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GROUPING

GROUPING​

ROLLUP and CUBE are modifiers to GROUP BY. Both of these calculate subtotals. ROLLUP takes an ordered list of columns, for example (day, month, year), and calculates subtotals at each level of the aggregation and then a grand total. CUBE calculates subtotals across all possible combinations of the columns specified. GROUPING identifies which rows returned by ROLLUP or CUBE are superaggregates, and which are rows that would be returned by an unmodified GROUP BY.

The GROUPING function takes multiple columns as an argument, and returns a bitmask.

  • 1 indicates that a row returned by a ROLLUP or CUBE modifier to GROUP BY is a subtotal
  • 0 indicates that a row returned by a ROLLUP or CUBE is a row that is not a subtotal

GROUPING SETS​

By default, the CUBE modifier calculates subtotals for all possible combinations of the columns passed to CUBE. GROUPING SETS allows you to specify the specific combinations to calculate.

Analyzing hierarchical data is a good use case for ROLLUP, CUBE, and GROUPING SETS modifiers. The sample here is a table containing data about what Linux distribution, and the version of that distribution is installed across two datacenters. It may be valuable to look at the data by distribution, version, and location.

Load sample data​

CREATE TABLE servers ( datacenter VARCHAR(255),
                         distro VARCHAR(255) NOT NULL,
                         version VARCHAR(50) NOT NULL,
                         quantity INT
                       )
                        ORDER BY (datacenter, distro, version)
INSERT INTO servers(datacenter, distro, version, quantity)
VALUES ('Schenectady', 'Arch','2022.08.05',50),
       ('Westport', 'Arch','2022.08.05',40),
       ('Schenectady','Arch','2021.09.01',30),
       ('Westport', 'Arch','2021.09.01',20),
       ('Schenectady','Arch','2020.05.01',10),
       ('Westport', 'Arch','2020.05.01',5),
       ('Schenectady','RHEL','9',60),
       ('Westport','RHEL','9',70),
       ('Westport','RHEL','7',80),
       ('Schenectady','RHEL','7',80)
SELECT 
    *
FROM
    servers;
┌─datacenter──┬─distro─┬─version────┬─quantity─┐
│ Schenectady │ Arch   │ 2020.05.01 │       10 │
│ Schenectady │ Arch   │ 2021.09.01 │       30 │
│ Schenectady │ Arch   │ 2022.08.05 │       50 │
│ Schenectady │ RHEL   │ 7          │       80 │
│ Schenectady │ RHEL   │ 9          │       60 │
│ Westport    │ Arch   │ 2020.05.01 │        5 │
│ Westport    │ Arch   │ 2021.09.01 │       20 │
│ Westport    │ Arch   │ 2022.08.05 │       40 │
│ Westport    │ RHEL   │ 7          │       80 │
│ Westport    │ RHEL   │ 9          │       70 │
└─────────────┴────────┴────────────┴──────────┘

10 rows in set. Elapsed: 0.409 sec.

Simple queries​

Get the count of servers in each data center by distribution: 

SELECT
    datacenter,
    distro, 
    SUM (quantity) qty
FROM
    servers
GROUP BY
    datacenter,
    distro;
┌─datacenter──┬─distro─┬─qty─┐
│ Schenectady │ RHEL   │ 140 │
│ Westport    │ Arch   │  65 │
│ Schenectady │ Arch   │  90 │
│ Westport    │ RHEL   │ 150 │
└─────────────┴────────┴─────┘

4 rows in set. Elapsed: 0.212 sec.
SELECT
    datacenter, 
    SUM (quantity) qty
FROM
    servers
GROUP BY
    datacenter;
┌─datacenter──┬─qty─┐
│ Westport    │ 215 │
│ Schenectady │ 230 │
└─────────────┴─────┘

2 rows in set. Elapsed: 0.277 sec. 
SELECT
    distro, 
    SUM (quantity) qty
FROM
    servers
GROUP BY
    distro;

┌─distro─┬─qty─┐
│ Arch   │ 155 │
│ RHEL   │ 290 │
└────────┴─────┘

2 rows in set. Elapsed: 0.352 sec. 
SELECT
    SUM(quantity) qty
FROM
    servers;
┌─qty─┐
│ 445 │
└─────┘

1 row in set. Elapsed: 0.244 sec.

