ClickHouse/docs/en/sql-reference/statements/watch.md

---
sidebar_position: 53
sidebar_label: WATCH
---

# WATCH Statement (Experimental) {#watch}

:::warning    
This is an experimental feature that may change in backwards-incompatible ways in the future releases. Enable live views and `WATCH` query using `set allow_experimental_live_view = 1`.
:::

``` sql
WATCH [db.]live_view
[EVENTS]
[LIMIT n]
[FORMAT format]
```

The `WATCH` query performs continuous data retrieval from a [LIVE VIEW](./create/view.md#live-view) table. Unless the `LIMIT` clause is specified it provides an infinite stream of query results from a [LIVE VIEW](./create/view.md#live-view).

```sql
WATCH [db.]live_view [EVENTS] [LIMIT n] [FORMAT format]
```

## Virtual columns {#watch-virtual-columns}

The virtual `_version` column in the query result indicates the current result version.

**Example:**

```sql
CREATE LIVE VIEW lv WITH REFRESH 5 AS SELECT now();
WATCH lv;
```

```bash
┌───────────────now()─┬─_version─┐
│ 2021-02-21 09:17:21 │        1 │
└─────────────────────┴──────────┘
┌───────────────now()─┬─_version─┐
│ 2021-02-21 09:17:26 │        2 │
└─────────────────────┴──────────┘
┌───────────────now()─┬─_version─┐
│ 2021-02-21 09:17:31 │        3 │
└─────────────────────┴──────────┘
...
```

By default, the requested data is returned to the client, while in conjunction with [INSERT INTO](../../sql-reference/statements/insert-into.md) it can be forwarded to a different table.

**Example:**

```sql
INSERT INTO [db.]table WATCH [db.]live_view ...
```

## EVENTS Clause {#events-clause}

The `EVENTS` clause can be used to obtain a short form of the `WATCH` query where instead of the query result you will just get the latest query result version.

```sql
WATCH [db.]live_view EVENTS;
```

**Example:**

```sql
CREATE LIVE VIEW lv WITH REFRESH 5 AS SELECT now();
WATCH lv EVENTS;
```

```bash
┌─version─┐
│       1 │
└─────────┘
┌─version─┐
│       2 │
└─────────┘
...
```

## LIMIT Clause {#limit-clause}

The `LIMIT n` clause specifies the number of updates the `WATCH` query should wait for before terminating. By default there is no limit on the number of updates and therefore the query will not terminate. The value of `0` indicates that the `WATCH` query should not wait for any new query results and therefore will return immediately once query result is evaluated.

```sql
WATCH [db.]live_view LIMIT 1;
```

**Example:**

```sql
CREATE LIVE VIEW lv WITH REFRESH 5 AS SELECT now();
WATCH lv EVENTS LIMIT 1;
```

```bash
┌─version─┐
│       1 │
└─────────┘
```

## FORMAT Clause {#format-clause}

The `FORMAT` clause works the same way as for the [SELECT](../../sql-reference/statements/select/format.md#format-clause).

:::note    
The [JSONEachRowWithProgress](../../interfaces/formats.md#jsoneachrowwithprogress) format should be used when watching [LIVE VIEW](./create/view.md#live-view) tables over the HTTP interface. The progress messages will be added to the output to keep the long-lived HTTP connection alive until the query result changes. The interval between progress messages is controlled using the [live_view_heartbeat_interval](./create/view.md#live-view-settings) setting.
:::