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Query Translation

Translation step:   SQL text → RA expression

Example: 

SQL: select name from Students where id=7654321;
-- is translated to
RA:  Proj[name](Sel[id=7654321]Students)

Processes:  lexer/parser,  mapping rules,  rewriting rules.

Mapping from SQL to RA may include some optimisations, e.g. 

select * from Students where id = 54321 and age > 50;
-- is translated to
Sel[age>50](Sel[id=54321]Students)
-- rather than ... because of index on id
Sel[id=54321&age>50](Students)