ALTER TABLE

Syntax Description

ALTER TABLE is used to modify the structure of an existing table.

Syntax Structure

ALTER TABLE tbl_name
    [alter_option [, alter_option] ...]

alter_option: {
    table_options
  | ADD [COLUMN] col_name column_definition
        [FIRST | AFTER col_name]
  | ADD [COLUMN] (col_name column_definition,...)
  | ADD {[INDEX | KEY] [index_name]
        [index_option] ...
  | ADD [CONSTRAINT] UNIQUE [INDEX | KEY]
        [index_name][index_option] ...
  | ADD [CONSTRAINT] FOREIGN KEY
        [index_name] (col_name,...)
        reference_definition
  | ADD [CONSTRAINT [symbol]] PRIMARY KEY
        [index_type] (key_part,...)
  | CHANGE [COLUMN] old_col_name new_col_name column_definition
        [FIRST | AFTER col_name]
  | ALTER INDEX index_name {VISIBLE | INVISIBLE}
  | DROP [COLUMN] col_name
  | DROP {INDEX | KEY} index_name
  | DROP FOREIGN KEY fk_symbol
  | DROP PRIMARY KEY
  | RENAME [TO | AS] new_tbl_name
  | MODIFY [COLUMN] col_name column_definition
        [FIRST | AFTER col_name]
  | RENAME COLUMN old_col_name TO new_col_name
    }

key_part: {col_name [(length)] | (expr)} [ASC | DESC]
index_option: {
  COMMENT[=]'string'
}
table_options:
    table_option [[,] table_option] ...
table_option: {
  COMMENT [=] 'string'
}

Syntax Explanation

Below are explanations for each parameter:

ALTER TABLE tbl_name: Modifies the table named tbl_name.
alter_option: Specifies one or more alteration options, separated by commas.
- table_options: Used to set or modify table options, such as table comments (COMMENT).
- ADD [COLUMN] col_name column_definition [FIRST | AFTER col_name]: Adds a new column to the table, optionally specifying its position (before or after another column).
- ADD [COLUMN] (col_name column_definition,...): Adds multiple columns simultaneously.
- ADD {[INDEX | KEY] [index_name] [index_option] ...: Adds an index, optionally specifying the index name and options (e.g., comments).
- ADD [CONSTRAINT] UNIQUE [INDEX | KEY] [index_name][index_option] ...: Adds a UNIQUE constraint or UNIQUE index.
- ADD [CONSTRAINT] FOREIGN KEY [index_name] (col_name,...) reference_definition: Adds a FOREIGN KEY constraint.
- ADD [CONSTRAINT [symbol]] PRIMARY KEY [index_type] (key_part,...): Adds a PRIMARY KEY constraint.
- CHANGE [COLUMN] old_col_name new_col_name column_definition [FIRST | AFTER col_name]: Modifies a column's definition, name, and position.
- ALTER INDEX index_name {VISIBLE | INVISIBLE}: Changes the visibility of an index.
- DROP [COLUMN] col_name: Drops a column.
- DROP {INDEX | KEY} index_name: Drops an index.
- DROP FOREIGN KEY fk_symbol: Drops a FOREIGN KEY constraint.
- DROP PRIMARY KEY: Drops the PRIMARY KEY.
- RENAME [TO | AS] new_tbl_name: Renames the table.
- MODIFY [COLUMN] col_name column_definition [FIRST | AFTER col_name]: Modifies a column's definition and position.
- RENAME COLUMN old_col_name TO new_col_name: Renames a column.
key_part: Specifies the components of an index. For text columns, you can optionally specify a length for the index. If no length is specified, the entire column value is used, which may impact performance for large text or binary columns.
index_option: Specifies index options, such as comments (COMMENT).
table_options: Specifies table options, such as comments (COMMENT).
table_option: Specific table options, such as comments (COMMENT).

Examples

Example 1: Dropping a FOREIGN KEY constraint

```sql -- Create table f1 with two integer columns: fa (PRIMARY KEY) and fb (UNIQUE KEY) CREATE TABLE f1(fa INT PRIMARY KEY, fb INT UNIQUE KEY); -- Create table c1 with two integer columns: ca and cb CREATE TABLE c1 (ca INT, cb INT); -- Add a FOREIGN KEY constraint named ffa to c1, linking column ca to f1.fa ALTER TABLE c1 ADD CONSTRAINT ffa FOREIGN KEY (ca) REFERENCES f1(fa); -- Insert a record into f1: (2, 2) INSERT INTO f1 VALUES (2, 2); -- Insert a record into c1: (1, 1) INSERT INTO c1 VALUES (1, 1); -- Insert a record into c1: (2, 2) INSERT INTO c1 VALUES (2, 2); -- Select all records from c1, ordered by ca mysql> select ca, cb from c1 order by ca; +------+------+ | ca | cb | +------+------+ | 2 | 2 | +------+------+ 1 row in set (0.01 sec) -- Drop the FOREIGN KEY constraint ffa from c1 ALTER TABLE c1 DROP FOREIGN KEY ffa; -- Insert a record into c1: (1, 1) INSERT INTO c1 VALUES (1, 1); -- Select all records from c1, ordered by ca mysql> select ca, cb from c1 order by ca; +------+------+ | ca | cb | +------+------+ | 1 | 1 | | 2 | 2 | +------+------+ 2 rows in set (0.01 sec)

- Example 2: Adding a PRIMARY KEY

