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The LIMIT .. OFFSET clause
While being extremely useful for every application that does pagination, or just to limit result sets to reasonable sizes, this clause is not yet part of any SQL standard (up until SQL:2008). Hence, there exist a variety of possible implementations in various SQL dialects, concerning this limit clause. jOOQ chose to implement the LIMIT .. OFFSET clause as understood and supported by MySQL, H2, HSQLDB, Postgres, and SQLite. Here is an example of how to apply limits with jOOQ:
This will limit the result to 1 books starting with the 2nd book (starting at offset 0!). limit() is supported in all dialects, offset() in all but Sybase ASE, which has no reasonable means to emulate it. This is how jOOQ trivially emulates the above query in various SQL dialects with native
OFFSET pagination support:
-- MySQL, H2, HSQLDB, Postgres, and SQLite SELECT * FROM BOOK LIMIT 1 OFFSET 2 -- CUBRID supports a MySQL variant of the LIMIT .. OFFSET clause SELECT * FROM BOOK LIMIT 2, 1 -- Derby, SQL Server 2012, Oracle 12c, the SQL:2008 standard SELECT * FROM BOOK OFFSET 2 ROWS FETCH NEXT 1 ROWS ONLY -- Informix has SKIP .. FIRST support SELECT SKIP 2 FIRST 1 * FROM BOOK -- Ingres (almost the SQL:2008 standard) SELECT * FROM BOOK OFFSET 2 FETCH FIRST 1 ROWS ONLY -- Firebird SELECT * FROM BOOK ROWS 2 TO 3 -- Sybase SQL Anywhere SELECT TOP 1 ROWS START AT 3 * FROM BOOK -- DB2 (almost the SQL:2008 standard, without OFFSET) SELECT * FROM BOOK FETCH FIRST 1 ROWS ONLY -- Sybase ASE, SQL Server 2008 (without OFFSET) SELECT TOP 1 * FROM BOOK
Things get a little more tricky in those databases that have no native idiom for
OFFSET pagination (actual queries may vary):
-- DB2 (with OFFSET), SQL Server 2008 (with OFFSET) SELECT * FROM ( SELECT BOOK.*, ROW_NUMBER() OVER (ORDER BY ID ASC) AS RN FROM BOOK ) AS X WHERE RN > 2 AND RN <= 3 -- DB2 (with OFFSET), SQL Server 2008 (with OFFSET) SELECT * FROM ( SELECT DISTINCT BOOK.ID, BOOK.TITLE DENSE_RANK() OVER (ORDER BY ID ASC, TITLE ASC) AS RN FROM BOOK ) AS X WHERE RN > 2 AND RN <= 3 -- Oracle 11g and less SELECT * FROM ( SELECT b.*, ROWNUM RN FROM ( SELECT * FROM BOOK ORDER BY ID ASC ) b WHERE ROWNUM <= 3 ) WHERE RN > 2
As you can see, jOOQ will take care of the incredibly painful ROW_NUMBER() OVER() (or ROWNUM for Oracle) filtering in subselects for you, you'll just have to write limit(1).offset(2) in any dialect.
Side-note: If you're interested in understanding why we chose ROWNUM for Oracle, please refer to this very interesting benchmark, comparing the different approaches of doing pagination in Oracle: http://www.inf.unideb.hu/~gabora/pagination/results.html.
As can be seen in the above example, writing correct SQL can be quite tricky, depending on the SQL dialect. For instance, with SQL Server, you cannot have an ORDER BY clause in a subquery, unless you also have a TOP clause. This is illustrated by the fact that jOOQ renders a TOP 100 PERCENT clause for you. The same applies to the fact that ROW_NUMBER() OVER() needs an ORDER BY windowing clause, even if you don't provide one to the jOOQ query. By default, jOOQ adds ordering by the first column of your projection.