2.2.6.1 ROLLUP Extension

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The ROLLUP extension produces a hierarchical set of groups with subtotals for each hierarchical group as well as a grand total. The order of the hierarchy is determined by the order of the expressions given in the ROLLUP expression list. The top of the hierarchy is the leftmost item in the list. Each successive item proceeding to the right moves down the hierarchy with the rightmost item being the lowest level.
The syntax for a single ROLLUP is as follows:
ROLLUP ( { expr_1 | ( expr_1a [, expr_1b ] ...) }
[, expr_2 | ( expr_2a [, expr_2b ] ...) ] ...)
Each expr is an expression that determines the grouping of the result set. If enclosed within parenthesis as ( expr_1a, expr_1b, ...) then the combination of values returned by expr_1a and expr_1b defines a single grouping level of the hierarchy.
In addition, a subtotal is returned for the first item in the list (expr_1 or the combination of ( expr_1a, expr_1b, ...), whichever is specified) for each unique value. A subtotal is returned for the second item in the list (expr_2 or the combination of ( expr_2a, expr_2b, ...), whichever is specified) for each unique value, within each grouping of the first item and so on. Finally a grand total is returned for the entire result set.
The ROLLUP extension specified within the context of the GROUP BY clause is shown by the following:
SELECT select_list FROM ...
GROUP BY [... ,] ROLLUP ( expression_list ) [, ...]
The items specified in select_list must also appear in the ROLLUP expression_list; or they must be aggregate functions such as COUNT, SUM, AVG, MIN, or MAX; or they must be constants or functions whose return values are independent of the individual rows in the group (for example, the SYSDATE function).
The GROUP BY clause may specify multiple ROLLUP extensions as well as multiple occurrences of other GROUP BY extensions and individual expressions.
The ORDER BY clause should be used if you want the output to display in a hierarchical or other meaningful structure. There is no guarantee on the order of the result set if no ORDER BY clause is specified.
The number of grouping levels or totals is n + 1 where n represents the number of items in the ROLLUP expression list. A parenthesized list counts as one item.
The following is the result of the query. There is a count of the number of employees for each unique combination of loc, dname, and job, as well as subtotals for each unique combination of loc and dname, for each unique value of loc, and a grand total displayed on the last line.
In the output, note that there are no subtotals for loc and dname combinations as in the prior example.
If the first two columns in the ROLLUP list are enclosed in parenthesis, the subtotal levels differ as well.
Now there is a subtotal for each unique loc and dname combination, but none for unique values of loc.


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