SELECT Clause¶
The SELECT clause indicates the attributes to be obtained from the
relations specified in the FROM clause.
If the character * is specified in the SELECT clause, the
Server will project all the attributes of the queried views. In this
case, the order of the columns in the result will be the order in which
they appear in the CREATE VIEW or CREATE TABLE statement used to
create the view.
Aliases may also be defined for the columns obtained, thus allowing the name of any attribute to be modified.
In the case of derived attributes (see section Derived Attributes), if an alias is not specified, Virtual DataPort will automatically generate a name for the new attribute.
In the queries and views, no two fields with the same name are allowed, so it would be necessary to rename any of them (by using aliases).
The DISTINCT modifier may be included. In this case, all
duplicated tuples will be deleted from the result.
You can project simple boolean expressions like
field = 'value'
Or
field_1 = field_2
Derived Attributes¶
The SELECT clause may include derived attributes. These attributes
are created by evaluating an expression that may use functions,
constants and the values of other attributes.
A description of the functions supported by Virtual DataPort can be found in the section Functions for Conditions and Derived Attributes. Detailed examples of the use of each function can be found in the section Functions.
Some examples of how to use derived attribute functions are shown below.
The following query obtains a column named newSalary containing the
result of adding 1000 to the values contained in the salary
column of the emp view.
SELECT SUM(1000, salary) newSalary
FROM emp;
And the following example shows how to use a nested function as parameter:
SELECT NAME, SUM(SALARY, DIV(SALARY,1000)) salaries
FROM emp;