How do I define a column type?

Your warehouse's SQL engine automatically assigns a datatype to every column, whether it's found in a source or model. To force SQL to treat a columns a certain datatype, use cast functions:

models/order_prices.sql
select
cast(order_id as integer),
cast(order_price as double(6,2)) -- a more generic way of doing type conversion
from {{ ref('stg_orders') }}

Many modern data warehouses now support :: syntax as a shorthand for cast( as ).

models/orders_prices_colon_syntax.sql
select
order_id::integer,
order_price::numeric(6,2) -- you might find this in Redshift, Snowflake, and Postgres
from {{ ref('stg_orders') }}

Be warned, reading in data and casting that data may not always yield expected results, and every warehouse has its own subtleties. Certain casts may not be allowed (e.g. on Bigquery, you can't cast a boolean-type value to a float64). Casts that involve a loss in precision loss (e.g. float to integer) rely on your SQL engine to make a best guess or follow a specific schema not used by competing services. When performing casts, it's imperative that you are familiar with your warehouse's casting rules to best label fields in your sources and models.

Thankfully, popular database services tend to have type docs--Redshift and Bigquery.