Craig Freedman's SQL Server Blog

A discussion of query processing, query execution, and query plans in SQL Server.

GROUPING SETS in SQL Server 2008

GROUPING SETS in SQL Server 2008

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In my last two posts, I gave examples of aggregation WITH ROLLUP and CUBE.  SQL Server 2008 continues to support this syntax, but also introduces new more powerful ANSI SQL 2006 compliant syntax.  In this post, I'll give an overview of the changes.

First, let's see how we rewrite simple WITH ROLLUP and CUBE queries using the new syntax.  I'll use the same schema and queries as in my previous posts:

CREATE TABLE Sales (EmpId INT, Yr INT, Sales MONEY)
INSERT Sales VALUES(1, 2005, 12000)
INSERT Sales VALUES(1, 2006, 18000)
INSERT Sales VALUES(1, 2007, 25000)
INSERT Sales VALUES(2, 2005, 15000)
INSERT Sales VALUES(2, 2006, 6000)
INSERT Sales VALUES(3, 2006, 20000)
INSERT Sales VALUES(3, 2007, 24000)

SELECT EmpId, Yr, SUM(Sales) AS Sales
FROM Sales
GROUP BY EmpId, Yr WITH ROLLUP

SELECT EmpId, Yr, SUM(Sales) AS Sales
FROM Sales
GROUP BY EmpId, Yr WITH CUBE

We can rewrite these two queries using the new syntax as:

SELECT EmpId, Yr, SUM(Sales) AS Sales
FROM Sales
GROUP BY ROLLUP(EmpId, Yr)

SELECT EmpId, Yr, SUM(Sales) AS Sales
FROM Sales
GROUP BY CUBE(EmpId, Yr)

These new queries are semantically equivalent to and use the same query plans as the original queries.  Note that the new ROLLUP and CUBE syntax is only available in compatibility level 100.  The more general GROUPING SETS syntax, which I will discuss next, is also available in earlier compatibility levels.

The new GROUPING SETS syntax is considerably more powerful.  It allows us to specify precisely which aggregations we want to compute.  As the following table illustrates, our simple two dimensional schema has a total of only four possible aggregations:

 

Yr

2005

2006

2007

ALL

EmpId

1

GROUP BY (EmpId, Yr)

GROUP BY (EmpId)

2

3

ALL

GROUP BY (Yr)

GROUP BY ()

ROLLUP and CUBE are just shorthand for two common usages of GROUPING SETS.  We can express the above ROLLUP query as:

SELECT EmpId, Yr, SUM(Sales) AS Sales
FROM Sales
GROUP BY GROUPING SETS((EmpId, Yr), (EmpId), ())

EmpId       Yr          Sales
----------- ----------- ---------------------
1           2005        12000.00
1           2006        18000.00
1           2007        25000.00
1           NULL        55000.00
2           2005        15000.00
2           2006        6000.00
2           NULL        21000.00
3           2006        20000.00
3           2007        24000.00
3           NULL        44000.00
NULL        NULL        120000.00

This query explicitly asks SQL Server to aggregate sales by employee and year, to aggregate by employee only, and to compute the total for all employees for all years.  The () syntax with no GROUP BY columns denotes the total.  Similarly, we can express the above CUBE query by asking SQL Server to compute all possible aggregate combinations:

SELECT EmpId, Yr, SUM(Sales) AS Sales
FROM Sales
GROUP BY GROUPING SETS((EmpId, Yr), (EmpId), (Yr), ())

EmpId       Yr          Sales
----------- ----------- ---------------------
1           2005        12000.00
2           2005        15000.00
NULL        2005        27000.00
1           2006        18000.00
2           2006        6000.00
3           2006        20000.00
NULL        2006        44000.00
1           2007        25000.00
3           2007        24000.00
NULL        2007        49000.00
NULL        NULL        120000.00
1           NULL        55000.00
2           NULL        21000.00
3           NULL        44000.00

We can also use GROUPING SETS to compute other results.  For example, we can perform a partial rollup aggregating sales by employee and year and by employee only but without computing the total for all employees for all years:

SELECT EmpId, Yr, SUM(Sales) AS Sales
FROM Sales
GROUP BY GROUPING SETS((EmpId, Yr), (EmpId))

EmpId       Yr          Sales
----------- ----------- ---------------------
1           2005        12000.00
1           2006        18000.00
1           2007        25000.00
1           NULL        55000.00
2           2005        15000.00
2           2006        6000.00
2           NULL        21000.00
3           2006        20000.00
3           2007        24000.00
3           NULL        44000.00

We can skip certain rollup levels.  For example, we can compute the total sales by employee and year and the total sales for all employees and all years without computing any of the intermediate results:

