Information Technology Reference
In-Depth Information
e. MODIFY record-name-i
The MODIFY statement is issued to change the contents of one or more data
items in a record. It can also change the set membership of a record. For exam-
ple, the following command changes a student's name from John Doe to John W.
Doe.
MOVE “John W. Doe” TO Name.
MODIFY Student.
f. ERASE [ALL] [record-name-i]
To delete a record by the ERSAE statement, the record must be located as the
current record of the run unit. The current record of the run unit is removed pro-
vided that all affected sets are null sets. If ALL is specified and the current of the
run unit is the owner of a non-null set, then all members of the set are removed.
If the ALL option is not specified, then an affected set with member records can
be removed only if its member records have FIXED or OPTIONAL membership
in the set. For example, the following is the command to erase the student record
with student# = 1234.
MOVE 1234 TO student#.
ERASE Student.
2.3
Relational Model
The relational model is a logical schema in the form of tables (relations) corre-
sponding to the representation of an entity type. A column (attribute) of the tables
represents the extension of attributes in the entity. The row (tuple) of the tables
represents instances of the entity. Such tables are commonly called record types
and consist of a non-null primary key that can uniquely identify a tuple. The par-
ent-child relationship of relations is represented in the foreign key residing in the
child relation referencing the primary key of parent relation.
The following are fundamental properties of a relational database:
• ColumnHomogeneous:Foranygivencolumnofarelation,allitemsmustbeof
the same kind whereas items in different columns may not be of the same kind.
• IndivisibleItems:Eachitemisasimplenumberoracharacterstring.Itshould
represent a data element with the simplest form.
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