basic-queries.md

Using Queries

Your Engines will query through various Entities. Now you can pass Schema object result from AddSchema to your preferred engine. Let's see how to iterate over Entities we built.

Basic Query from Table

In Schema extensions. Queries are formed like SQL. If you want to components of DamagableSet, which is Result Set for <HealthComponent, DefenseComponent>, think about form of SQL.

foreach (var result in _indexedDB.Select<DamagableSet>().From(_schema.Character))

Which resembles simple SQL like this.

SELECT IDamagbleRow FROM CharacterTable;

Note that CharacterRow, Descriptor Row of _schema.Character, have to implement DamagableSet. But don't worry, if it's not then query is not valid, it will result in compile-time error. Which gives type-safety layer to Schema extensions.

Also it is worth to mention that you'll have to do foreach loop for any queries even though there is only one Group in the Table. This way you can keep your code not dependent how the underlying Group is managed.

Basic Query from all Tables

Simply, if you want to query from all Tables, call FromAll() instead of From().

foreach (var result in _indexedDB.Select<DamagableSet>().FromAll())

In this case, you'll iterate any Table in your schema that their Row implementing IQueryable<DamagableSet>. Important note: If the their Row does not implements IQueryable<DamagableSet>, then that Table does not included in result of this query. Even though Row has both HealthComponent and DefenseComponent. This is important because it is different behavour from Svelto or other ECS frameworks. In Schema extensions Rows define behaviours, not the Components.

Basic Query to find Tables

But what if you want to find specific type of Tables only, not all DamagableSet? You can query like this.

var tables = _indexedDB.FindTables<CharacterRow>();

foreach (var result in _indexedDB.Select<DamagableSet>().From(tables))

You query Tables of CharacterRow, and pass it to From(). You can find Tables with any Rows including Interface Rows and Descriptor Rows.

There is also shortcut for above code, using FromAll<T>().

foreach (var result in _indexedDB.Select<DamagableSet>().FromAll<CharacterRow>())

Summary

We saw how to select Entities with Queries. In Next Document, we'll see how to define Primary Key, that splits a Table into multiple Group.