---
title: "ALTER Statements"
weight: 6
type: docs
aliases:
  - /dev/table/sql/alter.html
---
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# ALTER Statements



ALTER statements are used to modified a registered table/view/function definition in the [Catalog]({{< ref "docs/dev/table/catalogs" >}}).

Flink SQL supports the following ALTER statements for now:

- ALTER TABLE
- ALTER VIEW
- ALTER DATABASE
- ALTER FUNCTION

## Run an ALTER statement

{{< tabs "alter" >}}
{{< tab "Java" >}}
ALTER statements can be executed with the `executeSql()` method of the `TableEnvironment`. The `executeSql()` method returns 'OK' for a successful ALTER operation, otherwise will throw an exception.

The following examples show how to run an ALTER statement in `TableEnvironment`.
{{< /tab >}}
{{< tab "Scala" >}}
ALTER statements can be executed with the `executeSql()` method of the `TableEnvironment`. The `executeSql()` method returns 'OK' for a successful ALTER operation, otherwise will throw an exception.

The following examples show how to run an ALTER statement in `TableEnvironment`.
{{< /tab >}}
{{< tab "Python" >}}

ALTER statements can be executed with the `execute_sql()` method of the `TableEnvironment`. The `execute_sql()` method returns 'OK' for a successful ALTER operation, otherwise will throw an exception.

The following examples show how to run an ALTER statement in `TableEnvironment`.

{{< /tab >}}
{{< tab "SQL CLI" >}}

ALTER statements can be executed in [SQL CLI]({{< ref "docs/dev/table/sqlClient" >}}).

The following examples show how to run an ALTER statement in SQL CLI.

