Showing posts from May, 2018

What is basic SELECT statement?

SELECT Statement A SELECT statement retrieves data from the database. With SELECT statement PROJECTION, SELECTION and JOIN can be performed on database tables. As data in relational database is stored in TABLES. TABLE has ROW and COLUMN structure. With SELECT statement all rows and all columns can be retrieved, this is known as PROJECTION. But, if rows and columns are retrieved on the basis of some condition, it is known as SELECTION. And suppose if some information is needed and that is stored in two or more different tables but are related, use JOINS to retrieve it from two or more tables. In SQL, basic SELECT statements are as follows: SELECT  * FROM  [ table_name ] ; SELECT   [ column1 ] ,  [ column2 ] ,  [ column3 ] ,   .....   FROM   [ table_name ] ; SELECT and FROM are known as clauses.  SELECT clause allows to specify columns to be selected from the database table.  FROM clause allows to specify table name that has those column to be s

Define the terms: domain, tuple, attribute and relation

General Definitions Relation: A relation in a relational database is described as a table. Table is organized into rows and columns. Attribute: A column header of a table is known as attribute of a relation. Tuple: A row in a table represents the record of a relation and known as a tuple of a relation. Domain : A set of possible values for a given attribute is known as domain of a relation. A domain is atomic, that means values are indivisible. For example, set of values for attribute FirstName of an EMPLOYEE relation are atomic.