SELECT is the most commonly useed command in SQl. Used to retrieve the data.

The following is the syntax we use to pick records from table(s)

SELECT [DISTINCT] col-name [,col-name] ...
FROM table-name [,table-name]...
[WHERE predicate]
[GROUP BY col-name [,col-name] ... [HAVING predicate]]
[ORDER BY col-name [,col-name] ...];

* denotes all the attributes in a table. For example

SELECT   *
FROM       faculty
WHERE    dept = 'cis';

  is equivalent to

SELECT   facid, facname, dept, rank
FROM       faculty
WHERE    dept = 'cis';

We will use the enroll table for actual enrollment and grades.So to get the listing of courses 
for the corresponding students we have the fpllowing query:

SELECT                     course#
FROM                         enroll;

This will anyhow  yield duplicate courses;hence  to eliminate duplicates,  we use  DISTINCT

SELECT DISTINCT course#
FROM                         enroll; 
gives the listing of distint courses in the enroll table.Further to retreive the values for specific
records the where clause is used.
 
Conditional Selection:The WHERE clause is used to specify that only certain rows of the table are displayed, based on the criteria described in that WHERE clause. It is most easily
understood by looking at a couple of examples. 
Because grouping and aggregation are important in many database applications,SQL has features that incorporate these features.
The aggregate functions(COUNT, SUM, MAX, MIN , AVG) can be used in the SELECT clause or in a HAVING clause
.The functions  MAX and MIN can also be used with attributes that have nonumeric domains if the domain values have a 
"total ordering" among one another.

For example,

SELECT   COUNT (DISTINCT dept)
FROM       faculty;

finds the number of departments which have faculty in them.