{{< /tab >}}
{{< /tabs >}}

{{< tabs "a01e970b-5de2-44de-b5d0-595b53682c64" >}}
{{< tab "Java" >}}
```java
EnvironmentSettings settings = EnvironmentSettings.newInstance()...
TableEnvironment tableEnv = TableEnvironment.create(settings);

// register a table named "Orders"
tableEnv.executeSql("CREATE TABLE Orders (`user` BIGINT, product STRING, amount INT) WITH (...)");

// a string array: ["Orders"]
String[] tables = tableEnv.listTables();
// or tableEnv.executeSql("SHOW TABLES").print();

// add a new column `order` to the first position
tableEnv.executeSql("ALTER TABLE Orders ADD `order` INT COMMENT 'order identifier' FIRST");

// add more columns, primary key and watermark
tableEnv.executeSql("ALTER TABLE Orders ADD (ts TIMESTAMP(3), category STRING AFTER product, PRIMARY KEY(`order`) NOT ENFORCED, WATERMARK FOR ts AS ts - INTERVAL '1' HOUR)");

// modify column type, column comment and watermark
tableEnv.executeSql("ALTER TABLE Orders MODIFY (amount DOUBLE NOT NULL, category STRING COMMENT 'category identifier' AFTER `order`, WATERMARK FOR ts AS ts)");

// drop watermark
tableEnv.executeSql("ALTER TABLE Orders DROP WATERMARK");

// drop column
tableEnv.executeSql("ALTER TABLE Orders DROP (amount, ts, category)");

// rename column
tableEnv.executeSql("ALTER TABLE Orders RENAME `order` TO order_id");

// rename "Orders" to "NewOrders"
tableEnv.executeSql("ALTER TABLE Orders RENAME TO NewOrders");

// a string array: ["NewOrders"]
String[] tables = tableEnv.listTables();
// or tableEnv.executeSql("SHOW TABLES").print();
```
{{< /tab >}}
{{< tab "Scala" >}}
```scala
val tableEnv = TableEnvironment.create(...)

// register a table named "Orders"
tableEnv.executeSql("CREATE TABLE Orders (`user` BIGINT, product STRING, amount INT) WITH (...)")

// add a new column `order` to the first position
tableEnv.executeSql("ALTER TABLE Orders ADD `order` INT COMMENT 'order identifier' FIRST")

// add more columns, primary key and watermark
tableEnv.executeSql("ALTER TABLE Orders ADD (ts TIMESTAMP(3), category STRING AFTER product, PRIMARY KEY(`order`) NOT ENFORCED, WATERMARK FOR ts AS ts - INTERVAL '1' HOUR)")

// modify column type, column comment and watermark
tableEnv.executeSql("ALTER TABLE Orders MODIFY (amount DOUBLE NOT NULL, category STRING COMMENT 'category identifier' AFTER `order`, WATERMARK FOR ts AS ts)")

// drop watermark
tableEnv.executeSql("ALTER TABLE Orders DROP WATERMARK")

// drop column
tableEnv.executeSql("ALTER TABLE Orders DROP (amount, ts, category)")

// rename column
tableEnv.executeSql("ALTER TABLE Orders RENAME `order` TO order_id")

// a string array: ["Orders"]
val tables = tableEnv.listTables()
// or tableEnv.executeSql("SHOW TABLES").print()

// rename "Orders" to "NewOrders"
tableEnv.executeSql("ALTER TABLE Orders RENAME TO NewOrders")

// a string array: ["NewOrders"]
val tables = tableEnv.listTables()
// or tableEnv.executeSql("SHOW TABLES").print()
```
{{< /tab >}}
{{< tab "Python" >}}
```python
table_env = TableEnvironment.create(...)

# a string array: ["Orders"]
tables = table_env.list_tables()
# or table_env.execute_sql("SHOW TABLES").print()

# add a new column `order` to the first position
table_env.execute_sql("ALTER TABLE Orders ADD `order` INT COMMENT 'order identifier' FIRST");

# add more columns, primary key and watermark
table_env.execute_sql("ALTER TABLE Orders ADD (ts TIMESTAMP(3), category STRING AFTER product, PRIMARY KEY(`order`) NOT ENFORCED, WATERMARK FOR ts AS ts - INTERVAL '1' HOUR)");

# modify column type, column comment and watermark
table_env.execute_sql("ALTER TABLE Orders MODIFY (amount DOUBLE NOT NULL, category STRING COMMENT 'category identifier' AFTER `order`, WATERMARK FOR ts AS ts)");

# drop watermark
table_env.execute_sql("ALTER TABLE Orders DROP WATERMARK");

# drop column
table_env.execute_sql("ALTER TABLE Orders DROP (amount, ts, category)");

# rename column
table_env.execute_sql("ALTER TABLE Orders RENAME `order` TO order_id");

# rename "Orders" to "NewOrders"
table_env.execute_sql("ALTER TABLE Orders RENAME TO NewOrders");

# a string array: ["NewOrders"]
tables = table_env.list_tables()
# or table_env.execute_sql("SHOW TABLES").print()
```
{{< /tab >}}
{{< tab "SQL CLI" >}}
```sql
Flink SQL> CREATE TABLE Orders (`user` BIGINT, product STRING, amount INT) WITH (...);
[INFO] Execute statement succeed.

Flink SQL> ALTER TABLE Orders ADD `order` INT COMMENT 'order identifier' FIRST;
[INFO] Execute statement succeed.

Flink SQL> DESCRIBE Orders;
+---------+--------+------+-----+--------+-----------+------------------+
|    name |   type | null | key | extras | watermark |          comment |
+---------+--------+------+-----+--------+-----------+------------------+
|   order |    INT | TRUE |     |        |           | order identifier |
|    user | BIGINT | TRUE |     |        |           |                  |
| product | STRING | TRUE |     |        |           |                  |
|  amount |    INT | TRUE |     |        |           |                  |
+---------+--------+------+-----+--------+-----------+------------------+
4 rows in set

Flink SQL> ALTER TABLE Orders ADD (ts TIMESTAMP(3), category STRING AFTER product, PRIMARY KEY(`order`) NOT ENFORCED, WATERMARK FOR ts AS ts - INTERVAL '1' HOUR);
[INFO] Execute statement succeed. 

Flink SQL> DESCRIBE Orders;
+----------+------------------------+-------+------------+--------+--------------------------+------------------+
|     name |                   type |  null |        key | extras |                watermark |          comment |
+----------+------------------------+-------+------------+--------+--------------------------+------------------+
|    order |                    INT | FALSE | PRI(order) |        |                          | order identifier |
|     user |                 BIGINT |  TRUE |            |        |                          |                  |
|  product |                 STRING |  TRUE |            |        |                          |                  |
| category |                 STRING |  TRUE |            |        |                          |                  |
|   amount |                    INT |  TRUE |            |        |                          |                  |
|       ts | TIMESTAMP(3) *ROWTIME* |  TRUE |            |        | `ts` - INTERVAL '1' HOUR |                  |
+----------+------------------------+-------+------------+--------+--------------------------+------------------+
6 rows in set

Flink SQL> ALTER TABLE Orders MODIFY (amount DOUBLE NOT NULL, category STRING COMMENT 'category identifier' AFTER `order`, WATERMARK FOR ts AS ts);
[INFO] Execute statement succeed. 

Flink SQL> DESCRIBE Orders;
+----------+------------------------+-------+------------+--------+-----------+---------------------+
|     name |                   type |  null |        key | extras | watermark |             comment |
+----------+------------------------+-------+------------+--------+-----------+---------------------+
|    order |                    INT | FALSE | PRI(order) |        |           |    order identifier |
| category |                 STRING |  TRUE |            |        |           | category identifier |
|     user |                 BIGINT |  TRUE |            |        |           |                     |
|  product |                 STRING |  TRUE |            |        |           |                     |
|   amount |                 DOUBLE | FALSE |            |        |           |                     |
|       ts | TIMESTAMP(3) *ROWTIME* |  TRUE |            |        |      `ts` |                     |
+----------+------------------------+-------+------------+--------+-----------+---------------------+
6 rows in set

Flink SQL> ALTER TABLE Orders DROP WATERMARK;
[INFO] Execute statement succeed.

Flink SQL> DESCRIBE Orders;
+----------+--------------+-------+------------+--------+-----------+---------------------+
|     name |         type |  null |        key | extras | watermark |             comment |
+----------+--------------+-------+------------+--------+-----------+---------------------+
|    order |          INT | FALSE | PRI(order) |        |           |    order identifier |
| category |       STRING |  TRUE |            |        |           | category identifier |
|     user |       BIGINT |  TRUE |            |        |           |                     |
|  product |       STRING |  TRUE |            |        |           |                     |
|   amount |       DOUBLE | FALSE |            |        |           |                     |
|       ts | TIMESTAMP(3) |  TRUE |            |        |           |                     |
+----------+--------------+-------+------------+--------+-----------+---------------------+
6 rows in set

Flink SQL> ALTER TABLE Orders DROP (amount, ts, category);
[INFO] Execute statement succeed.

Flink SQL> DESCRIBE Orders;
+---------+--------+-------+------------+--------+-----------+------------------+
|    name |   type |  null |        key | extras | watermark |          comment |
+---------+--------+-------+------------+--------+-----------+------------------+
|   order |    INT | FALSE | PRI(order) |        |           | order identifier |
|    user | BIGINT |  TRUE |            |        |           |                  |
| product | STRING |  TRUE |            |        |           |                  |
+---------+--------+-------+------------+--------+-----------+------------------+
3 rows in set

Flink SQL> ALTER TABLE Orders RENAME `order` to `order_id`;
[INFO] Execute statement succeed.

Flink SQL> DESCRIBE Orders;
+----------+--------+-------+---------------+--------+-----------+------------------+
|     name |   type |  null |           key | extras | watermark |          comment |
+----------+--------+-------+---------------+--------+-----------+------------------+
| order_id |    INT | FALSE | PRI(order_id) |        |           | order identifier |
|     user | BIGINT |  TRUE |               |        |           |                  |
|  product | STRING |  TRUE |               |        |           |                  |
+----------+--------+-------+---------------+--------+-----------+------------------+
3 rows in set

Flink SQL> SHOW TABLES;
+------------+
| table name |
+------------+
|     Orders |
+------------+
1 row in set

Flink SQL> ALTER TABLE Orders RENAME TO NewOrders;
[INFO] Execute statement succeed.

Flink SQL> SHOW TABLES;
+------------+
| table name |
+------------+
|  NewOrders |
+------------+
1 row in set
```
{{< /tab >}}
{{< /tabs >}}