SELECT EmpId, Yr, SUM(Sales) AS Sales
FROM Sales
GROUP BY GROUPING SETS((EmpId, Yr), ())

EmpId       Yr          Sales
----------- ----------- ---------------------
1           2005        12000.00
1           2006        18000.00
1           2007        25000.00
2           2005        15000.00
2           2006        6000.00
3           2006        20000.00
3           2007        24000.00
NULL        NULL        120000.00

We can even compute multiple unrelated aggregations along disparate dimensions.  For example, we can compute the total sales by employee and the total sales by year:

SELECT EmpId, Yr, SUM(Sales) AS Sales
FROM Sales
GROUP BY GROUPING SETS((EmpId), (Yr))

EmpId       Yr          Sales
----------- ----------- ---------------------
NULL        2005        27000.00
NULL        2006        44000.00
NULL        2007        49000.00
1           NULL        55000.00
2           NULL        21000.00
3           NULL        44000.00

Note that we could also write GROUPING SETS (EmpId, Yr) without the extra set of parenthesis, but the extra parenthesis make the intent of the query more explicit and clearly differentiate the previous query from the following query which just performs a normal aggregation by employee and year:

SELECT EmpId, Yr, SUM(Sales) AS Sales
FROM Sales
GROUP BY GROUPING SETS((EmpId, Yr))

EmpId       Yr          Sales
----------- ----------- ---------------------
1           2005        12000.00
2           2005        15000.00
1           2006        18000.00
2           2006        6000.00
3           2006        20000.00
1           2007        25000.00
3           2007        24000.00

Here are some additional points worth noting about the GROUPING SETS syntax:

As with any other aggregation query, if a column appears in the SELECT list and is not part of an aggregate function, it must appear somewhere in the GROUP BY clause.  Thus, the following is not valid:

SELECT EmpId, Yr, SUM(Sales) AS Sales
FROM Sales
GROUP BY GROUPING SETS((EmpId), ())

Msg 8120, Level 16, State 1, Line 1
Column 'Sales.Yr' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.

The order of the columns within each GROUPING SET and the order of the GROUPING SETS does not matter.  So both of the following queries compute the same CUBE although the order that the rows are output differs:

SELECT EmpId, Yr, SUM(Sales) AS Sales
FROM Sales
GROUP BY GROUPING SETS((EmpId, Yr), (EmpId), (Yr), ())

SELECT EmpId, Yr, SUM(Sales) AS Sales
FROM Sales
GROUP BY GROUPING SETS((), (Yr), (EmpId), (Yr, EmpId))

If the order that the rows are output matters, use an explicit ORDER BY clause to enforce that order.

We can nest CUBE and ROLLUP within a GROUPING SETS clause as shorthand for expressing more complex GROUPING SETS.  This shorthand is most useful when we have more than three dimensions in our schema.  For example, suppose we add a month column to our sales table:

CREATE TABLE Sales (EmpId INT, Month INT, Yr INT, Sales MONEY)

Now, suppose we want to compute sales for each employee by month and year, by year, and total.  We could write out all of the GROUPING SETS explicitly:

SELECT EmpId, Month, Yr, SUM(Sales) AS Sales
FROM Sales
GROUP BY GROUPING SETS((EmpId, Yr, Month), (EmpId, Yr), (EmpId))

Or we can use ROLLUP to simplify the query:

SELECT EmpId, Month, Yr, SUM(Sales) AS Sales
FROM Sales
GROUP BY GROUPING SETS((EmpId, ROLLUP(Yr, Month)))

Note that once again the correct use of parenthesis is critical.  If we omit one set of parenthesis from the above query, the meaning changes significantly and we end up separately aggregating by employee and then computing the year and month ROLLUP for all employees.

The new GROUPING SETS syntax is available in all of SQL Server 2008 Community Technology Preview (CTP) releases.
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  • Looking at the table that shows "four possible aggregations" really helped me understand the concept of GROUPING SETS

  • Nota 10!!!

    Adorei essa forma de listar e somar os valores.

    A função CUBE já existe a anos no ORACLE e estava ancioso para ter ela rodando no SQL.

    Parabéns

    Robinson - TI - Phonoway Systemas - São Paulo - SP - Brazil

  • that is cool!

  • This post is very helpful

  • This was something knew to me. I played around with my tables and found it very usefull.Will impliment it the first chance i get.

    Thanks Craig

  • I am currently studying for a Microsft Certification and your posts have really helped me understand some of these key features, thanks.

  • I needed a quick explanation of GROUPING SETS and your post was perfect!  The examples you provided really helped make the point for me.

  • Great post! Very helpful and will practice at my work. Thanks Craig.

  • Excellent Articles. Thanks for sharing this. Thanks once again to Craig Freedman.

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