It is the query language in the system INGRES. It is based on relational calculus. The fundamental aspect of the query languages based on the relational calculus is tuple variable ( which is a variable that "ranges over" some named relation).
e.g. query "Get supplier number for supplier in London" can be expressed in QUEL as:
RANGE of SX is S
RETRIEVE(SE. S#) where SX .CITY='LONDON'.
The tuple here is SX that ranges over relation S.
In SEQUEL, the expression following WHERE can be any expression involving the attribute of the relation following FROM, arithmetic comparison and operations.
Boolean connectives (AND, OR NOT) set operations (UNION,INTERSECT,MINUS), set membership (X IN S, where S is a set or equivalently S CONTAINS X)
and negation of set membership (X NOT IN S OR S DOES NOT CONTAIN X).
When SEQUEL applies a mapping it does not eliminate duplicate unless told to do so by the keyword UNIQUE. In SEQUEL assignment is indicated by preceding a query by
ASSIGN TO R
To assign the result to relation R, any relational algebra expression in SEQUEL can be evaluated by applying one operator at a time and thereby computing each subexpression, hence SEQUEL is complete.
QBE is specifically designed for use with a visual display terminal. QBE has two-dimensional syntax since operation are specified in tabular form.
The basic idea is that the user formulates the query by entering an example of a possible answer in appropriate place in an empty table.
e.g. consider the query 'Get supplier numbers for supplier in Paris'. First by pressing a certain function key on the terminal, the user can have a blank "skeleton" table displaced on the screen. Then the user enters S as the table-name gets QBE to respond by filling in the column names . The query is expressed as
The "P" stands for "print" , indicating the target of the query. S7 is an "example element" , i.e. an example of a possible answer to the query (indicated by underlining). PARIS is a "constant element". QBE automatically eliminates redundant duplicates from a query result. To suppress each elimination the user can specify the keyword "ALL".
This language consist of commands, each command has an operational part and condition part. Operation is executed through a search in all relations defined in relational database. Required solution (list of data) is returned by the command. The data listed in solution satisfies all condition given in that command.
SYSTEM R SQL is now the standard language for commercial relational DBMS. A joint effort by ANSI and ISO has led to a standard version of SQL,called SQL-86 or SQL1. A revised and much expanded standard called SQL@ has subsequently been developed. Plans are already well underway called SQL2 has subsequently been developed. Plans are already well underway for SQL2 has subseqently been developed. Plans are already well underway for SQL3, which will further extend SQL with object-oriented and other recent database concepts.