Basic CASE
CASE adds a calculated category to your output. The rules should match the business definition, not just produce attractive labels.
Searched CASE
CASE WHEN condition THEN value ... ELSE value END checks conditions in order and returns the first matching result. It behaves like an ordered decision list.
Simple CASE
CASE status WHEN 'paid' THEN ... compares one expression with fixed values. For ranges such as sales >= 100, use searched CASE and write a complete condition.
Boundary rules
If High means 100 or more, use >= 100. Put the highest threshold first. Once a row matches it, later conditions are not evaluated for the result.
ELSE and output type
Without ELSE, unmatched rows return NULL. Choose outputs of compatible types and give the expression an informative alias. A label does not change the stored value.