Clean values, apply mappings and remove duplicate records.

CREATE TEMP TABLE silver_customers AS
WITH ranked AS(
  SELECT
    *,
    ROW_NUMBER() OVER(PARTITION BY TRIM(raw_id) ORDER BY updated_at DESC) AS rn
  FROM bronze_customers
  WHERE raw_id IS NOT NULL AND TRIM(raw_id)<>''
)
SELECT
  TRIM(raw_id) AS customer_id,
  TRIM(name) AS name,
  CASE UPPER(TRIM(country))
    WHEN 'US' THEN 'United States'
    WHEN 'USA' THEN 'United States'
    WHEN 'UK' THEN 'United Kingdom'
    WHEN 'GB' THEN 'United Kingdom'
    WHEN 'IND' THEN 'India'
    WHEN 'CA' THEN 'Canada'
    ELSE UPPER(TRIM(country))
  END AS country
FROM ranked
WHERE rn=1;
SELECT customer_id,name,country
FROM silver_customers;