LINQ [L1 ] stands for Language INtegrated Query, and is Microsoft's attempt to integrate a declarative query language into C# and Visual Basic. It basically provides an SQL-like mini-language for specifying queries over a variety of different data sources, such as relational databases, XML, and objects.
Similar things can be accomplished in Tcl (and other languages) via list comprehensions, and higher-order enumeration functions such as map, filter, zip and fold.
See also tcllib's treeql, a tree query language.