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                [TOC] # 集合 數據庫中的集合操作可以把多個查詢的結果組合成一個結果集。集合操作主要包含: * UNION * INTERSECT * EXCEPT/MINUS在Ocenabase中,我們同時支持EXCEPT與MINUS,這兩者的語義是相同的。這里需要注意的是參加集合操作的各查詢結果的列數必須相同,對應的數據類型也必須兼容。對與UNION來說用戶可以指定UNION的屬性為ALL和DISTINCT/UNIQUE。分別代表集合可重復,和集合不可重復。而其它幾種集合操作是不能指定ALL屬性的(它們只有DISTINCT屬性)。所有的集合操作默認的屬性是DISTINCT。在Oceanbase中,集合操作中可以指定order by和 limit子句,但是不允許其他子句的出現,如下圖所示 ~~~ 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語句中出現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語句中出現除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 該例子獲取t1和t2中所有不重復的行。 ~~~ 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 該例子獲取t1和t2中的所有行,不進行去重。 ~~~ 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 該例子獲取同時出現在t1和t2中行,并且去重。 ~~~ 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 該例子獲取出現在t1中,但是不出現在t2中的行,并且去重。 ~~~ 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) ~~~
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