GROUP BY - SQL Server to Aurora MySQL Migration Playbook
Services or capabilities described in Amazon Web Services documentation might vary by Region. To see the differences applicable to the China Regions, see Getting Started with Amazon Web Services in China (PDF).

GROUP BY

Feature compatibility Amazon SCT / Amazon DMS automation level Amazon SCT action code index Key differences


                              Four star feature compatibility


                              Four star automation level

GROUP BY

Basic syntax compatible. Advanced options such as ALL, CUBE, GROUPING SETS will require rewrites to use multiple queries with UNION.

SQL Server Usage

GROUP BY is an ANSI SQL query clause used to group individual rows that have passed the WHERE filter clause into groups to be passed on to the HAVING filter and then to the SELECT list. This grouping supports the use of aggregate functions such as SUM, MAX, AVG and others.

Syntax

ANSI compliant GROUP BY syntax:

GROUP BY
[ROLLUP | CUBE]
<Column Expression> ...n
[GROUPING SETS (<Grouping Set>)...n

Backward compatibility syntax:

GROUP BY
    [ ALL ] <Column Expression> ...n
    [ WITH CUBE | ROLLUP ]

The basic ANSI syntax for GROUP BY supports multiple grouping expressions, the CUBE and ROLLUP keywords, and the GROUPING SETS clause; all used to add super-aggregate rows to the output.

Up to SQL Server 2008 R2, the database engine supported a legacy, proprietary, and not ANSI-compliant syntax using the WITH CUBE and WITH ROLLUP clauses. These clauses added super-aggregates to the output.

Also, up to SQL Server 2008 R2, SQL Server supported the GROUP BY ALL syntax, which was used to create an empty group for rows that failed the WHERE clause.

SQL Server supports the following aggregate functions: AVG, CHECKSUM_AGG, COUNT, COUNT_BIG, GROUPING, GROUPING_ID, STDEV, STDEVP, STRING_AGG, SUM, MIN, MAX, VAR, VARP.

Examples

Legacy CUBE and ROLLUP Syntax

CREATE TABLE Orders
(
    OrderID INT IDENTITY(1,1) NOT NULL
    PRIMARY KEY,
    Customer VARCHAR(20) NOT NULL,
    OrderDate DATE NOT NULL
);
INSERT INTO Orders(Customer, OrderDate)
VALUES ('John', '20180501'), ('John', '20180502'), ('John', '20180503'),
    ('Jim', '20180501'), ('Jim', '20180503'), ('Jim', '20180504')
SELECT Customer,
    OrderDate,
    COUNT(*) AS NumOrders
FROM Orders AS O
GROUP BY Customer, OrderDate
WITH ROLLUP
Customer  OrderDate   NumOrders
Jim       2018-05-01  1
Jim       2018-05-03  1
Jim       2018-05-04  1
Jim       NULL        3
John      2018-05-01  1
John      2018-05-02  1
John      2018-05-03  1
John      NULL        3
NULL      NULL        6

The rows with NULL values were added as a result of the WITH ROLLUP clause and contain super aggregates for the following:

  • All orders for Jim and John regardless of OrderDate.

  • A super aggregated for all customers and all dates.

Using CUBE instead of ROLLUP adds super aggregates in all possible combinations, not only in group by expression order.

SELECT Customer,
    OrderDate,
    COUNT(*) AS NumOrders
FROM Orders AS O
GROUP BY Customer, OrderDate
WITH CUBE
Customer  OrderDate   NumOrders
Jim       2018-05-01  1
John      2018-05-01  1
NULL      2018-05-01  2
John      2018-05-02  1
NULL      2018-05-02  1
Jim       2018-05-03  1
John      2018-05-03  1
NULL      2018-05-03  2
Jim       2018-05-04  1
NULL      2018-05-04  1
NULL      NULL        6
Jim       NULL        3
John      NULL        3

The additional four rows where the value for Customer is set to NULL, were added by CUBE. These rows provide super aggregates for every date for all customers that were not part of the ROLLUP results.

Legacy GROUP BY ALL

Use the Orders table from the preceding example.

