It sounds simple. It can break everything. Schema changes in a live database touch queries, indexes, replication, migrations, and uptime. A new column is never just a new column. Decide wrong and you lock rows, slow writes, or trigger hours of downtime.
Start with purpose. Know why the column exists, what data type it needs, and how it will be used. Map out affected queries. Trace reads and writes. If the field will be indexed, understand the impact on storage and query plans before the first ALTER command runs.
Choose the safest migration strategy for your system size and load. For small tables in low-traffic environments, a blocking ALTER TABLE might be acceptable. For large, high-traffic systems, use online schema change tools like pt-online-schema-change or gh-ost. Stage changes to avoid locking. Consider adding nullable fields first, backfilling data in batches, then enforcing constraints once the backfill is complete.