Modify a table

After a table is created, you can use the ALTER TABLE statement to modify it.

Considerations

You cannot perform other DDL operations when you modify the primary key or column type of a table. Vice versa, you cannot modify the primary key or column type of a table when you perform other DDL operations.

Modify the schema of a table

OceanBase Database allows you to add a column to a table, modify a column and its attributes, and delete a column from a table.

• You can add columns except for a primary key column to a table. To add a primary key column, you can add a normal column and then add a primary key to the column. For more information, see Define column constraints.

  Example: Add a normal column.

  obclient> DESCRIBE test;
  +-------+--------------+------+-----+---------+-------+
  | FIELD | TYPE         | NULL | KEY | DEFAULT | EXTRA |
  +-------+--------------+------+-----+---------+-------+
  | C1    | NUMBER(38)   | NO   | PRI | NULL    | NULL  |
  | C2    | VARCHAR2(50) | YES  | NULL | NULL    | NULL  |
  +-------+--------------+------+-----+---------+-------+
  2 rows in set
  
  obclient> ALTER TABLE test ADD c3 int;
  Query OK, 0 rows affected
  
  obclient> DESCRIBE test;
  +-------+--------------+------+-----+---------+-------+
  | FIELD | TYPE         | NULL | KEY | DEFAULT | EXTRA |
  +-------+--------------+------+-----+---------+-------+
  | C1    | NUMBER(38)   | NO   | PRI | NULL    | NULL  |
  | C2    | VARCHAR2(50) | YES  | NULL | NULL    | NULL |
  | C3    | NUMBER(38)   | YES  | NULL | NULL    | NULL |
  +-------+--------------+------+-----+---------+-------+
  3 rows in set
  

• You can change the type of a column. OceanBase Database supports the conversion of column types.

  For more information about the rules for changing the data types of columns, see Column type change rules.

  The syntax is as follows:

  ALTER TABLE table_name MODIFY (column_name data_type);
  

  Here is an example:

  obclient> DESCRIBE test;
  +-------+--------------+------+-----+---------+-------+
  | FIELD | TYPE         | NULL | KEY | DEFAULT | EXTRA |
  +-------+--------------+------+-----+---------+-------+
  | C1    | NUMBER(38)   | NO   | PRI | NULL    | NULL  |
  | C2    | VARCHAR2(50) | YES  | NULL | NULL    | NULL  |
  +-------+--------------+------+-----+---------+-------+
  2 rows in set
  
  obclient> ALTER TABLE test MODIFY (c2 VARCHAR(70));
  Query OK, 0 rows affected
  
  obclient> DESCRIBE test;
  +-------+--------------+------+-----+---------+-------+
  | FIELD | TYPE         | NULL | KEY | DEFAULT | EXTRA |
  +-------+--------------+------+-----+---------+-------+
  | C1    | NUMBER(38)   | YES  | NULL | NULL    | NULL |
  | C2    | VARCHAR2(70) | YES  | NULL | NULL    | NULL |
  +-------+--------------+------+-----+---------+-------+
  2 rows in set
  

• You can rename a column. Here is an example:

  obclient> ALTER TABLE test RENAME COLUMN c1 TO c;
  Query OK, 0 rows affected
  
  obclient> DESCRIBE test;
  +-------+--------------+------+-----+---------+-------+
  | FIELD | TYPE         | NULL | KEY | DEFAULT | EXTRA |
  +-------+--------------+------+-----+---------+-------+
  | C     | NUMBER(38)   | YES  | NULL | NULL    | NULL  |
  | C2    | VARCHAR2(70) | YES  | NULL | NULL    | NULL  |
  +-------+--------------+------+-----+---------+-------+
  2 rows in set
  

• You can change the default value of a column. Here is an example:

  obclient> ALTER TABLE test MODIFY c DEFAULT 1;
  Query OK, 0 rows affected
  
  obclient> DESCRIBE test;
  +-------+--------------+------+-----+---------+-------+
  | FIELD | TYPE         | NULL | KEY | DEFAULT | EXTRA |
  +-------+--------------+------+-----+---------+-------+
  | C     | NUMBER(38)   | YES  | NULL | 1       | NULL  |
  | C2    | VARCHAR2(70) | YES  | NULL | NULL    | NULL  |
  +-------+--------------+------+-----+---------+-------+
  2 rows in set
  

• You can modify the NOT NULL constraint on a column. Here is an example:

  obclient> DESCRIBE test;
  +-------+--------------+------+-----+---------+-------+
  | FIELD | TYPE         | NULL | KEY | DEFAULT | EXTRA |
  +-------+--------------+------+-----+---------+-------+
  | C     | NUMBER(38)   | YES  | NULL | 1       | NULL  |
  | C2    | VARCHAR2(70) | YES  | NULL | NULL    | NULL  |
  +-------+--------------+------+-----+---------+-------+
  2 rows in set
  
  obclient> ALTER TABLE test MODIFY (c2 NOT NULL);
  Query OK, 0 rows affected
  
  obclient> DESCRIBE test;
  +-------+--------------+------+-----+---------+-------+
  | FIELD | TYPE         | NULL | KEY | DEFAULT | EXTRA |
  +-------+--------------+------+-----+---------+-------+
  | C     | NUMBER(38)   | YES  | NULL | 1       | NULL  |
  | C2    | VARCHAR2(70) | NO   | NULL | NULL    | NULL  |
  +-------+--------------+------+-----+---------+-------+
  2 rows in set
  

• You can delete columns except for the primary key column and indexed columns from a table. Here is an example:

  obclient> DESCRIBE test;
  +-------+--------------+------+-----+---------+-------+
  | FIELD | TYPE         | NULL | KEY | DEFAULT | EXTRA |
  +-------+--------------+------+-----+---------+-------+
  | C1    | NUMBER(38)   | NO   | PRI | NULL    | NULL  |
  | C2    | VARCHAR2(50) | YES  | NULL | NULL    | NULL  |
  +-------+--------------+------+-----+---------+-------+
  2 rows in set
  
  obclient> ALTER TABLE test DROP COLUMN c2;
  Query OK, 0 rows affected
  
  obclient> DESCRIBE test;
  +-------+--------------+------+-----+---------+-------+
  | FIELD | TYPE         | NULL | KEY | DEFAULT | EXTRA |
  +-------+--------------+------+-----+---------+-------+
  | C1    | NUMBER(38)   | NO   | PRI | NULL    | NULL  |
  +-------+--------------+------+-----+---------+-------+
  1 rows in set
  

Modify indexes

After you create a table, you can create a unique index on the table. Here is an example:

obclient> CREATE TABLE test (c1 int PRIMARY KEY, c2 VARCHAR(50));
Query OK, 0 rows affected

obclient> ALTER TABLE test ADD UNIQUE(c2);

Rename a table

You can rename an existing table.

OceanBase Database allows you to rename a table. Here is an example:

obclient> ALTER TABLE test RENAME TO t1;

Or

obclient> RENAME TABLE test TO t1;

Modify the PRIMARY KEY and FOREIGN KEY constraints for a table

After you create a table, you can modify the PRIMARY KEY and FOREIGN KEY constraints for the table. For more information, see Define column constraints.

Change the number of replicas of a table

After you create a table, you can change the number of replicas of the table.

Example: Change the number of replicas of a table to 2.

obclient> ALTER TABLE test SET REPLICA_NUM=2;
Query OK, 0 rows affected