Query Builder allows adding fields from multiple tables to a query. When you add two or more tables to a query, the Query Builder attempts to determine whether they are related. If they are, the Query Builder automatically adds join between these tables to the generated SELECT statement.
In most cases when you add the necessary columns from related tables to a query, Skyvia will build the necessary joins by the relations between tables automatically and you don't need to perform any additional actions. However in more complex cases Skyvia may choose relationship for join incorrectly or not to add joins at all. The example of such cases are when there are multiple relationships between the same tables or when two tables are joined via an intermediate table, and no fields are added from the intermediate table. In such cases you need to specify relations for creating joins explicitly.