What are query languages and why do they matter?

Query languages are special-purpose languages that allow users to interact with databases and retrieve data that matches certain criteria. Query languages are essential for data analysis, reporting, and decision-making, as they enable users to access and manipulate data in a fast and efficient way.

There are different types of query languages, each with its own syntax, semantics, and features. Some of the most common query languages are SQL, Cypher, SPARQL, and Gremlin. These query languages can be classified into two main paradigms: imperative and declarative.

What is the difference between imperative and declarative query languages?

The difference between imperative and declarative query languages lies in how they express the logic of the query. Imperative query languages describe how to perform the query, while declarative query languages describe what to retrieve from the database.

Imperative query languages require users to specify the exact steps and algorithms to execute the query. They give users more control over the execution of the task, but they also require more knowledge of the language and the underlying data structure. Imperative query languages are more prone to human error and may not be very user-friendly.

Declarative query languages abstract away the details of how the query is performed and focus on the desired outcome. They allow users to express their intentions without worrying about the implementation details. Declarative query languages are more concise, readable, and flexible than imperative query languages, but they may also be less efficient or optimal in some cases.

What is functional programming and how does it relate to declarative query languages?

Functional programming is a paradigm of programming that treats computation as the evaluation of mathematical functions. Functional programming avoids mutable states, side effects, and imperative control structures, and instead relies on pure functions, immutable data structures, and higher-order functions.

Functional programming is a form of declarative programming, as it expresses the logic of computation without describing its control flow. Functional programming can be used to implement declarative query languages, as it provides powerful abstractions and techniques to manipulate data.

Some examples of functional programming languages are Haskell, Lisp, Clojure, Scala, and F#. Some examples of functional features in query languages are lambda expressions, map-reduce operations, list comprehensions, and recursion.

What are some examples of imperative and declarative query languages?

One of the most widely used imperative query languages is Gremlin, which is a graph traversal language that operates on property graphs. Gremlin allows users to specify how to traverse a graph using a series of steps that modify a traversal object. Gremlin supports both imperative and declarative constructs, but it is mainly imperative in nature.

An example of a Gremlin query that finds all the people who have reviewed a book with a rating higher than 4 is:

g.V().hasLabel('book').has('rating', gt(4)).in('reviewed').values('name')

This query can be read as follows:

• Start from all the vertices (g.V())

• Filter those that have the label ‘book’ (hasLabel('book'))

• Filter those that have a property ‘rating’ with a value greater than 4 (has('rating', gt(4)))

• Traverse the incoming edges with the label ‘reviewed’ (in('reviewed'))

• Extract the property ‘name’ from the resulting vertices (values('name'))

One of the most widely used declarative query languages is SQL, which is a relational database management system (RDBMS) language that operates on tables. SQL allows users to specify what data they want to retrieve or manipulate using clauses such as SELECT, FROM, WHERE, GROUP BY, HAVING, ORDER BY, etc. SQL does not expose how the query is executed or optimized by the RDBMS.

An example of a SQL query that finds all the people who have reviewed a book with a rating higher than 4 is:

SELECT name FROM reviewers
JOIN reviews ON reviewers.id = reviews.reviewer_id
JOIN books ON reviews.book_id = books.id
WHERE books.rating > 4;

This query can be read as follows:

• Select the column ‘name’ from the table ‘reviewers’ (SELECT name FROM reviewers)

• Join the table ‘reviewers’ with the table ‘reviews’ on the condition that their ‘id’ columns match (JOIN reviews ON reviewers.id = reviews.reviewer_id)

• Join the resulting table with the table ‘books’ on the condition that their ‘id’ columns match (JOIN books ON reviews.book_id = books.id)

• Filter the rows that have a column ‘rating’ with a value greater than 4 (WHERE books.rating > 4)

Conclusion

Query languages are powerful tools for data analysis and manipulation. They can be classified into two main paradigms: imperative and declarative. Imperative query languages describe how to perform the query, while declarative query languages describe what to retrieve from the database. Functional programming is a form of declarative programming that can be used to implement declarative query languages. Some examples of imperative and declarative query languages are Gremlin and SQL, respectively.