# UPDATE keyword

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UPDATE SQL keyword reference documentation.

Updates data in a database table.

## Syntax

```questdb-sql
UPDATE tableName [alias]
SET columnName = expression [, columnName = expression ...]
    [FROM joinTable [JOIN joinTable2 ON joinCondition] ...]
    [WHERE filter];
```

:::note

- the same `columnName` cannot be specified multiple times after the SET keyword
  as it would be ambiguous
- the designated timestamp column cannot be updated as it would lead to altering
  history of the [time-series data](/blog/what-is-time-series-data/)
- If the target partition is
  [attached by a symbolic link](/docs/query/sql/alter-table-attach-partition/#symbolic-links),
  the partition is read-only. `UPDATE` operation on a read-only partition will
  fail and generate an error.
- On a WAL table, `UPDATE` is applied by the WAL apply job and counts against
  [`cairo.wal.apply.memory.limit.bytes`](/docs/configuration/cairo-engine/#cairowalapplymemorylimitbytes)
  rather than the query memory limit. This holds for every form of `UPDATE` a
  WAL table accepts, including one with a subquery in its `SET` or `WHERE`
  clause: the whole statement is written to the WAL and executed by the apply
  job. `UPDATE ... FROM`, which joins another table, is not supported on WAL
  tables and is rejected with
  `UPDATE statements with join are not supported yet for WAL tables`. On a
  non-WAL table, `UPDATE` runs on the caller's connection under the caller's
  [query memory limit](/docs/configuration/cairo-engine/#memory-limits).

:::

## Examples

```questdb-sql title="Update with constant"
UPDATE trades SET price = 125.34 WHERE symbol = 'AAPL';
```

```questdb-sql title="Update with function"
UPDATE book SET mid = (bid + ask)/2 WHERE symbol = 'AAPL';
```

```questdb-sql title="Update with subquery"
UPDATE spreads s SET spread = p.ask - p.bid FROM prices p WHERE s.symbol = p.symbol;
```

```questdb-sql title="Update with multiple joins"
WITH up AS (
    SELECT p.ask - p.bid AS spread, p.timestamp
    FROM prices p
    JOIN instruments i ON p.symbol = i.symbol
    WHERE i.type = 'BOND'
)
UPDATE spreads s
SET spread = up.spread
FROM up
WHERE s.timestamp = up.timestamp;
```

```questdb-sql title="Update with a sub-query"
WITH up AS (
    SELECT symbol, spread, ts
    FROM temp_spreads
    WHERE timestamp between '2022-01-02' and '2022-01-03'
)
UPDATE spreads s
SET spread = up.spread
FROM up
WHERE up.ts = s.ts AND s.symbol = up.symbol;
```