A SELECT list defines the columns of your result. Use an alias when a calculated value needs a useful name, and keep identifiers separate from values: backticks quote column or table names, while single quotes quote text. Spark SQL accepts STRING and DECIMAL types, including in casts.
Aster Supply stores amount as a decimal because its order amounts have two decimal places. Arithmetic keeps missing values missing unless you give them a default. That behavior matters: an unknown amount is different from a zero-dollar order. Use COALESCE only when the requested business rule treats missing amounts as zero. Selecting fewer columns changes the result schema; it does not change the source table. You can run several SELECT statements while exploring, but the last statement supplies the result you check.