Comparing multiple GROUP BY statements with GROUPING SETS​

Breaking down the data without CUBE, ROLLUP, or GROUPING SETS:

SELECT
    datacenter,
    distro, 
    SUM (quantity) qty
FROM
    servers
GROUP BY
    datacenter,
    distro
UNION ALL
SELECT
    datacenter, 
    null,
    SUM (quantity) qty
FROM
    servers
GROUP BY
    datacenter
UNION ALL
SELECT
    null,
    distro, 
    SUM (quantity) qty
FROM
    servers
GROUP BY
    distro
UNION ALL
SELECT
    null,
    null,
    SUM(quantity) qty
FROM
    servers;
┌─datacenter─┬─distro─┬─qty─┐
│ ᴺᵁᴸᴸ       │ ᴺᵁᴸᴸ   │ 445 │
└────────────┴────────┴─────┘
┌─datacenter──┬─distro─┬─qty─┐
│ Westport    │ ᴺᵁᴸᴸ   │ 215 │
│ Schenectady │ ᴺᵁᴸᴸ   │ 230 │
└─────────────┴────────┴─────┘
┌─datacenter──┬─distro─┬─qty─┐
│ Schenectady │ RHEL   │ 140 │
│ Westport    │ Arch   │  65 │
│ Schenectady │ Arch   │  90 │
│ Westport    │ RHEL   │ 150 │
└─────────────┴────────┴─────┘
┌─datacenter─┬─distro─┬─qty─┐
│ ᴺᵁᴸᴸ       │ Arch   │ 155 │
│ ᴺᵁᴸᴸ       │ RHEL   │ 290 │
└────────────┴────────┴─────┘

9 rows in set. Elapsed: 0.527 sec.

Getting the same information using GROUPING SETS:

SELECT
    datacenter,
    distro, 
    SUM (quantity) qty
FROM
    servers
GROUP BY
    GROUPING SETS(
        (datacenter,distro),
        (datacenter),
        (distro),
        ()
    )
┌─datacenter──┬─distro─┬─qty─┐
│ Schenectady │ RHEL   │ 140 │
│ Westport    │ Arch   │  65 │
│ Schenectady │ Arch   │  90 │
│ Westport    │ RHEL   │ 150 │
└─────────────┴────────┴─────┘
┌─datacenter──┬─distro─┬─qty─┐
│ Westport    │        │ 215 │
│ Schenectady │        │ 230 │
└─────────────┴────────┴─────┘
┌─datacenter─┬─distro─┬─qty─┐
│            │        │ 445 │
└────────────┴────────┴─────┘
┌─datacenter─┬─distro─┬─qty─┐
│            │ Arch   │ 155 │
│            │ RHEL   │ 290 │
└────────────┴────────┴─────┘

9 rows in set. Elapsed: 0.427 sec.

Comparing CUBE with GROUPING SETS​

The CUBE in the next query, CUBE(datacenter,distro,version) provides a hierarchy that may not make sense. It does not make sense to look at Version across the two distributions (as Arch and RHEL do not have the same release cycle or version naming standards). The GROUPING SETS example following this one is more appropriate as it groups distro and version in the same set. 

