Mapping the GROUP BY clause: GroupByClauseMapper element

To map the GROUP BY clause, use the GroupByClauseMapper element in mappings.xml. The GroupByClauseMapper element has two attributes, UseSelectItemIndexes and PushComplexExprs.

UseSelectItemIndexes attribute

Most databases support expressions in the GROUP BY clause of a query, for example:

SELECT contact_last

FROM CUSTOMERS

GROUP BY city, contact_last || ', ' || contact_first

Some databases, however, do not. The SELECT clause must contain the expression and the GROUP BY clause must reference the expression by index, for example:

SELECT city, contact_last || ', ' || contact_first

FROM CUSTOMERS

GROUP BY 1, 2

If your database does not support expressions in the GROUP BY clause, set the UseSelectItemIndexes attribute to true, for example:

<GroupByClauseMapper UseSelectItemIndexes="true" />

PushComplexExprs attribute

Some databases do not support GROUP BY expressions other than column references. For such databases, a GROUP BY expression that is not a column reference should not be sent to the database. For example, GROUP BY contact_last should be sent to the database, but GROUP BY contact_last || ', ' || contact_first should not.

If your database does not support GROUP BY expressions other than column references, set the PushComplexExprs attribute to false, for example:

<GroupByClauseMapper PushComplexExprs="false" />

 

 

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