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Annotation of 42BSD/ingres/doc/quel/view.nr, revision 1.1.1.1

1.1       root        1: .th VIEW QUEL 2/7/79
                      2: .sh NAME
                      3: view \- define a virtual relation
                      4: .sh SYNOPSIS
                      5: .bd define
                      6: .bd view
                      7: name (target-list) [
                      8: .bd where
                      9: qual ]
                     10: .sh DESCRIPTION
                     11: The syntax of the
                     12: .it view
                     13: statement is almost identical
                     14: to the
                     15: .it retrieve
                     16: .it into
                     17: statment;
                     18: however,
                     19: the data is not retrieved.
                     20: Instead,
                     21: the definition is stored.
                     22: When the relation
                     23: .it name
                     24: is later used,
                     25: the query is converted
                     26: to operate on the relations specified
                     27: in the
                     28: .it target-list.
                     29: .s1
                     30: All forms of retrieval on the view
                     31: are fully supported,
                     32: but only a limited set of updates
                     33: are supported
                     34: because of anomolies which can appear.
                     35: Almost no updates are supported
                     36: on views
                     37: which span more than one relation.
                     38: No updates are supported
                     39: that affect a domain in the qualification of the view or
                     40: that affect a domain
                     41: which does not translate into a simple attribute.
                     42: .s3
                     43: In general,
                     44: updates are supported
                     45: if and only if it can be guaranteed
                     46: (without looking at the actual data)
                     47: that the result of updating the view
                     48: is identical to that of updating the corresponding real relation.
                     49: .s1
                     50: The person who defines a view
                     51: must own all relations
                     52: upon which the view is based.
                     53: .sh EXAMPLE
                     54: range of e is employee
                     55: .br
                     56: range of d is dept
                     57: .br
                     58: define view empdpt (ename = e.name, e.sal, dname = d.name)
                     59: .br
                     60:        where e.mgr = d.mgr
                     61: .sh "SEE ALSO"
                     62: retrieve(quel), destroy(quel)

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