What is the difference between a union and a union in MySQL?

When querying data for multiple tables with identical table structures:

1. When the query condition is exactly the same and does not include the primary key, then the union query will filter out the duplicate records of the query, and the record is omitted, and the query with union ALL will find all the eligible records and ensure the integrity of the data.

2. When the query condition is exactly the same and contains the primary key, there is no difference between the same records that are queried with union and union all.

What is the difference between a union and a union in MySQL?