{{< top >}}

## ALTER TABLE

The following grammar gives an overview about the available syntax:
```text
ALTER TABLE [IF EXISTS] table_name {
    ADD { <schema_component> | (<schema_component> [, ...]) }
  | MODIFY { <schema_component> | (<schema_component> [, ...]) }
  | DROP {column_name | (column_name, column_name, ....) | PRIMARY KEY | CONSTRAINT constraint_name | WATERMARK}
  | RENAME old_column_name TO new_column_name
  | RENAME TO new_table_name
  | SET (key1=val1, ...)
  | RESET (key1, ...)
}

<schema_component>:
  { <column_component> | <constraint_component> | <watermark_component> }

<column_component>:
  column_name <column_definition> [FIRST | AFTER column_name]

<constraint_component>:
  [CONSTRAINT constraint_name] PRIMARY KEY (column_name, ...) NOT ENFORCED

<watermark_component>:
  WATERMARK FOR rowtime_column_name AS watermark_strategy_expression

<column_definition>:
  { <physical_column_definition> | <metadata_column_definition> | <computed_column_definition> } [COMMENT column_comment]

<physical_column_definition>:
  column_type

<metadata_column_definition>:
  column_type METADATA [ FROM metadata_key ] [ VIRTUAL ]

<computed_column_definition>:
  AS computed_column_expression
```

**IF EXISTS**

If the table does not exist, nothing happens.

### ADD
Use `ADD` clause to add [columns]({{< ref "docs/dev/table/sql/create" >}}#columns), [constraints]({{< ref "docs/dev/table/sql/create" >}}#primary-key), [watermark]({{< ref "docs/dev/table/sql/create" >}}#watermark) to an existing table. 

To add a column at the specified position, use `FIRST` or `AFTER col_name`. By default, the column is appended at last.

