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The `Merge` engine (not to be confused with `MergeTree`) does not store data itself, but allows reading from any number of other tables simultaneously.

Reading is automatically parallelized. Writing to a table is not supported. When reading, the indexes of tables that are actually being read are used, if they exist.

## Creating a table[​](#creating-a-table "Direct link to Creating a table")

```
CREATE TABLE ... Engine=Merge(db_name, tables_regexp)
```

## Engine parameters[​](#engine-parameters "Direct link to Engine parameters")

### `db_name`[​](#db_name "Direct link to db_name")

`db_name` — Possible values:

- database name,
- constant expression that returns a string with a database name, for example, `currentDatabase()`,
- `REGEXP(expression)`, where `expression` is a regular expression to match the DB names.

### `tables_regexp`[​](#tables_regexp "Direct link to tables_regexp")

`tables_regexp` — A regular expression to match the table names in the specified DB or DBs.

Regular expressions — [re2](https://github.com/google/re2) (supports a subset of PCRE), case-sensitive.
See the notes about escaping symbols in regular expressions in the "match" section.

## Usage[​](#usage "Direct link to Usage")

When selecting tables to read, the `Merge` table itself is not selected, even if it matches the regex. This is to avoid loops.
It is possible to create two `Merge` tables that will endlessly try to read each others' data, but this is not a good idea.

The typical way to use the `Merge` engine is for working with a large number of `TinyLog` tables as if with a single table.

## Examples[​](#examples "Direct link to Examples")

**Example 1**

Consider two databases `ABC_corporate_site` and `ABC_store`. The `all_visitors` table will contain IDs from the tables `visitors` in both databases.

```
CREATE TABLE all_visitors (id UInt32) ENGINE=Merge(REGEXP('ABC_*'), 'visitors');
```

**Example 2**

Let's say you have an old table `WatchLog_old` and decided to change partitioning without moving data to a new table `WatchLog_new`, and you need to see data from both tables.

```
CREATE TABLE WatchLog_old(
    date Date,
    UserId Int64,
    EventType String,
    Cnt UInt64
)
ENGINE=MergeTree
ORDER BY (date, UserId, EventType);

INSERT INTO WatchLog_old VALUES ('2018-01-01', 1, 'hit', 3);

CREATE TABLE WatchLog_new(
    date Date,
    UserId Int64,
    EventType String,
    Cnt UInt64
)
ENGINE=MergeTree
PARTITION BY date
ORDER BY (UserId, EventType)
SETTINGS index_granularity=8192;

INSERT INTO WatchLog_new VALUES ('2018-01-02', 2, 'hit', 3);

CREATE TABLE WatchLog AS WatchLog_old ENGINE=Merge(currentDatabase(), '^WatchLog');

SELECT * FROM WatchLog;
```

```
┌───────date─┬─UserId─┬─EventType─┬─Cnt─┐
│ 2018-01-01 │      1 │ hit       │   3 │
└────────────┴────────┴───────────┴─────┘
┌───────date─┬─UserId─┬─EventType─┬─Cnt─┐
│ 2018-01-02 │      2 │ hit       │   3 │
└────────────┴────────┴───────────┴─────┘
```

## Virtual columns[​](#virtual-columns "Direct link to Virtual columns")

- `_table` — The name of the table from which data was read. Type: [String](/docs/sql-reference/data-types/string).

  If you filter on `_table`, (for example `WHERE _table='xyz'`) only tables which satisfy the filter condition are read.
- `_database` — Contains the name of the database from which data was read. Type: [String](/docs/sql-reference/data-types/string).

**See Also**

- [Virtual columns](/docs/engines/table-engines#table_engines-virtual_columns)
- [merge](/docs/sql-reference/table-functions/merge) table function

- [Creating a table](#creating-a-table)- [Engine parameters](#engine-parameters)
    - [`db_name`](#db_name)- [`tables_regexp`](#tables_regexp)- [Usage](#usage)- [Examples](#examples)- [Virtual columns](#virtual-columns)

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