range of t
is r range of t
is r ...
range of t
is r retrieve (t
.A
, t
.A
, ..., t
.A
)where P
QBE is a DML whose query language:
The QBE system displays "skeleton tables" - relations with just the attributes (column headers of the tables). The user specifies a query by filling in fields of one or more skeleton tables.
Microsoft Access supports a (very nonstandard) variant of QBE.
Quel is the original query language for the Ingres database system. It is based on tuple relational calculus.
Basic form of queries:
range of t is r
range of t is r
...
range of t is r
retrieve (t.A , t.A , ..., t.A )
where P
This is equivalent to the trc query:
t 
t
r
t
r
t
r
P
t[A
t[A
t[A
t[A
...
t[A
t[A
except that the predicate P would be slightly different.
Examples:
Find the name and address of all customers who have purchased a flyrod with flyrod-stock-num of 3.
range of t is customer
range of s is purchased
retrieve (t.name, t.address)
where t.cust-num = s.cust-num and
s.flyrod-stock-num = 3
Find the name and address of everyone with the same address as Tim Wahls.
range of t is customer
range of s is customer
retrieve (t.name, t.address)
where s.name = "Tim Wahls" and
s.address = t.address
Quel also has aggregate functions:
count, sum, avg, max,
min, countu, sumu, avgu,
any
Function any is related to count - it always
returns 0 when count would return 0, and 1 when
count would return a number larger than 0.
Example: Find the average length of all flyrods stocked by the flyshop that are not 9 weights.
range of t is flyrod
retrieve avg(t.length where not
(t.line-weight = 9))