5.4. Query Language (QUEL)
Query language (QUEL) is a tuple relational calculus language of a relational database system INGRESS (Interactive Graphics and Retrieval System). INGRESS runs under UNIX operating system developed at AT and T Bell Laboratories, USA. ‘C’ programming language has been used for implementation of both Ingress and UNIX. The language can be used either in a stand-alone manner by typing commands to QUEL processor, or embedded in the ‘C’ programming language. In case it is embedded in ‘C’, QUEL statements are preceded by hash (# #) and handled by a processor. INGRESS statements are used for implementing QUEL.
Let us consider a tuple relational calculus statement given as
| {q(r) |(∃t1)...(∃tn)(R1 (t1) ∧...∧ Rn(tn) ∧ q[1] | = ... |
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