CONNECT XCOL Table Type

The CONNECT storage engine has been deprecated.

This storage engine has been deprecated.

XCOL tables are based on another table or view, like PROXY tables. This type can be used when the object table has a column that contains a list of values.

Suppose we have a 'children' table that can be displayed as:

name	childlist
Sophie	Vivian, Antony
Lisbeth	Lucy,Charles,Diana
Corinne
Claude	Marc
Janet	Arthur, Sandra, Peter, John

We can have a different view on these data, where each child are associated with his/her mother by creating an XCOL table by:

CREATE TABLE xchild (
  mother CHAR(12) NOT NULL,
  child CHAR(12) DEFAULT NULL flag=2
) ENGINE=CONNECT table_type=XCOL tabname='chlist'
option_list='colname=child';

The COLNAME option specifies the name of the column receiving the list items. This will return from:

SELECT * FROM xchild;

The requested view:

mother	child
Sophia	Vivian
Sophia	Antony
Lisbeth	Lucy
Lisbeth	Charles
Lisbeth	Diana
Corinne	NULL
Claude	Marc
Janet	Arthur
Janet	Sandra
Janet	Peter
Janet	John

Several things should be noted here:

• When the original children field is void, what happens depends on the NULL specification of the "multiple" column. If it is nullable, like here, a void string will generate a NULL value. However, if the column is not nullable, no row are generated at all.

• Blanks after the separator are ignored.

• No copy of the original data was done. Both tables use the same source data.

• Specifying the column definitions in the CREATE TABLE statement is optional.

The "multiple" column child can be used as any other column. For instance:

SELECT * FROM xchild WHERE substr(child,1,1) = 'A';

This will return:

Mother	Child
Sophia	Antony
Janet	Arthur

If a query does not involve the "multiple" column, no row multiplication will be done. For instance:

SELECT mother FROM xchild;

This will just return all the mothers:

mother
Sophia
Lisbeth
Corinne
Claude
Janet

The same occurs with other types of select statements, for instance:

SELECT COUNT(*) FROM xchild;      -- returns 5
SELECT COUNT(child) FROM xchild;  -- returns 10
SELECT COUNT(mother) FROM xchild; -- returns 5

Grouping also gives different result:

SELECT mother, COUNT(*) FROM xchild GROUP BY mother;

Replies:

mother	count(*)
Claude	1
Corinne	1
Janet	1
Lisbeth	1
Sophia	1

While the query:

SELECT mother, COUNT(child) FROM xchild GROUP BY mother;

Gives the more interesting result:

mother	count(child)
Claude	1
Corinne	0
Janet	4
Lisbeth	3
Sophia	2

Some more options are available for this table type:

Option	Description
Sep_char	The separator character used in the "multiple" column, defaults to the comma.
Mult	Indicates the max number of multiple items. It is used to internally calculate the max size of the table and defaults to 10. (To be specified in OPTION_LIST).

Using Special Columns with XCOL

Special columns can be used in XCOL tables. The mostly useful one is ROWNUM that gives the rank of the value in the list of values. For instance:

CREATE TABLE xchild2 (
rank INT NOT NULL SPECIAL=ROWID,
mother CHAR(12) NOT NULL,
child CHAR(12) NOT NULL flag=2
) ENGINE=CONNECT table_type=XCOL tabname='chlist' option_list='colname=child';

This table are displayed as:

rank	mother	child
1	Sophia	Vivian
2	Sophia	Antony
1	Lisbeth	Lucy
2	Lisbeth	Charles
3	Lisbeth	Diana
1	Claude	Marc
1	Janet	Arthur
2	Janet	Sandra
3	Janet	Peter
4	Janet	John

To list only the first child of each mother you can do:

SELECT mother, child FROM xchild2 WHERE rank = 1 ;

returning:

mother	child
Sophia	Vivian
Lisbeth	Lucy
Claude	Marc
Janet	Arthur

However, note the following pitfall: trying to get the names of all mothers having more than 2 children cannot be done by:

SELECT mother FROM xchild2 WHERE rank > 2;

This is because with no row multiplication being done, the rank value is always 1. The correct way to obtain this result is longer but cannot use the ROWNUM column:

SELECT mother FROM xchild2 GROUP BY mother HAVING COUNT(child) > 2;

XCOL tables based on specified views

Instead of specifying a source table name via the TABNAME option, it is possible to retrieve data from a “view” whose definition is given in a new option SRCDEF . For instance:

CREATE TABLE xsvars ENGINE=CONNECT table_type=XCOL
srcdef='show variables like "optimizer_switch"'
option_list='Colname=Value';

Then, for instance:

SELECT value FROM xsvars LIMIT 10;

This will display something like:

value
index_merge=on
index_merge_union=on
index_merge_sort_union=on
index_merge_intersection=on
index_merge_sort_intersection=off
engine_condition_pushdown=off
index_condition_pushdown=on
derived_merge=on
derived_with_keys=on
firstmatch=on

Note: All XCOL tables are read only.

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