Linq Tutorial (Part 2): Query Sintax

Description
Summary

The Linq Query Sintax is a SQL-like syntax provided as a short hand method for invocation, that at compile time will be parsed and converted to the Standard Query Operators.

Description
query-expression ::= from-clause query-body

query-body ::= 

      query-body-clause* final-query-clause query-continuation?

query-body-clause ::=
  (from-clause
      | join-clause
      | let-clause
      | where-clause
      | orderby-clause)

from-clause ::= from itemName in srcExpr

join-clause ::= join itemName in srcExpr on keyExpr equals keyExpr
       (into itemName)?

let-clause ::= let itemName = selExpr

where-clause ::= where predExpr

orderby-clause ::= orderby (keyExpr (ascending | descending)?)*

final-query-clause ::=
  (select-clause | groupby-clause)

select-clause ::= select selExpr

groupby-clause ::= group selExpr by keyExpr

query-continuation ::= into itemName query-body
Template issue: INTO keyword

The INTO keyword essentially allows one to continue a query after a projection by taking the result retrieved from GROUP, JOIN or SELECT clauses and places it into the a new temporary variable which then allows one to introduce additional WHERE, ORDERBY, and SELECT clauses.

Once the INTO variable is used then a new context/scope is used and the previous variable(s) are no longer available in the query.

In the Methods Sintax the INTO keyword is converted to a GROUPJOIN operator.

Template issue: LET keyword

The LET keyword uses temporary anonymous types that are injected into the context/scope and allow to reuse more than one variable at a time: The variable value can be passed forward to the WHERE and ORDERBY operations without duplicate calls.

Examples of use:

1. To create an enumerable type that can itself be queried in subsequent clauses;
2. Apply a function to an enumerable instead of each predicate in the where clause.

In the Methods Sintax the LET keyword is equivalent to the use of a intermediate SELECT operator that will create an anonynous tuple with:
   1) the original selected item; and
   2) the new injected value.