Lesson 3: Reading data

Fetching simple data

Fetch queries are used to retrieve data in JSON format. They comprise a match clause and fetch clause. Fetch queries are run using a data session and read transaction.
```
match
# match clause
fetch
# fetch clause
```
Variables are declared with a $ prefix.
An isa statement is used to specify the type of a variable.
```
$entity isa entity-type;
```
Data objects can be either entities or relations.
Relations have roles. An entity that plays a role is a roleplayer. All relations must have at least one roleplayer.
```
($role-1: $a, $role-2: $b) isa relation-type;
```
A has statement is used to specify the value of an entity or relation’s attribute.
```
$entity has attribute-type "attribute value";
```

Type inference allows the database to infer the possible types of a variable without having to explicitly specify them all.
Inheritance polymorphism allows us to query data of multiple types through a common supertype.
Interface polymorphism allows us to query data of multiple types through a common implemented interface: either a role in a relation or ownership of an attribute.
Parametric polymorphism allows us to query data of multiple types through the structure of the data alone. Purely parametric queries can be run on any schema.
Different types of polymorphism can be combined to produce complex queries.

Rule inference allows the database to infer new data based on existing data and rules defined as part of the schema.
Rule inference must be enabled for inferred data to be returned.

Types can be variablized in the same way as data instances, by using a variable in place of a type label in statements.
```
$entity isa $entity-type;
```
An isa! statement is used to specify the exact type of a data variable.
```
$entity isa! exact-type;
```
A sub statement is used to specify a supertype of a type.
```
$type sub supertype;
```
A sub! statement is used to specify the direct supertype of a type.
```
$type sub! direct-supertype;
```
An owns statement is used to specify an owner of an attribute type.
```
$entity-type owns attribute-type;
```
A relates statement is used to specify a role of a relation type.
```
relation-type relates $role;
```
A plays statement is used to specify a roleplayer of a role.
```
$entity-type plays $role;
```

All queries are validated against the schema.
Running a query that does not conform to the schema will cause an exception to be thrown.

Learn how to write data to TypeDB, covering insert, delete, and update operations, including polymorphic data writes.

Learn more about reading data from TypeDB, covering Fetch queries, Get queries, and inference.

Read the Fetch query reference, including syntax, behaviour, and advanced usage.