ALTER TABLE

Purpose

You can use this statement to modify the structure of an existing table. For example, you can use this statement to modify a table and its attributes, add columns to it, modify its columns and attributes, and delete its columns.

Syntax

alter_table_stmt:
      ALTER TABLE table_name alter_table_action_list;

alter_table_action_list:
    alter_table_action [, alter_table_action ...]

alter_table_action:
      ADD [COLUMN] column_definition  
          [FIRST | BEFORE | AFTER column_name]
    | ADD [COLUMN] (column_definition_list)
    | ADD [CONSTRAINT [constraint_name]] UNIQUE {INDEX | KEY}
          [index_name] index_desc
    | ADD [CONSTRAINT [constraint_name]] FOREIGN KEY
          [index_name] index_desc
          REFERENCES reference_definition
          [match_action][opt_reference_option_list]
    | ADD PRIMARY KEY (column_name)
    | ADD CONSTRAINT [constraint_name] CHECK (expression)[[NOT] ENFORCED]
    | ADD PARTITION (range_partition_list)
    | ALTER [COLUMN] column_name {
          SET DEFAULT const_value
        | DROP DEFAULT
      }
    | ALTER {CHECK | CONSTRAINT} symbol [NOT] ENFORCED
    | ALTER INDEX index_name
          [VISIBLE | INVISIBLE]
    | CHANGE [COLUMN] column_name column_definition
    | DROP [COLUMN] column_name
    | DROP {PARTITION | SUBPARTITION} partition_name_list
    | DROP TABLEGROUP
    | DROP FOREIGN KEY fk_name
    | DROP PRIMARY KEY [, ADD PRIMARY KEY (column)]
    | DROP CHECK constraint_name
    | MODIFY [COLUMN] column_definition
    | RENAME [TO] table_name
    | RENAME {INDEX | KEY} old_index_name TO new_index_name
    | REORGANIZE PARTITION name_list INTO partition_range_or_list
    | [SET] table_option_list
    | TRUNCATE {PARTITION | SUBPARTITION} partition_name_list

column_definition_list:
    column_definition [, column_definition ...]

column_definition:
    column_name data_type
    [DEFAULT const_value] [AUTO_INCREMENT]
    [NULL | NOT NULL] [[PRIMARY] KEY] [UNIQUE [KEY]] comment

index_desc:
   (column_desc_list) [index_type] [index_option_list]

match_action:
   MATCH {SIMPLE | FULL | PARTIAL}

opt_reference_option_list:
   reference_option [,reference_option ...]

reference_option:
   ON {DELETE | UPDATE} {RESTRICT | CASCADE | SET NULLX | NO ACTION | SET DEFAULT}

column_desc_list:
    column_desc [, column_desc ...]

column_desc:
     column_name [(length)] [ASC | DESC]

index_type:
    USING BTREE

index_option_list:
    index_option [ index_option ...]

index_option:
     [GLOBAL | LOCAL]
    | block_size
    | compression
    | STORING(column_name_list)
    | comment

table_option_list:
    table_option [ table_option ...]

table_option:
      table_group
    | block_size
    | compression
    | AUTO_INCREMENT [=] INT_VALUE
    | comment
    | DUPLICATE_SCOPE [=] "none|zone|region|cluster"
    | parallel_clause

parallel_clause:
    {NOPARALLEL | PARALLEL integer}

partition_option:
      PARTITION BY HASH(expression)
      [subpartition_option] PARTITIONS partition_count
    | PARTITION BY KEY([column_name_list])
      [subpartition_option] PARTITIONS partition_count
    | PARTITION BY RANGE {(expression) | COLUMNS (column_name_list)}
      [subpartition_option] (range_partition_list)

subpartition_option:
      SUBPARTITION BY HASH(expression)
      SUBPARTITIONS subpartition_count
    | SUBPARTITION BY KEY(column_name_list)
      SUBPARTITIONS subpartition_count
    | SUBPARTITION BY RANGE {(expression) | COLUMNS (column_name_list)}
      (range_subpartition_list)

range_partition_list:
    range_partition [, range_partition ...]

range_partition:
    PARTITION partition_name
    VALUES LESS THAN {(expression_list) | MAXVALUE}

range_subpartition_list:
    range_subpartition [, range_subpartition ...]

