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Support NVARCHAR Type in Avatica when searching Oracle.

[CALCITE-4868] Elasticsearch adapter fails if GROUP BY is followed by ORDER BY

3

The PR supports sorting aggregation result in Elasticsearch Adapter. Like: **select val2, max(val3) as MAX_VAL3 from view group by val2 order by MAX_VAL3 desc** Quoted from ES: "Buckets can only...

[CALCITE-5685] Support MySQL CONVERT function that works on data types

5

Enable MySQL convert(expr, type) function that works on converting data types. Noted: SIGNED type is equivalent with BigInt in Calcite type system, and this PR use `SqlAlienSystemTypeNameSpec` to represent UNSIGNED,...

[CALCITE-815] Add an option to allow empty strings to represent null values

3

The following functions will return null if the result is an empty string under Oracle semantics: - concat (||), UPPER, LOWER, TRIM, OVERLAY, SUBSTRING, INITCAP, REPLACE - JDBC functions: INSERT,...