Main concepts about database tables |
For one table several files are created which contain data, indexes, keys etc. Name of the main file of the table (file containing the data) - it is a table name - is set at creation of the table. The names of remaining files of the table are assigned automatically, and the names of all these files coincide with the table name, except of extensions of names; the different extensions indicate onto contents of the appropriate file. Each DB table consisting of lines and columns is intended for a storage of information about one-type objects of the system. The table line is named as record, and table column - field. Keys and indexes can be defined in the tables. Key is a combination of fields, the data in which uniquely define each record in the table. The simple key consists of one field, and complex (composite) key - from several fields. Fields, on which the key is built, are named key fields. The key serves for unique identification of the table records, and also for preventing repetitions of the key values. Thus the values of separate fields of a composite key can repeat. Such key is named as a primary key. Index, as well as a key, is created by the table fields, however it can admit the values repetition of components of its fields. Fields, on which the index is built, are named index fields. The indexes are named at their creation. The index serves for tables sorting according index fields. Besides at usage of indexing, the search rate of data in the table become s higher. The following table lists the types of fields of dBASE tables, and also the description of values that can contain a field of viewed type.
The following table lists the fields types of MS SQL tables.
The table contains a list of fields types of MySQL tables.
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