Database schemas, domains and constraints |
There is more to a database than just a collection of tables. In addition to tables, there are structural aspects that maintain the integrity of data stored in databases. This tutorial will cover the structural aspect of any relational database. These include schemas, domains and constraints.SchemasA schema is basically the structure of an entire database structure. Some also calls it the conceptual or logical view of the entire database. Basically a schema is made up of tables, relationships and data describing the structure of a database. In computer science jargon, this is called metadata (which in layman terms means "Data about Data"). DomainsEvery attribute or column of a database can only assume a finite number of values. The set of such values is what is called a domain of that attribute or table column. For example; assuming that you sell television sets, and have a database that keeps track of the number of television sets you have in stock and those sold. This table has a column called tv_color where you would store the exterior color of the television such as "black", "grey" and "white". The domain of such a field is the set of these three colors. ConstraintsThese are perhaps one of the most overlooked aspects of a good database. Constraints are very important in making sure that the data stored in the database keeps its integrity. You can do stuff such as preventing people from entering invalid data into a field. For example, if you had the field called tv_color and you only want it to have one of the three colors (Black, Grey and White) as valid data, you could use what is called a check constraint to make sure that the data you are about to enter into the table is one of those colors. The general syntax to do that is as follows:
If somebody attempts to insert the value "Blue" into tv_color, the database system will raise an error, reminding you that the value you entered is not valid. Other constraintsThere are several database constraints that will be discussed in other postings. These include the
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