The following examples illustrate the usage of the `ADD` statements.

```sql
-- add a new column 
ALTER TABLE MyTable ADD category_id STRING COMMENT 'identifier of the category';

-- add columns, constraint, and watermark
ALTER TABLE MyTable ADD (
    log_ts STRING COMMENT 'log timestamp string' FIRST,
    ts AS TO_TIMESTAMP(log_ts) AFTER log_ts,
    PRIMARY KEY (id) NOT ENFORCED,
    WATERMARK FOR ts AS ts - INTERVAL '3' SECOND
);
```
<span class="label label-danger">Note</span> Add a column to be primary key will change the column's nullability to false implicitly.

### MODIFY
Use `MODIFY` clause to change column's position, type, comment or nullability, change primary key columns and watermark strategy to an existing table. 

To modify an existent column to a new position, use `FIRST` or `AFTER col_name`. By default, the position remains unchanged. 

The following examples illustrate the usage of the `MODIFY` statements.

```sql
-- modify a column type, comment and position
ALTER TABLE MyTable MODIFY measurement double COMMENT 'unit is bytes per second' AFTER `id`;

-- modify definition of column log_ts and ts, primary key, watermark. They must exist in table schema
ALTER TABLE MyTable MODIFY (
    log_ts STRING COMMENT 'log timestamp string' AFTER `id`,  -- reorder columns
    ts AS TO_TIMESTAMP(log_ts) AFTER log_ts,
    PRIMARY KEY (id) NOT ENFORCED,
    WATERMARK FOR ts AS ts -- modify watermark strategy
);
```

<span class="label label-danger">Note</span> Modify a column to be primary key will change the column's nullability to false implicitly.

### DROP
Use `DROP` clause to drop columns, primary key and watermark strategy to an existing table.

The following examples illustrate the usage of the `DROP` statements.

```sql
-- drop a column
ALTER TABLE MyTable DROP measurement;

-- drop columns
ALTER TABLE MyTable DROP (col1, col2, col3);

-- drop primary key
ALTER TABLE MyTable DROP PRIMARY KEY;

-- drop a watermark
ALTER TABLE MyTable DROP WATERMARK;
```

### RENAME
Use `RENAME` clause to rename column or an existing table.

The following examples illustrate the usage of the `RENAME` statements.
```sql
-- rename column
ALTER TABLE MyTable RENAME request_body TO payload;

-- rename table
ALTER TABLE MyTable RENAME TO MyTable2;
```

### SET

Set one or more properties in the specified table. If a particular property is already set in the table, override the old value with the new one.

The following examples illustrate the usage of the `SET` statements.

```sql
-- set 'rows-per-second'
ALTER TABLE DataGenSource SET ('rows-per-second' = '10');
```

### RESET

Reset one or more properties to its default value.

The following examples illustrate the usage of the `RESET` statements.

```sql
-- reset 'rows-per-second' to the default value
ALTER TABLE DataGenSource RESET ('rows-per-second');
```

{{< top >}}

## ALTER VIEW

```sql
ALTER VIEW [catalog_name.][db_name.]view_name RENAME TO new_view_name
```

Renames a given view to a new name within the same catalog and database.

```sql
ALTER VIEW [catalog_name.][db_name.]view_name AS new_query_expression
```

Changes the underlying query defining the given view to a new query.

{{< top >}}

## ALTER DATABASE

```sql
ALTER DATABASE [catalog_name.]db_name SET (key1=val1, key2=val2, ...)
```

Set one or more properties in the specified database. If a particular property is already set in the database, override the old value with the new one.

{{< top >}}

## ALTER FUNCTION

```sql
ALTER [TEMPORARY|TEMPORARY SYSTEM] FUNCTION 
  [IF EXISTS] [catalog_name.][db_name.]function_name 
  AS identifier [LANGUAGE JAVA|SCALA|PYTHON]
```

Alter a catalog function with the new identifier and optional language tag. If a function doesn't exist in the catalog, an exception is thrown.

If the language tag is JAVA/SCALA, the identifier is the full classpath of the UDF. For the implementation of Java/Scala UDF, please refer to [User-defined Functions]({{< ref "docs/dev/table/functions/udfs" >}}) for more details.

If the language tag is PYTHON, the identifier is the fully qualified name of the UDF, e.g. `pyflink.table.tests.test_udf.add`. For the implementation of Python UDF, please refer to [Python UDFs]({{< ref "docs/dev/python/table/udfs/python_udfs" >}}) for more details.

**TEMPORARY**

Alter temporary catalog function that has catalog and database namespaces and overrides catalog functions.

**TEMPORARY SYSTEM**

Alter temporary system function that has no namespace and overrides built-in functions

**IF EXISTS**

If the function doesn't exist, nothing happens.

**LANGUAGE JAVA\|SCALA\|PYTHON**

Language tag to instruct flink runtime how to execute the function. Currently only JAVA, SCALA and PYTHON are supported, the default language for a function is JAVA.

