Database Basics BCIS 3680 Enterprise Programming

Database Concepts  A database usually consists of a number of tables.  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.  MySQL is a DBMS. 2

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.  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. 3

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. 4

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). 5

Example of Schema 6

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 –  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. 7

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 addresses are bad PKs. 8

“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).  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 …  It enhances readability of programming code and stored procedures. 9

Keywords and Clause 10 SELECT prod_name, prod_price FROM products WHERE prod_price = 2.50;  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.