Database Properties
The heart of any database is the tables, because the tables contain the data that is used to provide information to the end users. Tables are contained in databases, and in MySQL each database has an associated "data directory" to store all the physical components of the database, to include tables. This physical representation on disk of the tables is dependent on the type of storage engine that the table is utilizing; however, every table, no matter of the type of storage engine that is being used, contains a format file (table_name.frm) which describes the table definition and structure. In addition to the physical representation of the table, there is also the better known logical aspect of the table. This logical aspect is represented in rows and columns. Rows within a table, consist of a collection of values that describe an entity (ex. an employee). Columns within a table, consist of a collection of similar data among rows (ex. employee surnames). The intersection of row and column contains individual data items called values. Values are always atomic, in that each position in a table may contain only one datum
(piece of data). Without the ability to obtain information from the database, there is no need for the data to be stored.
However, the tables are of little value unless they are contained in a unified and associated manner that allow for the storage, retrieval and manipulation of the data. This is where the database comes in. For its part, the database can be thought of as the container for the tables (which are the containers for the data). In MySQL, databases manage their associated components in locations called "data directories". These data directories (meaning there can be more than one database that the MySQL server can manage) are located in a central sub-directory of the mysql installation and can be seen through the operating system on which the
MySQL Server is running. MySQL "data directories" have the following common structures:
• Data Directory - In MySQL, there is a parent directory for all the databases that are managed by that instance of the MySQL Server.
• Database Name - The database name given to the represent the database in the MySQL server is also the name of the associated data sub-directory. This limits the naming conventions that can be used when giving databases a name.
• Database Components - The data directory is responsible for managing the majority of
component associated with the associated database in the MySQL server. This includes such
components as the tables, stored procedures, triggers, views, etc.
• Database Character Set Support - Each database is defined with a default character set and
collation (a character set is a set of symbols and encodings and a collation is a set of rules for
comparing characters in a character set). This behavior ensures that each table created within the database takes on the same characteristics associated with the default values; however, tables themselves can also be created with different character sets and collations than their database container.
• Database Hierarchy - Databases are "flat" in the sense that they can not be nested, meaning a database can not contain a database itself.
• Unlimited Databases - The MySQL server has no limit on the number of databases that it can
contain; however, the operating system may limit the number of sub-directories that a directory
can contain thus limiting the number of databases that can be created.
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