Remove Nulls & Duplicates (MySQL)

I have a MySQL database that holds a mix of text and numerical fields, and it needs a quick yet thorough clean-up. The goals are simple:

• Identify every record containing NULL values and either replace those NULLs with the appropriate defaults or remove the rows when a sensible default is impossible.
• Detect and eliminate duplicate rows without breaking referential integrity.

I will give you direct access to a staging copy of the database along with its schema. Please supply:

1. The SQL scripts you use (well-commented) so I can rerun them later.
2. A brief before-and-after row count plus any caveats you found during the process.

Successful delivery is a database free of NULLs in mandatory fields, no duplicate records, and a clear script trail so the work is fully reproducible.