Note the Union feature in Oracle SQL statements.

In Oracle SQL, union is also a common statement. Note that the Union statement of Oracle filters out duplicate data by default.

That is to say, Oracle union has a distinct Statement by default. So how can we not need this attribute? We need to add the all statement after union. That is, Union all. In this way, the same data will not be automatically filtered out. Although this knowledge point is very simple, if you are not familiar with this feature of Oracle, it will easily lead to mistakes during the development process.