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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

  • INTERSECT

  • EXCEPT/MINUS

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。对于 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 数据库中,集合操作中可以指定 ORDER BYLIMIT 子句,但是不允许其他子句的出现,如下所示:

obclient> create table t1(a int primary key, b int, c int);
Query OK, 0 rows affected (0.16 sec)
obclient> 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
obclient> (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
obclient> (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 'group by a limit 10' at line 1

UNION 例子

该例子获取 t1 和 t2 中所有不重复的行。

obclient> create table t1(a int, b int, c int);
Query OK, 0 rows affected (0.12 sec)
obclient> create table t2(a int, b int, c int);
Query OK, 0 rows affected (0.11 sec)
obclient> 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
obclient> 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
obclient> 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。

obclient> create table t1(a int, b int, c int);
Query OK, 0 rows affected (0.12 sec)
obclient> create table t2(a int, b int, c int);
Query OK, 0 rows affected (0.11 sec)
obclient> 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
obclient> 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
obclient> 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 中的行,并且去重。

obclient> create table t1(a int, b int, c int);
Query OK, 0 rows affected (0.12 sec)
obclient> create table t2(a int, b int, c int);
Query OK, 0 rows affected (0.12 sec)
obclient> 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
obclient> 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
obclient> 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。

obclient> create table t1(a int, b int, c int);
Query OK, 0 rows affected (0.12 sec)
obclient> create table t2(a int, b int, c int);
Query OK, 0 rows affected (0.12 sec)
obclient> 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
obclient> 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
obclient> select * from t1 except select * from t2;
+------+------+------+
| a    | b    | c    |
+------+------+------+
|    1 |    1 |    1 |
+------+------+------+
1 row in set (0.02 sec)