range_subpartition:
    SUBPARTITION subpartition_name
    VALUES LESS THAN {(expression_list) | MAXVALUE}

expression_list:
    expression [, expression ...]

column_name_list:
    column_name [, column_name ...]

partition_name_list:
    partition_name [, partition_name ...]

partition_count | subpartition_count:
    INT_VALUE

Parameters

Examples

Add and drop columns

Create a table named tbl1 and execute the DESCRIBE statement to view the table information.

obclient> CREATE TABLE tbl1 (c1 INT(11) PRIMARY KEY,c2 VARCHAR(50));
Query OK, 0 rows affected

obclient> DESCRIBE tbl1;
+-------+------------+----------+--------+---------+-------+
| Field | Type       | Null     | Key    | Default | Extra |
+-------+------------+----------+--------+---------+-------+
| c1    | int(11)    | NO       | PRI    | NULL    |       |    
| c2    | varchar(50)| YES      |        | NULL    |       |   
+-------+------------+----------+--------+---------+-------+

• Add a column named c3 and execute the DESCRIBE statement to view the table information.

  obclient> ALTER TABLE tbl1 ADD c3 INT;
  Query OK, 1 row affected
  
  obclient> DESCRIBE tbl1;
  +-------+------------+----------+--------+---------+-------+
  | Field | Type       | Null     | Key    | Default | Extra |
  +-------+------------+----------+--------+---------+-------+
  | c1    | int(11)    | NO       | PRI    | NULL    |       |    
  | c2    | varchar(50)| YES      |        | NULL    |       |  
  | c3    | int(11)    | YES      |        | NULL    |       |  
  +-------+------------+----------+--------+---------+-------+
  3 row affected
  

• Drop the c3 column and execute the DESCRIBE statement to view the table information.

  obclient> ALTER TABLE tbl1 DROP c3;
  Query OK, 1 row affected
  
  obclient> DESCRIBE tbl1;
  +-------+------------+----------+--------+---------+-------+
  | Field | Type       | Null     | Key    | Default | Extra |
  +-------+------------+----------+--------+---------+-------+
  | c1    | int(11)    | NO       | PRI    | NULL    |       |    
  | c2    | varchar(50)| YES      |        | NULL    |       |  
  +-------+------------+----------+--------+---------+-------+
  2 row affected
  

• Rename the c2 column in the tbl1 table to c3 and execute the DESCRIBE statement to view the table information.

  obclient> ALTER TABLE tbl1 CHANGE COLUMN c2 c3 VARCHAR(50);
  Query OK, 1 row affected
  
  obclient> DESCRIBE tbl1;
  +-------+------------+----------+--------+---------+-------+
  | Field | Type       | Null     | Key    | Default | Extra |
  +-------+------------+----------+--------+---------+-------+
  | c1    | int(11)    | NO       | PRI    | NULL    |       |    
  | c3    | varchar(50)| YES      |        | NULL    |       |  
  +-------+------------+----------+--------+---------+-------+
  2 rows affected
  

• Add a column named c4 to the tbl1 table, set this column as the first column of the table, and execute the DESCRIBE statement to view the table information.

  obclient> ALTER TABLE tbl1 ADD COLUMN c4 INTEGER FIRST;
  Query OK, 1 row affected
  
  obclient> DESCRIBE tbl1;
  +-------+------------+----------+--------+---------+-------+
  | Field | Type       | Null     | Key    | Default | Extra |
  +-------+------------+----------+--------+---------+-------+
  | c4    | int(11)    | YES      |        | NULL    |       |  
  | c1    | int(11)    | NO       | PRI    | NULL    |       |    
  | c3    | varchar(50)| YES      |        | NULL    |       |  
  +-------+------------+----------+--------+---------+-------+
  3 row affected
  

• Add a column named c2 behind the c1 column and execute the DESCRIBE statement to view the table information.

  obclient> ALTER TABLE tbl1 ADD COLUMN c2 INTEGER AFTER c1;
  Query OK, 1 row affected
  
  obclient> DESCRIBE tbl1;
  +-------+------------+----------+--------+---------+-------+
  | Field | Type       | Null     | Key    | Default | Extra |
  +-------+------------+----------+--------+---------+-------+
  | c4    | int(11)    | YES      |        | NULL    |       |  
  | c1    | int(11)    | NO       | PRI    | NULL    |       |
  | c2    | int(11)    | YES      |        | NULL    |       |    
  | c3    | varchar(50)| YES      |        | NULL    |       |  
  +-------+------------+----------+--------+---------+-------+
  4 row affected
  

