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Set operators
Set operators are used to connect different queries and produce a resulting relation. Of course these set operations are only allowed if the attributes match (eg. the domain or the number of attributes etc.). The following set operators are used to join together sets of tuples. They might already be known from the algebra of sets.
unionproduces a union of different setsintersectproduces an intersection of different sets.minussubtracts one set from another one.
By default, duplicates are not returned using a request with these set operators.
If duplicates need to be returned, the keyword ALL must be put behind
UNION,
INTERSECT or EXCEPT.
This query is used to find all names of customers who have a magazine subscription and who have placed an advertising in this magazine.
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