Maintainability
SQL is an explanation that another person must be able to inspect, test and change. Write for that person as well as for the database.
Naming and formatting
Use meaningful aliases, one clear expression per output, and consistent formatting. Avoid names such as qry9 when the step has a useful business meaning. Format complex joins and conditions so their relationships are visible.
Modular layers
Separate source cleaning, business rules and final presentation when complexity warrants it. A longer chain of trivial CTEs is not automatically clearer. Each layer should have a defined grain and responsibility.
Tests and documentation
Test key uniqueness, non-null requirements, accepted values and metric reconciliation. Document timeframe, timezone, population and exclusion rules. Comments should explain the reason for a choice, not repeat each keyword.
Review and version control
Store changes in version control, review the result and plan, and compare outputs before deployment. A reviewer should be able to reproduce the check using the same data and stated assumptions.