SQL subquery, multiple table query, federated Query method

subqueries, multiple table queries, federated queries
These three concepts are interpreted slightly differently in different versions of SQL, roughly as follows:

Join connection
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1, such as: SELECT * from TAB1 where ID in (SELECT id form tab2 where ...)
The query sentence in parentheses above belongs to "subquery".

2, select tab1.*, tab2.* from Tab1, tab2 where TAB1. [Field]=tab2. Field
This is a "multiple table query."

3, such as: (SELECT * from TAB1 where ...) union (SELECT * from TaB2 where ...)
This is a "joint query", of course, the so-called "joint query" there are many forms.