集合 
=======================



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




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* INTERSECT




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* 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




```javascript
OceanBase (root@test)> create table t1(a int primary key, b int, c int);
Query OK, 0 rows affected (0.16 sec)
OceanBase (root@test)> create table t2(a int primary key, b int, c int);
Query OK, 0 rows affected (0.10 sec)
--支持union语句中出现order by和limit子句
OceanBase (root@test)> (select * from t1 union all select * from t2) order by a limit 10;
Empty set (0.02 sec)
--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
OceanBase (root@test)> OceanBase (root@test)> (select * from t1 union all select * from t2) group by a limit 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 例子 
-----------------

该例子获取t1和t2中所有不重复的行。

```javascript
OceanBase (root@test)> create table t1(a int, b int, c int);
Query OK, 0 rows affected (0.12 sec)
OceanBase (root@test)> create table t2(a int, b int, c int);
Query OK, 0 rows affected (0.11 sec)
OceanBase (root@test)> insert into t1 values (1,1,1),(2,2,2),(3,3,3);
Query OK, 3 rows affected (0.07 sec)
Records: 3  Duplicates: 0  Warnings: 0
OceanBase (root@test)> insert into t2 values (2,2,2),(3,3,3),(4,4,4);
Query OK, 3 rows affected (0.02 sec)
Records: 3  Duplicates: 0  Warnings: 0
OceanBase (root@test)> select * from t1 union select * from t2;
+------+------+------+
| a    | b    | c    |
+------+------+------+
|    1 |    1 |    1 |
|    2 |    2 |    2 |
|    3 |    3 |    3 |
|    4 |    4 |    4 |
+------+------+------+
4 rows in set (0.01 sec)
```





UNION All 例子 
---------------------

该例子获取t1和t2中的所有行，不进行去重。

```javascript
OceanBase (root@test)> create table t1(a int, b int, c int);
Query OK, 0 rows affected (0.12 sec)
OceanBase (root@test)> create table t2(a int, b int, c int);
Query OK, 0 rows affected (0.11 sec)
OceanBase (root@test)> insert into t1 values (1,1,1),(2,2,2),(3,3,3);
Query OK, 3 rows affected (0.07 sec)
Records: 3  Duplicates: 0  Warnings: 0
OceanBase (root@test)> insert into t1 values (2,2,2),(3,3,3),(4,4,4);
Query OK, 3 rows affected (0.02 sec)
Records: 3  Duplicates: 0  Warnings: 0
OceanBase (root@test)> select * from t1 union all select * from t2;
+------+------+------+
| a    | b    | c    |
+------+------+------+
|    1 |    1 |    1 |
|    2 |    2 |    2 |
|    3 |    3 |    3 |
|    2 |    2 |    2 |
|    3 |    3 |    3 |
|    4 |    4 |    4 |
+------+------+------+
6 rows in set (0.02 sec)
```





INTERSECT 例子 
---------------------

该例子获取同时出现在t1和t2中行，并且去重。

```javascript
OceanBase (root@test)> create table t1(a int, b int, c int);
Query OK, 0 rows affected (0.12 sec)
OceanBase (root@test)> create table t2(a int, b int, c int);
Query OK, 0 rows affected (0.12 sec)
OceanBase (root@test)> insert into t1 values (1,1,1),(2,2,2),(3,3,3);
Query OK, 3 rows affected (0.02 sec)
Records: 3  Duplicates: 0  Warnings: 0
OceanBase (root@test)> insert into t2 values (2,2,2),(3,3,3),(3,3,3),(4,4,4);
Query OK, 4 rows affected (0.01 sec)
Records: 4  Duplicates: 0  Warnings: 0
OceanBase (root@test)> select * from t1 intersect select * from t2;
+------+------+------+
| a    | b    | c    |
+------+------+------+
|    2 |    2 |    2 |
|    3 |    3 |    3 |
+------+------+------+
2 rows in set (0.01 sec)
```





EXCEPT/MINUS 例子 
------------------------

该例子获取出现在t1中，但是不出现在t2中的行，并且去重。

```javascript
OceanBase (root@test)> create table t1(a int, b int, c int);
Query OK, 0 rows affected (0.12 sec)
OceanBase (root@test)> create table t2(a int, b int, c int);
Query OK, 0 rows affected (0.12 sec)
OceanBase (root@test)> insert into t1 values (1,1,1),(2,2,2),(3,3,3);
Query OK, 3 rows affected (0.02 sec)
Records: 3  Duplicates: 0  Warnings: 0
OceanBase (root@test)> insert into t2 values (2,2,2),(3,3,3),(3,3,3),(4,4,4);
Query OK, 4 rows affected (0.01 sec)
Records: 4  Duplicates: 0  Warnings: 0
OceanBase (root@test)> select * from t1 except select * from t2;
+------+------+------+
| a    | b    | c    |
+------+------+------+
|    1 |    1 |    1 |
+------+------+------+
1 row in set (0.02 sec)
```


