different union all and join and Oracle more

 1. what is different union all and join
UNION combines the results of two or more queries into a single result set that includes all the rows that belong to all queries in the union. By using JOINs, you can retrieve data from two or more tables based on logical relationships between the tables.

2. What is the difference between union and union all?

The difference between Union and Union all is that Union all will not eliminate duplicate rows, instead it just pulls all rows from all tables fitting your query specifics and combines them into a table. A UNION statement effectively does a SELECT DISTINCT on the results set.

3. What is a union in SQL?

The SQL UNION Operator. The UNION operator is used to combine the result-set of two or more SELECT statements. Notice that each SELECT statement within the UNION must have the same number of columns. The columns must also have similar data types.

4. What are the set operators in SQL?

Set operators combine the results of two component queries into a single result. Queries containing set operators are called compound queries. Table 4-4 lists SQL set operators. They are fully described, including examples and restrictions on these operators, in "The UNION [ALL], INTERSECT, MINUS Operators"

Operator	Returns
UNION	All distinct rows selected by either query
UNION ALL	All rows selected by either query, including all duplicates
INTERSECT	All distinct rows selected by both queries
MINUS	All distinct rows selected by the first query but not the second