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SEMI JOIN
Applies to ✅ Open Source Edition ✅ Express Edition ✅ Professional Edition ✅ Enterprise Edition
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 EXISTS or 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 EXISTS or IN are semantically more precise and should be preferred.
SEMI JOIN is the inverse of the ANTI JOIN operator.
Dialect support
This example using jOOQ:
select(AUTHOR.ID).from(AUTHOR).leftSemiJoin(BOOK).on(BOOK.AUTHOR_ID.eq(AUTHOR.ID))
Translates to the following dialect specific expressions:
ASE, Access, Aurora MySQL, Aurora Postgres, BigQuery, CockroachDB, DB2, Derby, Exasol, Firebird, H2, HSQLDB, Hana, Informix, MariaDB, MemSQL, MySQL, Oracle, Postgres, Redshift, SQLDataWarehouse, SQLServer, SQLite, Snowflake, Sybase, Teradata, Trino, Vertica, YugabyteDB
SELECT AUTHOR.ID FROM AUTHOR WHERE EXISTS ( SELECT 1 one FROM BOOK WHERE BOOK.AUTHOR_ID = AUTHOR.ID )
ClickHouse
SELECT AUTHOR.ID
FROM AUTHOR
LEFT SEMI JOIN BOOK
ON BOOK.AUTHOR_ID = AUTHOR.ID
DuckDB
SELECT AUTHOR.ID
FROM AUTHOR
SEMI JOIN BOOK
ON BOOK.AUTHOR_ID = AUTHOR.ID
Generated with jOOQ 3.20. Translate your own SQL on our website
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