntax is not case sensitive. However, it is conventional
to capitalize the keywords, e.g., SELECT… FROM…
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It enhances readability of programming code and stored
procedures.
Keywords and Clause
SELECT prod_name, prod_price
FROM products
WHERE prod_price = 2.50;
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The keywords are SELECT, FROM, and WHERE.
The SELECT clause tells us that this is for data retrieval.
The FROM clause specifies which table to look for
information.
The WHERE clause defines the criterion for retrieval.
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