1. Static column-level constraint:
Is a description of a range, including the following aspects, data type constraints, including data type, length, unit, precision and so on. Restrictions on data formats; Constraints on the range or set of values; Constraints on null values; Other constraints.
Second, the static tuple constraints:
Is to specify the constraint relationship between columns that make up a tuple, and the static tuple constraint is limited to a single tuple. Static relation constraint means that there are often various connections or constraints between tuples of a relationship or between several relationships.
Third, dynamic column-level constraints:
It means that when modifying the value of a tuple, it is necessary to refer to its old value, and certain constraints must be met between the old and new values. It is also a constraint on the state before and after the relationship changes, such as transaction consistency and atomicity. It is a constraint that should be satisfied when modifying column definitions or column values, including two aspects, namely, the constraint when modifying column definitions and the constraint when modifying column values.
The definition and development status of database;
I. Definition:
A database is a warehouse for storing data. Its storage space is very large, and it can store millions, tens of millions and hundreds of millions of pieces of data. But the database does not store data at will, and there are certain rules, otherwise the query efficiency will be very low. Today's world is an internet world full of data, full of a lot of data.
That is, this internet world is the data world. There are many sources of data, such as travel records, consumption records, web pages visited, messages sent and so on. Besides text data, images, music and sounds are all data.
Second, the development status:
In the history of database development, database has experienced various stages of development, such as hierarchical database, network database and relational database, and database technology has developed rapidly in all aspects. In particular, relational database has become the most important member of database products.
This kind of database is very different from the traditional relational database in design and data structure. They emphasize the high concurrent reading and writing of database data and the storage of big data. This kind of database is generally called NoSQL (not just SQL) database. However, the traditional relational database still maintains strong vitality in some traditional fields.