QUEL query languages

QUEL statements are always defined by tuple variables, which can be used to limit queries or return result sets. Consider this example, taken from one of the first original Ingres papers:[2]

range of E is EMPLOYEE
retrieve into W
(COMP = E.Salary / (E.Age - 18))
where E.Name = "Jones"

Here E is a tuple variable that ranges over the EMPLOYEE relation, and all tuples in that relation are found which satisfy the qualification `E.Name = "Jones"`. The result of the query is a new relation W, which has a single domain COMP that has been calculated for each qualifying tuple. Additional queries can then be made against the relation W.

An equivalent SQL statement is:

create table W as
select (E.salary / (E.age - 18)) as COMP
from employee as E
where E.name = 'Jones'

In this example, the relation is being stored in a new table W. This is not a direct analog of the QUEL version; relations in QUEL are more similar to temporary tables seen in most modern SQL implementations.

Here is a sample of a simple session that creates a table, inserts a row into it, and then retrieves and modifies the data inside it and finally deletes the row that was added (assuming that name is a unique field).

create student(name = c10, age = i4, sex = c1, state = c2)

range of s is student
append to s (name = "philip", age = 17, sex = "m", state = "FL")

retrieve (s.all) where s.state = "FL"

replace s (age=s.age+1)

retrieve (s.all)

delete s where s.name="philip"

create table student(name char(10), age int, sex char(1), state char(2));

insert into student (name, age, sex, state) values ('philip', 17, 'm', 'FL');

select * from student where state = 'FL';

update student set age=age+1;

select * from student;

delete from student where name='philip';

Another feature of QUEL was a built-in system for moving records en-masse into and out of the system. Consider this command:

copy student(name=c0, comma=d1, age=c0, comma=d1, sex=c0, comma=d1, address=c0, nl=d1)
into "/student.txt"

which creates a comma-delimited file of all the records in the student table. The d1 indicates a delimiter, as opposed to a data type. Changing the into to a from reverses the process. Similar commands are available in many SQL systems, but usually as external tools, as opposed to being internal to the SQL language. This makes them unavailable to stored procedures.

QUEL has an extremely powerful aggregation capability. Aggregates can be nested, and different aggregates can have independent by-lists and/or restriction clauses. For example:

retrieve (a=count(y.i by y.d where y.str = "ii*" or y.str = "foo"), b=max(count(y.i by y.d)))

This example illustrates one of the arguably less desirable quirks of QUEL, namely that all string comparisons are potentially pattern matches. y.str = "ii*" matches all y.str values starting with ii. In contrast, SQL uses = only for exact matches, while like is used when pattern matching is required.

• D4 (programming language) (an implementation of D)
• Relational algebra
• Relational calculus

• C. J. Date: A Critique of the SQL Database Language. SIGMOD Record 14(3): 8-54, 1984.