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Relational algebra defines a
SEMI JOIN operation that regrettably didn't make it into standard SQL (yet), though it is easy to emulate using the EXISTS predicate or IN predicate, which is what most people are doing.
jOOQ offers a convenient
LEFT SEMI JOIN operator to match the relational algebra semantics. The following query will produce all authors that have books (but doesn't produce any books):
SELECT AUTHOR.FIRST_NAME, AUTHOR.LAST_NAME FROM AUTHOR WHERE EXISTS ( SELECT * FROM BOOK WHERE BOOK.AUTHOR_ID = AUTHOR.ID )
create.select( AUTHOR.FIRST_NAME, AUTHOR.LAST_NAME ) .from(AUTHOR) .leftSemiJoin(BOOK).on(BOOK.AUTHOR_ID.eq(AUTHOR.ID)) .fetch();
The result might look like this:
+------------+-----------+ | FIRST_NAME | LAST_NAME | +------------+-----------+ | George | Orwell | | Paulo | Coelho | +------------+-----------+
Of course, you can form an equivalent query using
IN as well in jOOQ. It is also possible to achieve
SEMI JOIN semantics by using an INNER JOIN, and possibly the
SELECT DISTINCT clause, but chances are, that query is slower and incorrect (e.g. removing too many distinct rows). A
SEMI JOIN both using jOOQ's convenience syntax or the equivalent SQL emulation using
IN are semantically more precise and should be preferred.
SEMI JOIN is the inverse of the ANTI JOIN operator.
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