Querying with SQL

Introduction

Data stored in the database can be queried for with SQL queries. The query is passed as a string to the Db.SQL method. The allowed SQL syntax is covered in the SQL section.

In addition to querying the database with SQL, you can also use LINQ by using the package Starcounter.Linq. Documentation for Starcounter.Linq is in in the Starcounter.Linq repository on GitHub,

Return Types

SQL queries are executed with the Db.SQL method. If the SQL command is SELECT, the function returns Starcounter.QueryResultRows<T> : IEnumerable<T>, it otherwise returns null.

Db.SQL("SELECT p FROM Person p"); // => QueryResultRows<Person>
Db.SQL("DELETE FROM Person"); // => null

T in QueryResultRows<T> is the type of the object retrieved if the whole object is retrieved, otherwise, it's Starcounter.Query.Execution.Row.

Db.SQL("SELECT p FROM Person p"); // => QueryResultRows<Person>
Db.SQL("SELECT p.Name FROM Person p"); // => QueryResultRows<Query.Execution.Row>

We recommend avoiding Starcounter.Query.Execution.Row when possible and instead retrieve the whole object and filter out the needed properties with Linq.

Db.SQL("SELECT p.Name FROM Person p"); // Not recommended
Db.SQL("SELECT p FROM Person p")
.Select(p => new { p.Name }); // Recommended

In addition to traditional SQL, Starcounter allows you to select objects in addition to primitive types such as strings and numbers. Also, it allows you to use C# style path expressions such as person.FullName.

When writing queries with Db.SQL, keep in mind that there are certain reserved words that should be escaped. That is done by surrounding the reserved word in quotation marks.

The QueryResultRows class is deprecated from Starcounter 2.3.1. Read about the changes here: https://starcounter.io/reducing-magic-increasing-familiarity-obsoleting-queryresultrows/

Making Queries Less Fragile

Since the queries are strings, they are sensitive to refactorings, for example if a database class is renamed. To make the queries less fragile, typeof and nameof can be used:

Db.SQL($"SELECT p FROM {typeof(Person)} p");
Db.SQL($"SELECT p.{nameof(Person.Name)} FROM {typeof(Person)} p");

If the query is executed many times, constructing the string every time can become a performance problem. In that case, the string can be constructed once and stored in a static property.

Using Variables

SQL variables are represented by question marks (?) in the query string, and you pass the current values of the variables as parameters to the method Db.SQL.

var employees = Db.SQL<Employee>(
"SELECT e FROM Employee e WHERE e.FirstName = ?", "Joe");
foreach (var employee in employees)
{
Console.WriteLine($"{employee.FirstName} {employee.LastName}");
}

Db.SQLtakes an arbitrary number of variables as long as the number of variable values are the same as the number of variables in the query string. Otherwise, an ArgumentException will be thrown.

var lastName = "Smith";
Employee manager; //Assume some value is assigned to the variable manager.
var employees = Db.SQL<Employee>(
"SELECT e FROM Employee e WHERE e.LastName = ? AND e.Manager = ?",
lastName, manager);
foreach (var employee in employees)
{
Console.WriteLine($"{employee.LastName}; {emp.Manager.LastName}");
}

Each variable has an implicit type depending on its context in the query string. For example, a variable that is compared with a property of type string will implicitly be of the type string. If a variable is given a value of some incompatible type, then an InvalidCastException will be thrown. All numerical types are compatible with each other.

You can only use ? for variables after the WHERE clause. You can't, for instance, use ? to replace the class name of a query.

Query Processing Error

If a query cannot be processed due to some syntax or type checking error then the method Db.SQL will throw the SqlException ScErrSQLIncorrectSyntax (SCERR7021).

try
{
var people = Db.SQL<Person>("SELECT e.NonExistingProperty FROM Person p");
foreach(Person person in people)
{
Console.WriteLine(person.Name);
}
}
catch (SqlException exception)
{
Console.WriteLine("Incorrect query: " + exception.Message);
}