SELECT Customer,
    OrderDate,
    COUNT(*) AS NumOrders
FROM Orders AS O
WHERE OrderDate <= '20180503'
GROUP BY ALL Customer, OrderDate
Customer  OrderDate   NumOrders
Jim       2018-05-01  1
John      2018-05-01  1
John      2018-05-02  1
Jim       2018-05-03  1
John      2018-05-03  1
Jim       2018-05-04  0
Warning: Null value is eliminated by an aggregate or other SET operation.

The last row for 2018-05-04 failed the WHERE clause and was returned as an empty group as indicated by the warning for the empty COUNT(*) = 0.

Use GROUPING SETS

The following query uses the ANSI compliant GROUPING SETS syntax to provide all possible aggregate combinations for the Orders table, similar to the result of the CUBE syntax. This syntax requires specifying each dimension that needs to be aggregated.

SELECT Customer,
    OrderDate,
    COUNT(*) AS NumOrders
FROM Orders AS O
GROUP BY GROUPING SETS (
    (Customer, OrderDate),
    (Customer),
    (OrderDate),
    ()
    )
Customer  OrderDate   NumOrders
Jim       2018-05-01  1
John      2018-05-01  1
NULL      2018-05-01  2
John      2018-05-02  1
NULL      2018-05-02  1
Jim       2018-05-03  1
John      2018-05-03  1
NULL      2018-05-03  2
Jim       2018-05-04  1
NULL      2018-05-04  1
NULL      NULL        6
Jim       NULL        3
John      NULL        3

For more information, see Aggregate Functions (Transact-SQL) and SELECT - GROUP BY- Transact-SQL in the SQL Server documentation.

MySQL Usage

Amazon Aurora MySQL-Compatible Edition (Aurora MySQL) supports only the basic ANSI syntax for GROUP BY and doesn’t support GROUPING SETS or the standard GROUP BY CUBE and GROUP BY ROLLUP. Aurora MySQL supports the WITH ROLLUP non-ANSI syntax like SQL Server, but not the CUBE option.

Aurora MySQL supports a wider range of aggregate functions than SQL Server: AVG, BIT_AND, BIT_OR, BIT_XOR, COUNT, GROUP_CONCAT, JSON_ARRAYAGG, JSON_OBJECTAGG, MAX, MIN, STD, STDDEV, STDDEV_POP, STDDEV_SAMP, SUM, VAR_POP, VAR_SAMP, VARIANCE.

The bitwise aggregates and the JSON aggregates not available in SQL Server may prove to be very useful in many scenarios. For more information, see MySQL Handling of GROUP BY in the MySQL documentation.

Unlike SQL Server, in Aurora MySQL you can’t use ROLLUP and ORDER BY clauses in the same query. As a workaround, encapsulate the ROLLUP query as a derived table and add the ORDER BY clause to the outer query.

SELECT *
FROM (
    SELECT Customer,
        OrderDate,
        COUNT(*) AS NumOrders
    FROM Orders AS O
    GROUP BY Customer, OrderDate
    WITH ROLLUP
)
ORDER BY OrderDate, Customer;

Additionally, rows produced by ROLLUP can’t be referenced in a WHERE clause or in a FROM clause as a join condition because the super aggregates are added late in the processing phase.

Even more problematic is the lack of a function equivalent to the GROUPING_ID function in SQL Server, which can be used to distinguish super aggregate rows from the base groups. Unfortunately, it is currently not possible to distinguish rows that have NULLs due to being super aggregates from rows where the NULL is from the base set.

Until SQL92, column expressions not appearing in the GROUP BY list were not allowed in the HAVING, SELECT, and ORDER BY clauses. This limitation still applies in SQL Server today. For example, the following query isn’t legal in SQL Serve since a customer group may contain multiple order dates.

SELECT Customer,
    OrderDate,
    COUNT(*) AS NumOrders
FROM Orders AS O
GROUP BY Customer

However, in some cases, when the columns that don’t appear in the GROUP BY clause are functionally dependent on the GROUP BY columns, it does make sense to allow it and ANSI SQL optional feature T301 does allow it. Aurora MySQL can detect such functional dependencies and allows such queries to run.

Note

To use non-aggregate columns in the HAVING, SELECT, and ORDER BY clauses, turn on the ONLY_FULL_GROUP_BY SQL mode.

