166 lines
5.0 KiB
Markdown
166 lines
5.0 KiB
Markdown
集合
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=======================
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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：
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* UNION
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<!-- -->
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* INTERSECT
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<!-- -->
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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
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```javascript
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OceanBase (root@test)> create table t1(a int primary key, b int, c int);
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Query OK, 0 rows affected (0.16 sec)
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OceanBase (root@test)> create table t2(a int primary key, b int, c int);
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Query OK, 0 rows affected (0.10 sec)
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--支持union语句中出现order by和limit子句
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OceanBase (root@test)> (select * from t1 union all select * from t2) order by a limit 10;
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Empty set (0.02 sec)
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--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
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OceanBase (root@test)> OceanBase (root@test)> (select * from t1 union all select * from t2) group by a limit 10;
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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
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```
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UNION 例子
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-----------------
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该例子获取t1和t2中所有不重复的行。
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```javascript
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OceanBase (root@test)> create table t1(a int, b int, c int);
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Query OK, 0 rows affected (0.12 sec)
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OceanBase (root@test)> create table t2(a int, b int, c int);
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Query OK, 0 rows affected (0.11 sec)
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OceanBase (root@test)> insert into t1 values (1,1,1),(2,2,2),(3,3,3);
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Query OK, 3 rows affected (0.07 sec)
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Records: 3 Duplicates: 0 Warnings: 0
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OceanBase (root@test)> insert into t2 values (2,2,2),(3,3,3),(4,4,4);
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Query OK, 3 rows affected (0.02 sec)
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Records: 3 Duplicates: 0 Warnings: 0
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OceanBase (root@test)> select * from t1 union select * from t2;
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+------+------+------+
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| a | b | c |
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+------+------+------+
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| 1 | 1 | 1 |
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| 2 | 2 | 2 |
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| 3 | 3 | 3 |
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| 4 | 4 | 4 |
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+------+------+------+
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4 rows in set (0.01 sec)
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```
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UNION All 例子
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---------------------
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该例子获取t1和t2中的所有行，不进行去重。
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```javascript
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OceanBase (root@test)> create table t1(a int, b int, c int);
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Query OK, 0 rows affected (0.12 sec)
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OceanBase (root@test)> create table t2(a int, b int, c int);
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Query OK, 0 rows affected (0.11 sec)
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OceanBase (root@test)> insert into t1 values (1,1,1),(2,2,2),(3,3,3);
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Query OK, 3 rows affected (0.07 sec)
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Records: 3 Duplicates: 0 Warnings: 0
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OceanBase (root@test)> insert into t1 values (2,2,2),(3,3,3),(4,4,4);
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Query OK, 3 rows affected (0.02 sec)
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Records: 3 Duplicates: 0 Warnings: 0
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OceanBase (root@test)> select * from t1 union all select * from t2;
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+------+------+------+
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| a | b | c |
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+------+------+------+
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| 1 | 1 | 1 |
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| 2 | 2 | 2 |
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| 3 | 3 | 3 |
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| 2 | 2 | 2 |
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| 3 | 3 | 3 |
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| 4 | 4 | 4 |
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+------+------+------+
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6 rows in set (0.02 sec)
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```
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INTERSECT 例子
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---------------------
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该例子获取同时出现在t1和t2中行，并且去重。
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```javascript
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OceanBase (root@test)> create table t1(a int, b int, c int);
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Query OK, 0 rows affected (0.12 sec)
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OceanBase (root@test)> create table t2(a int, b int, c int);
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Query OK, 0 rows affected (0.12 sec)
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OceanBase (root@test)> insert into t1 values (1,1,1),(2,2,2),(3,3,3);
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Query OK, 3 rows affected (0.02 sec)
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Records: 3 Duplicates: 0 Warnings: 0
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OceanBase (root@test)> insert into t2 values (2,2,2),(3,3,3),(3,3,3),(4,4,4);
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Query OK, 4 rows affected (0.01 sec)
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Records: 4 Duplicates: 0 Warnings: 0
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OceanBase (root@test)> select * from t1 intersect select * from t2;
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+------+------+------+
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| a | b | c |
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+------+------+------+
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| 2 | 2 | 2 |
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| 3 | 3 | 3 |
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+------+------+------+
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2 rows in set (0.01 sec)
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```
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EXCEPT/MINUS 例子
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------------------------
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该例子获取出现在t1中，但是不出现在t2中的行，并且去重。
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```javascript
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OceanBase (root@test)> create table t1(a int, b int, c int);
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Query OK, 0 rows affected (0.12 sec)
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OceanBase (root@test)> create table t2(a int, b int, c int);
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Query OK, 0 rows affected (0.12 sec)
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OceanBase (root@test)> insert into t1 values (1,1,1),(2,2,2),(3,3,3);
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Query OK, 3 rows affected (0.02 sec)
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Records: 3 Duplicates: 0 Warnings: 0
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OceanBase (root@test)> insert into t2 values (2,2,2),(3,3,3),(3,3,3),(4,4,4);
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Query OK, 4 rows affected (0.01 sec)
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Records: 4 Duplicates: 0 Warnings: 0
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OceanBase (root@test)> select * from t1 except select * from t2;
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+------+------+------+
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| a | b | c |
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+------+------+------+
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| 1 | 1 | 1 |
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+------+------+------+
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1 row in set (0.02 sec)
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```
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