# Terms and Concepts #### Database **Databases** are the core data storage structures in XINA. A database essentially defines a MySQL table, with additional features managed by the XINA server system. Each database is defined by a set of **fields**, which specify the columns of the table. Fields are primarily defined by: * Name, unique to the field in the database * Static data type * Whether a value must be provided by each record (an empty value being null) A single unit of data in a database is a **record**, corresponding to a row of the table. Each record contains a value for each field of the database. Structural database changes (adding / changing / removing fields) are very slow (hours to days for very large databases) so initial time investment to optimize database requirements is worthwhile. #### Group Databases in XINA are organized into a heirarchical structure of **groups**, which can each contain any number of groups and databases. For example: * The `model` group contains a `journal` database and `data` group * The `data` group contains a `housekeeping` database and `science` database
A dot notation is used to reference groups and databases. For example, `moma.data.science` refers to the science database in the above configuration.