Tuesday, November 26, 2013

MySQL-style LIMIT and OFFSET in DB2

An "ancient" but not yet well-known feature in DB2 is the support of MySQL/PostgreSQL-style LIMIT and OFFSET in SELECT statements and searched UPDATE/DELETE. Unfortunately, it is not really documented and I am working on getting some more documentation added (think about the "mostly harmless" as in the Hitchhiker's Guide to the Galaxy).

To demonstrate what is possible, I created a small table full of good stuff:
db2 => create table sweets(id int, desc varchar(60), quant int)
DB20000I  The SQL command completed successfully.
db2 => insert into sweets values(1,'dark chocolate',4)
DB20000I  The SQL command completed successfully.
db2 => insert into sweets values(2,'marzipan bar',1)
DB20000I  The SQL command completed successfully.
db2 => insert into sweets values(3,'almond cookies',10)
DB20000I  The SQL command completed successfully.
db2 => insert into sweets values(4,'granola bar',1)
DB20000I  The SQL command completed successfully.
db2 => insert into sweets values(5,'nut chocolate',1)
DB20000I  The SQL command completed successfully.
db2 => insert into sweets values(6,'chocolate-covered peanuts',1)
DB20000I  The SQL command completed successfully.
db2 => select * from sweets limit 5
SQL0104N  An unexpected token "limit" was found following "".  Expected tokens
may include:  "FETCH FIRST ROWS ONLY".  SQLSTATE=42601


By default, DB2 does not know about LIMIT and OFFSET either. But setting the DB2_COMPATIBILITY_VECTOR to MYS (as in MySQL) or "04000" enables the syntactic alternative to the FETCH FIRST n ROWS ONLY. After restarting DB2 the SELECT works:

db2 => select * from sweets limit 5

ID          DESC                                                QUANT
----------- --------------------------------------------------- -----------
          1 dark chocolate                                                4
          2 marzipan bar                                                  1
          3 almond cookies                                               10
          4 granola bar                                                   1
          5 nut chocolate                                                 1

  5 record(s) selected.


The following two examples show the use of the OFFSET, i.e., where to start in the result set. I combined it with ordering on the quantity.

db2 => select quant,desc from sweets order by num limit 3 offset 2

QUANT       DESC                                                       
----------- ------------------------------------------------------------
          1 nut chocolate                                              
          1 chocolate-covered peanuts                                  
          4 dark chocolate                                             

  3 record(s) selected.


db2 => select quant,desc from sweets order by num limit 2,3

QUANT       DESC                                                       
----------- ------------------------------------------------------------
          1 nut chocolate                                              
          1 chocolate-covered peanuts                                  
          4 dark chocolate                                             

  3 record(s) selected.


Note that in the second example a special abbreviated syntax is used. The first number is the offset where to start, followed by the number of rows to return.

As said, there is not much in the DB2 Information Center (yet). Serge has some more background in his blog about different ways of paging through result sets.

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