A database is a collection of information that is organized so that it can easily be accessed, managed, and updated. In one view, databases can be classified according to types of content: bibliographic, full-text, numeric, and images.
In computing, databases are sometimes classified according to their organizational approach. The most prevalent approach is the relational database, a tabular database in which data is defined so that it can be reorganized and accessed in a number of different ways. A distributed database is one that can be dispersed or replicated among different points in a network. An object-oriented programming database is one that is congruent with the data defined in object classes and subclasses.
Computer databases typically contain aggregations of data records or files, such as sales transactions, product catalogs and inventories, and customer profiles. Typically, a database manager provides users the capabilities of controlling read/write access, specifying report generation, and analyzing usage. Databases and database managers are prevalent in large mainframe systems, but are also present in smaller distributed workstation and mid-range systems such as the AS/400 and on personal computers. SQL (Structured Query Language) is a standard language for making interactive queries from and updating a database such as IBM's DB2, Microsoft's SQL Server, and database products from Oracle, Sybase, and Computer Associates.
Two types of storage systems are available:
Relational Database Management System (RDBMS) and Hierarchical Storage Management System (HSM)
Join methods are of mainly 3 types.
Merge Join: Sorting both the tables using join key and then merge the rows which are sorted.
Nested loop join: It gets a result set after applying filter conditions based on the outer table.Then it joins the inner table with the respective result set.
Hash join: It uses hash algorithm first on smaller table and then on the other table to produce joined columns. After that matching rows are returned.
Components of logical data model are
Entity- Entity refers to an object that we use to store information. It has its own table.
Attribute- It represents the information of the entity that we are interested in. It is stored as a column of the table and has specific datatype associated with it.
Record- It refers to a collection of all the properties associated with an entity for one specific condition, represented as a row in a table.
Domain- It is the set of all the possible values for a particular attribute.
Relation- Represents a relation between two entities.
The logical data model represents database in terms of logical objects, such as entities and relationships.
The physical data model represents database in terms of physical objects, such as tables and constraints.
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