```sql <!-- validator-ignore-exec -->
-- Create table t1 with columns a (INTEGER), b (CHAR(10)), c (DATE), d (DECIMAL(7,2)), and a UNIQUE KEY on (a, b)
CREATE TABLE t1(a INTEGER, b CHAR(10), c DATE, d DECIMAL(7,2), UNIQUE KEY(a, b));

-- View the structure of t1
mysql> desc t1;
+-------+--------------+------+------+---------+-------+---------+
| Field | Type         | Null | Key  | Default | Extra | Comment |
+-------+--------------+------+------+---------+-------+---------+
| a     | INT(32)      | YES  |      | NULL    |       |         |
| b     | CHAR(10)     | YES  |      | NULL    |       |         |
| c     | DATE(0)      | YES  |      | NULL    |       |         |
| d     | DECIMAL64(7) | YES  |      | NULL    |       |         |
+-------+--------------+------+------+---------+-------+---------+
4 rows in set (0.01 sec)

-- Insert three records into t1
INSERT INTO t1 VALUES(1, 'ab', '1980-12-17', 800);
INSERT INTO t1 VALUES(2, 'ac', '1981-02-20', 1600);
INSERT INTO t1 VALUES(3, 'ad', '1981-02-22', 500);

-- Display all records from t1
mysql> select * from t1;
+------+------+------------+---------+
| a    | b    | c          | d       |
+------+------+------------+---------+
|    1 | ab   | 1980-12-17 |  800.00 |
|    2 | ac   | 1981-02-20 | 1600.00 |
|    3 | ad   | 1981-02-22 |  500.00 |
+------+------+------------+---------+
3 rows in set (0.01 sec)

-- Add a PRIMARY KEY named pk1 on columns (a, b)
mysql> alter table t1 add primary key pk1(a, b);
Query OK, 0 rows affected (0.02 sec)

-- View the modified structure of t1
mysql> desc t1;
+-------+--------------+------+------+---------+-------+---------+
| Field | Type         | Null | Key  | Default | Extra | Comment |
+-------+--------------+------+------+---------+-------+---------+
| a     | INT(32)      | NO   | PRI  | null    |       |         |
| b     | CHAR(10)     | NO   | PRI  | null    |       |         |
| c     | DATE(0)      | YES  |      | null    |       |         |
| d     | DECIMAL64(7) | YES  |      | null    |       |         |
+-------+--------------+------+------+---------+-------+---------+
4 rows in set (0.01 sec)

-- Display all records from t1 after adding the PRIMARY KEY
mysql> select * from t1;
+------+------+------------+---------+
| a    | b    | c          | d       |
+------+------+------------+---------+
|    1 | ab   | 1980-12-17 |  800.00 |
|    2 | ac   | 1981-02-20 | 1600.00 |
|    3 | ad   | 1981-02-22 |  500.00 |
+------+------+------------+---------+
3 rows in set (0.00 sec)

Example 3: Renaming a Column

```sql CREATE TABLE t1 (a INTEGER PRIMARY KEY, b CHAR(10)); mysql> desc t1; +-------+----------+------+------+---------+-------+---------+ | Field | Type | Null | Key | Default | Extra | Comment | +-------+----------+------+------+---------+-------+---------+ | a | INT(32) | NO | PRI | NULL | | | | b | CHAR(10) | YES | | NULL | | | +-------+----------+------+------+---------+-------+---------+ 2 rows in set (0.01 sec)

insert into t1 values(1, 'ab'); insert into t1 values(2, 'ac'); insert into t1 values(3, 'ad');

mysql> select * from t1; +------+------+ | a | b | +------+------+ | 1 | ab | | 2 | ac | | 3 | ad | +------+------+ 3 rows in set (0.01 sec)

-- Rename column a to x and change its data type to VARCHAR(20) mysql> alter table t1 change a x VARCHAR(20); Query OK, 0 rows affected (0.01 sec)

mysql> select * from t1; +------+------+ | x | b | +------+------+ | 1 | ab | | 2 | ac | | 3 | ad | +------+------+ 3 rows in set (0.00 sec)

- Example 4: Renaming a Table

```sql <!-- validator-ignore-exec -->
CREATE TABLE t1 (a INTEGER PRIMARY KEY, b CHAR(10));

mysql> show tables;
+---------------+
| Tables_in_db1 |
+---------------+
| t1            |
+---------------+
1 row in set (0.01 sec)

mysql> alter table t1 rename to t2;
Query OK, 0 rows affected (0.03 sec)

mysql> show tables;
+---------------+
| Tables_in_db1 |
+---------------+
| t2            |
+---------------+
1 row in set (0.01 sec)

Limitations

The following clauses: CHANGE [COLUMN], MODIFY [COLUMN], RENAME COLUMN, ADD [CONSTRAINT [symbol]] PRIMARY KEY, DROP PRIMARY KEY, and ALTER COLUMN ORDER BY can be freely combined in an ALTER TABLE statement but are currently not supported with other clauses.
Temporary tables do not currently support structural modifications via ALTER TABLE.
Tables created with CREATE TABLE ... CLUSTER BY... cannot be modified using ALTER TABLE.