Database and Expert System Technology - Information Systems Reengineering, Integration and Normalization - page 46

2.3.2 Structured Query Language (SQL)

SQL was introduced as the standard query language for relational DBMS. The basic
structure of an SQL retrieval command, a Select statement, is as follows:

1 , A 2 , … A n
1 , r 2 …. r n
[where P]
[order by O]
[group by G]
[having H]

All classes within the square brackets are optional. The A i represents attributes, the
r i represents relations, and P is a predicate, and is default to be true. The attribute
A i may be replaced with a asterisk (*) to select all attributes of all relations appear-
ing in the form clause. O is the sort order of the target tuples based upon attribute
values. G is the display group of the target attributes. H is the selection criteria of
the display groups.
For example, if we use the normalized relations as source, we can issue the fol-
lowing select statements:

• Toretrievethestudent#ofallstudents

Select Student# from Student

• Toretrievethestudent#ofallstudentswhoaretakingCS101

Select Student# from Enroled-Student

where Class# = CS101

• Toretrievethestudent#ofallstudentswhoaretakingCS101andwhosegrade
is A

Select Student# from Enroled-Student

where Class# = CS101 and Grade = A