1. Queries in general-purpose
programming languages

2. Languages
Theory
 Relational algebra
 Monads and comprehensions
Haskell and Links
 Monad comprehensions
 Comprehensions with group and order
C# and F#
 Language integrated query (LINQ)
 Extensible query syntax in F# 3.0

3. Problems
Expressing queries
 What operations can we use?
 What is the right syntax for queries?
 Should the syntax be extensible?
Compiling queries
 How to turn normal code to SQL?
 How to turn normal code to efficient SQL?
 Dealing with functions in SQL code

4. Theory

5. Monads and monad
comprehensions in Haskell

6. Monad comprehensions
[ p.Name | p <- db.Products, p.Price > 10 ]
db.Products `bind` (p ->
if (p.Price > 10) then return p.Name
else zero)

7. Monadic bind and joins
Join using comprehensions
[ (p.Name, c.CategoryName)
| p <- db.Products, c <- db.Categories
, p.CategoryID = c.ID ]
Translation using monads
db.Products `bind` (p ->
db.Categories `bind` (c ->
if (p.CategoryID = c.ID) then
return (p.Name, c.CategoryName)
else zero))
 Multiple uses of bind are nested
 Second data source may depend on values from first!

8. Comparing bind and join
Standard join
 Two independent data sources
 Equality condition on some projected values
[ (p, c) | p <- db.Products, c <- db.Categories
, p.CategoryID = c.ID ]
But bind allows more
 Second data source may depend on first
 Can use arbitrary conditions
[ (p1.Name, p2.Name)
| p1 <- Products
, p2 <- [ p | p <- Products, p.Price > p1.Price ] ]

9. Adding ordering and grouping

10. Adding ordering and grouping
[ p | p <- db.Products, then reverse ]
[ p | p <- db.Products, then order by p.Price ]
[ p | …, then group by p.CategoryID ]
[ p | …, then group by p.CategoryID using groupAdj ]

11. SQL-style relational
queries in LINQ using C#

12. LINQ Overview
Query example
from p in db.Products
join c in db.Categories on p.CategoryID equals c.ID
select new { p.Name, c.CategoryName }
Translating to SQL
 All operations have a corresponding method
db.Products.Where(p => p.Price > 10)
db.Products.Select(p => p.Name)
 Lambda function translated as expression tree
 Expression trees composed and processed

13. Expressing queries
Selection and projection
from p in db.Products where p.Price > 10 select p
from p in db.Products select p.Name
Ordering
from p in db.Products orderby p.Price select p
Grouping
from p in db.Products group p by p.CategoryID
Joins (products with selection)
from p in db.Products
join c in db.Categories on p.CategoryID equals c.ID
select new { ... }

14. Customizing LINQ operators
Operations depend on the data source
 Syntax is translated to method calls
from p in db.Products group p by p.CategoryID
db.Products.GroupBy(p => p.CategoryID)
 Wrap collection and then use different operator
from p in db.Products.WithAdjacentGrouping()
group p by p.CategoryID
WithAdjacentGrouping : IE<T> -> IEWithGroup<T>
GroupBy : IEWithGroup<T> -> IEWithGroup<IEWithGroup<T>>
Various applications
 LINQ over events, Parallel implementation

15. Writing joins in LINQ
Syntax for joins
 Independent sources; equality condition only
from p in db.Products
join c in db.Categories on p.CategoryID equals c.ID
select …
Using multiple from clauses
 Later from can depend on previous variables
 We can write arbitrary conditions
from p in db.Products
from c in db.Categories.Where(c => p.CategoryID == c.ID)
where c.Name != "Stuff"

16. Joins and theory
Multiple from clauses
 Uses an operation similar to Bind
SelectMany : M<S> -> (S -> M<C>) -> (S * C -> R) -> M<R>
 Can be implemented using Bind
Bind : M<S> -> (S -> M<R>) -> M<R>
StrengthL : V * M<S> -> M<V * S>
Syntax for joins
 Can be defined using SelectMany, but not vice versa
Join : M<A> -> M<B> -> (A -> K) -> (B -> K)
-> (A * B -> R) -> M<R>
 Resembles operation of “applicative functors”

17. More flexible
queries in F# 3.0

18. Queries in F# 3.0
Design between Haskell and C#
 Can define custom operations
query { for p in db.Products do
orderBy p.Price
takeWhile (p.Price > 100) }
 Here orderBy and takeWhile are not built-in
Two kinds of custom operations
 Preserve variable scope (orderBy, where, …)
 Require re-binding (groupBy)

19. What can be done in F# 3.0?
Better queries for OData
 Open standard for REST based queries
 Supports only subset of query operations
Expanding dependent tables explicitly
query { for title in db.Movies do
select title.Awards }
 Wrong! We did not say we will use Awards
query { for title in db.Movies do
expand title.Awards
select title.Awards }
Other interesting areas: NoSQL, etc.

20. Some questions about queries
that I’m interested in…

21. Possible questions
Advanced type checking
 Translating custom functions to SQL
 Limiting operations and allowed patterns (OData)
Expressivity
 What are other join-like operations?
 Aren’t monads and bind too powerful?
 Are we missing other important operations?
 What else is there than just lists…
Classical problems
 Optimizing queries, efficient compilation

22. References
Haskell and comprehensions
 Comprehensive Comprehensions:
comprehensions with "Order by" and "Group by"
 Bringing Back Monad Comprehensions
 Monad Comprehensions:
A Versatile Representation for Queries
LINQ, C# and F#
 C# Language Specification
 msdn.microsoft.com/en-us/library/bb397676.aspx
 tomasp.net/blog/idioms-in-linq.aspx