SELECT
   datacenter,
   distro,
   version,
   SUM(quantity)
FROM
   servers
GROUP BY
   CUBE(datacenter,distro,version)
ORDER BY
   datacenter,
   distro;
┌─datacenter──┬─distro─┬─version────┬─sum(quantity)─┐
│             │        │ 7          │           160 │
│             │        │ 2020.05.01 │            15 │
│             │        │ 2021.09.01 │            50 │
│             │        │ 2022.08.05 │            90 │
│             │        │ 9          │           130 │
│             │        │            │           445 │
│             │ Arch   │ 2021.09.01 │            50 │
│             │ Arch   │ 2022.08.05 │            90 │
│             │ Arch   │ 2020.05.01 │            15 │
│             │ Arch   │            │           155 │
│             │ RHEL   │ 9          │           130 │
│             │ RHEL   │ 7          │           160 │
│             │ RHEL   │            │           290 │
│ Schenectady │        │ 9          │            60 │
│ Schenectady │        │ 2021.09.01 │            30 │
│ Schenectady │        │ 7          │            80 │
│ Schenectady │        │ 2022.08.05 │            50 │
│ Schenectady │        │ 2020.05.01 │            10 │
│ Schenectady │        │            │           230 │
│ Schenectady │ Arch   │ 2022.08.05 │            50 │
│ Schenectady │ Arch   │ 2021.09.01 │            30 │
│ Schenectady │ Arch   │ 2020.05.01 │            10 │
│ Schenectady │ Arch   │            │            90 │
│ Schenectady │ RHEL   │ 7          │            80 │
│ Schenectady │ RHEL   │ 9          │            60 │
│ Schenectady │ RHEL   │            │           140 │
│ Westport    │        │ 9          │            70 │
│ Westport    │        │ 2020.05.01 │             5 │
│ Westport    │        │ 2022.08.05 │            40 │
│ Westport    │        │ 7          │            80 │
│ Westport    │        │ 2021.09.01 │            20 │
│ Westport    │        │            │           215 │
│ Westport    │ Arch   │ 2020.05.01 │             5 │
│ Westport    │ Arch   │ 2021.09.01 │            20 │
│ Westport    │ Arch   │ 2022.08.05 │            40 │
│ Westport    │ Arch   │            │            65 │
│ Westport    │ RHEL   │ 9          │            70 │
│ Westport    │ RHEL   │ 7          │            80 │
│ Westport    │ RHEL   │            │           150 │
└─────────────┴────────┴────────────┴───────────────┘

39 rows in set. Elapsed: 0.355 sec.
Note

Version in the above example may not make sense when it is not associated with a distro, if we were tracking the kernel version it might make sense because the kernel version can be associated with either distro. Using GROUPING SETS, as in the next example, may be a better choice.

SELECT
    datacenter,
    distro,
    version,
    SUM(quantity)
FROM servers
GROUP BY
    GROUPING SETS (
        (datacenter, distro, version),
        (datacenter, distro))
┌─datacenter──┬─distro─┬─version────┬─sum(quantity)─┐
│ Westport    │ RHEL   │ 9          │            70 │
│ Schenectady │ Arch   │ 2022.08.05 │            50 │
│ Schenectady │ Arch   │ 2021.09.01 │            30 │
│ Schenectady │ RHEL   │ 7          │            80 │
│ Westport    │ Arch   │ 2020.05.01 │             5 │
│ Westport    │ RHEL   │ 7          │            80 │
│ Westport    │ Arch   │ 2021.09.01 │            20 │
│ Westport    │ Arch   │ 2022.08.05 │            40 │
│ Schenectady │ RHEL   │ 9          │            60 │
│ Schenectady │ Arch   │ 2020.05.01 │            10 │
└─────────────┴────────┴────────────┴───────────────┘
┌─datacenter──┬─distro─┬─version─┬─sum(quantity)─┐
│ Schenectady │ RHEL   │         │           140 │
│ Westport    │ Arch   │         │            65 │
│ Schenectady │ Arch   │         │            90 │
│ Westport    │ RHEL   │         │           150 │
└─────────────┴────────┴─────────┴───────────────┘

14 rows in set. Elapsed: 1.036 sec.