Using a summary function with a GROUP BY clause in PROC SQL may cause
the following message to be written to the SAS Log:

  NOTE: The query requires remerging summary statistics back with the
        original data.

This note is generated when a column, listed on the select clause,
is not being used in a summary function and the column is not listed on
the GROUP BY clause.  If the column selected is not used in a summary
function, adding it to a GROUP BY clause prevents the note.

When this situation occurs, summary groups are not collapsed into a
single row per group, even if a GROUP BY clause is specified and the
rows within the groups are identical.  This is due to PROC SQL
choosing a course of action based on the query, not the values of the
columns.

If no GROUP BY clause is coded in the query, the complete table is
treated as one group.

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