# UPDATE
`UPDATE` changes values stored in tables.
`UPDATE` changes values stored in [user-created tables](../create-table).

## Syntax

```mzsql
UPDATE <table_name> [ AS <table_alias> ]
   SET <column_name> = <expression> [, <column2_name> = <expression2>, ...]
[WHERE <condition(s)> ];
```

Syntax element                | Description
------------------------------|------------
**AS** <table_alias>          | If specified, you can only use the alias to refer to the table within that `UPDATE` statement.
**WHERE** <condition(s)>      | If specified, only update rows that meet the condition(s).

## Details

### Known limitations

* `UPDATE` cannot be used inside [transactions](../begin).
* `UPDATE` can reference [read-write tables](../create-table) but not
  [sources](../create-source) or read-only tables.
* **Low performance.** While processing an `UPDATE` statement, Materialize cannot
  process other `INSERT`, `UPDATE`, or `DELETE` statements.

## Privileges

The privileges required to execute this statement are:

- `USAGE` privileges on the schemas that all relations and types in the query are contained in.
- `UPDATE` privileges on the table being updated.
- `SELECT` privileges on all relations in the query.
  - NOTE: if any item is a view, then the view owner must also have the necessary privileges to
    execute the view definition. Even if the view owner is a _superuser_, they still must explicitly be
    granted the necessary privileges.
- `USAGE` privileges on all types used in the query.
- `USAGE` privileges on the active cluster.

## Examples

All examples below will use the `example_table` table:

```mzsql
CREATE TABLE example_table (a int, b text);
INSERT INTO example_table VALUES (1, 'hello'), (2, 'goodbye');
```

To verify the initial state of the table, run the following `SELECT` statement:

```mzsql
SELECT * FROM example_table;
```

The `SELECT` statement above should return two rows:

```
 a |    b
---+---------
 1 | hello
 2 | goodbye
```

### Update based on a condition

The following `UPDATE` example includes a `WHERE` clause to specify which rows
to update:

```mzsql
UPDATE example_table
SET a = a + 2
WHERE b = 'hello';
```

Only one row should be updated, namely the row with `b = 'hello'`. To verify
that the operation updated the `a` column only for that row, run the following
`SELECT` statement:

```mzsql
SELECT * FROM example_table;
```

The returned results show that column `a` was updated only for the row with `b
= 'hello'`:

```
 a |    b
---+---------
 3 | hello         -- Previous value: 1
 2 | goodbye
```

### Update all rows

The following `UPDATE` example updates all rows in the table to set `a` to 0 and
`b` to `aloha`:

```mzsql
UPDATE example_table
SET a = 0, b = 'aloha';
```

To verify the results, run the following `SELECT` statement:

```mzsql
SELECT * FROM example_table;
```

The returned results show that all rows were updated:

```
 a |   b
---+-------
 0 | aloha
 0 | aloha
```

## Related pages

- [`CREATE TABLE`](../create-table)
- [`INSERT`](../insert)
- [`SELECT`](../select)