Syntax

SELECT <Select List>
FROM <Table Source>
WHERE <Row Filter>
GROUP BY <Column Name> | <Expression> | <Position>
    [ASC | DESC], ...
    [WITH ROLLUP]]

Migration Considerations

For most aggregate queries that use only grouping expressions without modifiers, the migration should be straightforward. Even the WITH ROLLUP syntax is supported as is in Aurora MySQL. For more complicated aggregates such as CUBE and GROUPING SETS, a rewrite to include all sub-aggregate queries as UNION ALL sets is required.

Because Aurora MySQL supports a wider range of aggregate functions, the migration shouldn’t present major challenges. Some minor syntax changes, for example replacing STDEVP with STDDEV_POP, can be performed automatically by the Amazon Schema Conversion Tool (Amazon SCT. Some may need manual intervention such as rewriting the STRING_AGG syntax to GROUP_CONCAT. Also consider using Aurora MySQL additional aggregate functions for query optimizations.

If you plan to keep using the WITH ROLLUP groupings, you must consider how to handle NULLS since Aurora MySQL doesn’t support an equivalent function to GROUPING_ID in SQL Server.

Examples

Rewrite SQL Server WITH CUBE modifier for migration.

CREATE TABLE Orders
(
    OrderID INT NOT NULL AUTO_INCREMENT
    PRIMARY KEY,
    Customer VARCHAR(20) NOT NULL,
    OrderDate DATE NOT NULL
);
INSERT INTO Orders(Customer, OrderDate)
VALUES ('John', '20180501'), ('John', '20180502'), ('John', '20180503'),
    ('Jim', '20180501'), ('Jim', '20180503'), ('Jim', '20180504')
SELECT Customer,
    OrderDate,
    COUNT(*) AS NumOrders
FROM Orders AS O
GROUP BY Customer, OrderDate
WITH ROLLUP
UNION ALL -- Add the super aggregate rows for each OrderDate
SELECT NULL,
    OrderDate,
    COUNT(*) AS NumOrders
FROM Orders AS O
GROUP BY OrderDate
Customer  OrderDate   NumOrders
Jim       2018-05-01  1
Jim       2018-05-03  1
Jim       2018-05-04  1
Jim       NULL        3
John      2018-05-01  1
John      2018-05-02  1
John      2018-05-03  1
John      NULL        3
NULL      NULL        6
NULL      2018-05-01  2
NULL      2018-05-02  1
NULL      2018-05-03  2
NULL      2018-05-04  1

Rewrite SQL Server GROUP BY ALL for migration.

SELECT Customer,
    OrderDate,
    COUNT(*) AS NumOrders
FROM Orders AS O
WHERE OrderDate <= '20180503'
GROUP BY Customer, OrderDate
UNION ALL -- Add the empty groups
SELECT DISTINCT Customer,
    OrderDate,
    NULL
FROM Orders AS O
WHERE OrderDate > '20180503';
Customer  OrderDate   NumOrders
Jim       2018-05-01  1
Jim       2018-05-03  1
John      2018-05-01  1
John      2018-05-02  1
John      2018-05-03  1
Jim       2018-05-04  NULL

Summary

Table of similarities, differences, and key migration considerations.

SQL Server feature Aurora MySQL feature Comments

MAX, MIN, AVG, COUNT, COUNT_BIG

MAX, MIN, AVG, COUNT

In Aurora MySQL, COUNT returns a BIGINT and is compatible with COUNT and COUNT_BIG in SQL Server.

CHECKSUM_AGG

N/A

Use a loop to calculate checksums.

GROUPING, GROUPING_ID

N/A

Reconsider query logic to avoid having NULL groups that are ambiguous with the super aggregates.

STDEV, STDEVP, VAR, VARP

STDDEV, STDDEV_POP, VARIANCE, VAR_POP

Rewrite keywords only.

STRING_AGG

GROUP_CONCAT

Rewrite syntax.

WITH ROLLUP

WITH ROLLUP

Compatible

WITH CUBE

N/A

Rewrite using UNION ALL.

ANSI CUBE / ROLLUP

N/A

Rewrite using WITH ROLLUP and using UNION ALL queries.

GROUPING SETS

N/A

Rewrite using UNION ALL queries.

N/A

Non-aggregate columns in HAVING, SELECT, ORDER BY

Requires to turn off the ONLY_FULL_GROUP_BY SQL mode. Functional dependencies are evaluated by the engine.

For more information, see MySQL Handling of GROUP BY in the MySQL documentation.