• Add a column named c5 before the c4 column and execute the DESCRIBE statement to view the table information.

  obclient> ALTER TABLE tbl1 ADD COLUMN c5 INT BEFORE c4;
  Query OK, 1 row affected
  
  obclient> DESCRIBE tbl1;
  +-------+------------+----------+--------+---------+-------+
  | Field | Type       | Null     | Key    | Default | Extra |
  +-------+------------+----------+--------+---------+-------+
  | c5    | int(11)    | YES      |        | NULL    |       |  
  | c4    | int(11)    | YES      |        | NULL    |       |  
  | c1    | int(11)    | NO       | PRI    | NULL    |       |
  | c2    | int(11)    | YES      |        | NULL    |       |    
  | c3    | varchar(50)| YES      |        | NULL    |       |  
  +-------+------------+----------+--------+---------+-------+
  5 row affected
  

• Add a FOREIGN KEY constraint fk1 to the tbl2 table.

  obclient> CREATE TABLE tbl2(c1 INT PRIMARY KEY,c2 INT);
  Query OK, 0 row affected
  obclient> CREATE TABLE tbl3(c1 INT PRIMARY KEY,c2 INT);
  Query OK, 0 row affected
  obclient> ALTER TABLE tbl2 ADD CONSTRAINT fk1 FOREIGN KEY (c2) REFERENCES tbl3(c1);
  Query OK, 0 row affected
  

• Drop the FOREIGN KEY constraint fk1 from the tbl3 table.

  obclient> ALTER TABLE tbl2 DROP FOREIGN KEY fk1;
  Query OK, 0 row affected
  

• Drop the primary key of the tbl2 table.

  obclient> ALTER TABLE tbl2 DROP PRIMARY KEY;
  Query OK, 0 row affected
  

Index operations

• Rename the ind1 index on the tbl2 table to ind2.

  obclient> CREATE TABLE tbl2(c1 INT(11) PRIMARY KEY,c2 INT(50));
  Query OK, 0 row affected
  
  obclient> CREATE INDEX ind1 ON tbl2 (c2 ASC);
  Query OK, 0 row affected
  obclient> ALTER TABLE tbl2 RENAME INDEX ind1 TO ind2;
  Query OK, 0 row affected
  

• Create an index named ind3 on the tbl2 table to reference the c1 and c2 columns.

  obclient> ALTER TABLE tbl2 ADD INDEX ind3 (c1,c2);
  Query OK, 0 row affected
  

  You can execute the SHOW INDEX statement to view the created indexes.

  obclient> SHOW INDEX FROM tbl2;
  +-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+-----------+---------------+---------+
  | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment   | Index_comment | Visible |
  +-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+-----------+---------------+---------+
  | tbl2  |          0 | PRIMARY  |            1 | c1          | A         |        NULL | NULL     | NULL   |      | BTREE      | available |               | YES     |
  | tbl2  |          1 | ind2     |            1 | c2          | A         |        NULL | NULL     | NULL   |  YES | BTREE      | available |               | YES     |
  | tbl2  |          1 | ind3     |            1 | c1          | A         |        NULL | NULL     | NULL   |      | BTREE      | available |               | YES     |
  | tbl2  |          1 | ind3     |            2 | c2          | A         |        NULL | NULL     | NULL   |  YES | BTREE      | available |               | YES     |
  +-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+-----------+---------------+---------+
  4 rows in set
  

• Drop the ind2 index from the tbl2 table.

  obclient> ALTER TABLE tbl2 DROP INDEX ind2;
  Query OK, 0 row affected
  

  Note

  You can use the preceding method to implement an atomic change to an index in actual O&M scenarios.

