SQL Queries and Subqueries

GaussDB is compatible with SQL queries and subqueries except hierarchical queries.

**Table 1** SQL queries and subqueries
No.	Oracle	GaussDB	Difference
1	Creating simple queries	Supported.	-
2	Hierarchical queries	Not supported.	-
3	UNION [ALL], INTERSECT, MINUS	Supported.	-
4	Sorting query results	Supported, with differences.	If the GaussDB query does not contain groups and the target column contains both an aggregate function and a set return function, the sorting of the set return function column is not ignored.
5	Joins	Supported, with differences.	GaussDB supports only the same join types as Oracle, such as left/right, self, natural, and full outer join. Join optimization methods such as IN-MEMORY JOIN GROUPS are not supported.
6	Using subqueries	Supported.	-
7	Unnesting of nested subqueries	Supported, with differences.	HASH_AJ or MERGE_AJ cannot be explicitly specified in GaussDB.
8	Distributed queries	Supported, with differences.	GaussDB requires explicit database link query.
9	Aggregate function nesting	Supported.	-