Similarly to the XCOL table type, OCCUR is an extension to the PROXY type when referring to a table or view having several columns containing the same kind of data. It enables having a different view of the table where the data from these columns are put in a single column, eventually causing several rows to be generated from one row of the object table. For example, supposing we have a pets table:

namedogcatrabbitbirdfish
John20000
Bill01000
Mary11000
Lisbeth00200
Kevin02060
Donald10003

We can create an occur table by:

create table xpet (
  name varchar(12) not null,
  race char(6) not null,
  number int not null)
engine=connect table_type=occur tabname=pets
option_list='OccurCol=number,RankCol=race'
Colist='dog,cat,rabbit,bird,fish';

When displaying it by

select * from xpet;

We will get the result:

nameracenumber
Johndog2
Billcat1
Marydog1
Marycat1
Lisbethrabbit2
Kevincat2
Kevinbird6
Donalddog1
Donaldfish3

First of all, the values of the column listed in the Colist option have been put in a unique column whose name is given by the OccurCol option. When several columns have non null (or pseudo-null) values, several rows are generated, with the other normal columns values repeated.

In addition, an optional special column was added whose name is given by the RankCol option. This column contains the name of the source column from which the value of the OccurCol column comes from. It permits here to know the race of the pets whose number is given in number.

This table type permit to make queries that would be more complicated to make on the original tables. For instance to know who as more than 1 pet of a kind, you can simply ask:

select * from xpet where number > 1;

You will get the result:

nameracenumber
Johndog2
Lisbethrabbit2
Kevincat2
Kevinbird6
Donaldfish3

Note 1: Like for XCOL tables, no row multiplication for queries not implying the Occur column.

Note 2: Because the OccurCol was declared "not null" no rows were generated for null or pseudo-null values of the column list. If the OccurCol is declared as nullable, rows are also generated for columns containing null or pseudo-null values.

Occur tables can be also defined from views or source definition. Also, CONNECT is able to generate the column definitions if not specified. For example:

create table ocsrc engine=connect table_type=occur
colist='january,february,march,april,may,june,july,august,september,
october,november,december' option_list='rankcol=month,occurcol=day'
srcdef='select ''Foo'' name, 8 january, 7 february, 2 march, 1 april,
  8 may, 14 june, 25 july, 10 august, 13 september, 22 october, 28
  november, 14 december';

This table is displayed as:

namemonthday
Foojanuary8
Foofebruary7
Foomarch2
Fooapril1
Foomay8
Foojune14
Foojuly25
Fooaugust10
Fooseptember13
Foooctober22
Foonovember28
Foodecember14

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