Partition operations

• Truncate partitions M202001 and M202002 of the t_log_part_by_range table.

  obclient> CREATE TABLE t_log_part_by_range (
      log_id      bigint NOT NULL
      , log_value varchar(50)
      , log_date  timestamp NOT NULL  
  ) PARTITION BY RANGE(UNIX_TIMESTAMP(log_date))
  (
      PARTITION M202001 VALUES LESS THAN(UNIX_TIMESTAMP('2020/02/01'))
      , PARTITION M202002 VALUES LESS THAN(UNIX_TIMESTAMP('2020/03/01'))
      , PARTITION M202003 VALUES LESS THAN(UNIX_TIMESTAMP('2020/04/01'))
      , PARTITION M202004 VALUES LESS THAN(UNIX_TIMESTAMP('2020/05/01'))
      , PARTITION M202005 VALUES LESS THAN(UNIX_TIMESTAMP('2020/06/01'))
  );
  Query OK, 0 rows affected
  
  obclient> ALTER TABLE t_log_part_by_range TRUNCATE PARTITION M202001, M202002;
  Query OK, 0 rows affected
  

• Add the M202006 partition to the t_log_part_by_range table.

  obclient> CREATE TABLE t_log_part_by_range (
      log_id      bigint NOT NULL
      , log_value varchar(50)
      , log_date  timestamp NOT NULL  
  ) PARTITION BY RANGE(UNIX_TIMESTAMP(log_date))
  (
      PARTITION M202001 VALUES LESS THAN(UNIX_TIMESTAMP('2020/02/01'))
      , PARTITION M202002 VALUES LESS THAN(UNIX_TIMESTAMP('2020/03/01'))
      , PARTITION M202003 VALUES LESS THAN(UNIX_TIMESTAMP('2020/04/01'))
      , PARTITION M202004 VALUES LESS THAN(UNIX_TIMESTAMP('2020/05/01'))
      , PARTITION M202005 VALUES LESS THAN(UNIX_TIMESTAMP('2020/06/01'))
  );
  Query OK, 0 rows affected
  
  obclient> ALTER TABLE t_log_part_by_range ADD PARTITION
           (PARTITION M202006 VALUES LESS THAN(UNIX_TIMESTAMP('2020/07/01'))
           );
  

Change the DOP

• Change the DOP of the tbl1 table to 2.

  obclient> ALTER TABLE tbl3 PARALLEL 2;
  

Operations related to column types

• Change the ordinary column col1 of the tbl4 table to an auto-increment column.

  obclient> CREATE TABLE tbl4 (col1 BIGINT(10) NOT NULL,col2 INT);
  Query OK, 0 rows affected
  
  obclient> ALTER TABLE tbl4 MODIFY col1 BIGINT(10) AUTO_INCREMENT;
  Query OK, 0 rows affected
  

• Add the primary key column col1 to the tbl4 table, and then change the primary key to col2.

  obclient> ALTER TABLE tbl4 ADD PRIMARY KEY (col1);
  Query OK, 0 rows affected
  
  obclient> ALTER TABLE tbl4 DROP PRIMARY KEY,ADD PRIMARY KEY (`col2`);
  Query OK, 0 rows affected
  

• Add a CHECK constraint.

  obclient> CREATE TABLE tbl7(col1 VARCHAR(10),col2 VARCHAR(10));
  Query OK, 0 rows affected
  
  obclient> ALTER TABLE tbl7 ADD CONSTRAINT my_check CHECK (col1> col2) ;
  Query OK, 0 rows affected
  

• Drop the CHECK constraint from the tbl7 table.

  obclient> ALTER TABLE tbl7 DROP CHECK my_check;
  Query OK, 0 rows affected
  

• Change the column type to NOT NULL.

  obclient> CREATE TABLE dept(  
      deptno           NUMBER(2,0),  
      dname            VARCHAR(14),  
      location         VARCHAR(13),   
      CONSTRAINT pk_dept PRIMARY KEY(deptno)  
  );
  Query OK, 0 rows affected
  
  obclient> CREATE TABLE emp(  
      empno         NUMBER(4,0) NOT NULL,  
      empname       VARCHAR(10) NOT NULL,  
      job           VARCHAR(9) NOT NULL,  
      mgr           NUMBER(4,0) NOT NULL,  
      hiredate      DATE NOT NULL,  
      sal           NUMBER(7,2) DEFAULT NULL,
      comm          NUMBER(7,2) DEFAULT NULL,        
      deptno        NUMBER(2,0) DEFAULT NULL,   
      CONSTRAINT PK_emp PRIMARY KEY (empno),
      CONSTRAINT FK_deptno  FOREIGN KEY (deptno)  REFERENCES dept (deptno)  
  );
  Query OK, 0 rows affected
  
  obclient> ALTER TABLE emp MODIFY deptno NUMBER(2,0) DEFAULT '12' NOT NULL;
  Query OK, 0 rows affected