数据库中的集合操作可以把多个查询的结果组合成一个结果集。集合操作主要包含：

    • UNION

    • INTERSECT

    • EXCEPT/MINUS在Ocenabase中，我们同时支持EXCEPT与MINUS，这两者的语义是相同的。这里需要注意的是参加集合操作的各查询结果的列数必须相同，对应的数据类型也必须兼容。对与UNION来说用户可以指定UNION的属性为ALL和DISTINCT/UNIQUE。分别代表集合可重复，和集合不可重复。而其它几种集合操作是不能指定ALL属性的(它们只有DISTINCT属性)。所有的集合操作默认的属性是DISTINCT。在Oceanbase中，集合操作中可以指定order by和 limit子句，但是不允许其他子句的出现，如下图所示

    1. OceanBase (root@test)> create table t1(a int primary key, b int, c int);
    2. Query OK, 0 rows affected (0.16 sec)
    3. OceanBase (root@test)> create table t2(a int primary key, b int, c int);
    4. Query OK, 0 rows affected (0.10 sec)
    5. --支持union语句中出现order by和limit子句
    6. OceanBase (root@test)> (select * from t1 union all select * from t2) order by a limit 10;
    7. Empty set (0.02 sec)
    8. --不支持union语句中出现除order by和limit子句的其他子句，比如group by
    9. OceanBase (root@test)> OceanBase (root@test)> (select * from t1 union all select * from t2) group by a limit 10;
    10. ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your OceanBase version for the right syntax to use near 'OceanBase (root@test)> (select * from t1 union all select * from t2) group by a ' at line 1

    UNION 示例

    该例子获取t1和t2中所有不重复的行。

    1. OceanBase (root@test)> create table t1(a int, b int, c int);
    2. Query OK, 0 rows affected (0.12 sec)
    3. OceanBase (root@test)> create table t2(a int, b int, c int);
    4. Query OK, 0 rows affected (0.11 sec)
    5. OceanBase (root@test)> insert into t1 values (1,1,1),(2,2,2),(3,3,3);
    6. Query OK, 3 rows affected (0.07 sec)
    7. Records: 3  Duplicates: 0  Warnings: 0
    8. OceanBase (root@test)> insert into t2 values (2,2,2),(3,3,3),(4,4,4);
    9. Query OK, 3 rows affected (0.02 sec)
    10. Records: 3  Duplicates: 0  Warnings: 0
    11. OceanBase (root@test)> select * from t1 union select * from t2;
    12. +------+------+------+
    13. | a    | b    | c    |
    14. +------+------+------+
    15. |    1 |    1 |    1 |
    16. |    2 |    2 |    2 |
    17. |    3 |    3 |    3 |
    18. |    4 |    4 |    4 |
    19. +------+------+------+
    20. 4 rows in set (0.01 sec)

    UNION All 示例

    该例子获取t1和t2中的所有行，不进行去重。

    1. OceanBase (root@test)> create table t1(a int, b int, c int);
    2. Query OK, 0 rows affected (0.12 sec)
    3. OceanBase (root@test)> create table t2(a int, b int, c int);
    4. Query OK, 0 rows affected (0.11 sec)
    5. OceanBase (root@test)> insert into t1 values (1,1,1),(2,2,2),(3,3,3);
    6. Query OK, 3 rows affected (0.07 sec)
    7. Records: 3  Duplicates: 0  Warnings: 0
    8. OceanBase (root@test)> insert into t1 values (2,2,2),(3,3,3),(4,4,4);
    9. Query OK, 3 rows affected (0.02 sec)
    10. Records: 3  Duplicates: 0  Warnings: 0
    11. OceanBase (root@test)> select * from t1 union all select * from t2;
    12. +------+------+------+
    13. | a    | b    | c    |
    14. +------+------+------+
    15. |    1 |    1 |    1 |
    16. |    2 |    2 |    2 |
    17. |    3 |    3 |    3 |
    18. |    2 |    2 |    2 |
    19. |    3 |    3 |    3 |
    20. |    4 |    4 |    4 |
    21. +------+------+------+
    22. 6 rows in set (0.02 sec)

    INTERSECT 示例

    该例子获取同时出现在t1和t2中行，并且去重。

    1. OceanBase (root@test)> create table t1(a int, b int, c int);
    2. Query OK, 0 rows affected (0.12 sec)
    3. OceanBase (root@test)> create table t2(a int, b int, c int);
    4. Query OK, 0 rows affected (0.12 sec)
    5. OceanBase (root@test)> insert into t1 values (1,1,1),(2,2,2),(3,3,3);
    6. Query OK, 3 rows affected (0.02 sec)
    7. Records: 3  Duplicates: 0  Warnings: 0
    8. OceanBase (root@test)> insert into t2 values (2,2,2),(3,3,3),(3,3,3),(4,4,4);
    9. Query OK, 4 rows affected (0.01 sec)
    10. Records: 4  Duplicates: 0  Warnings: 0
    11. OceanBase (root@test)> select * from t1 intersect select * from t2;
    12. +------+------+------+
    13. | a    | b    | c    |
    14. +------+------+------+
    15. |    2 |    2 |    2 |
    16. |    3 |    3 |    3 |
    17. +------+------+------+
    18. 2 rows in set (0.01 sec)

    EXCEPT/MINUS 示例

    该例子获取出现在t1中，但是不出现在t2中的行，并且去重。

    1. OceanBase (root@test)> create table t1(a int, b int, c int);
    2. Query OK, 0 rows affected (0.12 sec)
    3. OceanBase (root@test)> create table t2(a int, b int, c int);
    4. Query OK, 0 rows affected (0.12 sec)
    5. OceanBase (root@test)> insert into t1 values (1,1,1),(2,2,2),(3,3,3);
    6. Query OK, 3 rows affected (0.02 sec)
    7. Records: 3  Duplicates: 0  Warnings: 0
    8. OceanBase (root@test)> insert into t2 values (2,2,2),(3,3,3),(3,3,3),(4,4,4);
    9. Query OK, 4 rows affected (0.01 sec)
    10. Records: 4  Duplicates: 0  Warnings: 0
    11. OceanBase (root@test)> select * from t1 except select * from t2;
    12. +------+------+------+
    13. | a    | b    | c    |
    14. +------+------+------+
    15. |    1 |    1 |    1 |
    16. +------+------+------+
    17. 1 row in set (0.02 sec)