ClickHouse/docs/en/engines/table-engines/special/merge.md

---
slug: /en/engines/table-engines/special/merge
sidebar_position: 30
sidebar_label: Merge
---

# Merge Table Engine

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}

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

## Engine Parameters

### 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` — 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}

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}

**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.

``` sql
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.

``` sql
CREATE TABLE WatchLog_old(date Date, UserId Int64, EventType String, Cnt UInt64)
    ENGINE=MergeTree(date, (UserId, EventType), 8192);
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;
```

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

## Virtual Columns {#virtual-columns}

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

    You can set the constant conditions on `_table` in the `WHERE/PREWHERE` clause (for example, `WHERE _table='xyz'`). In this case the read operation is performed only for that tables where the condition on `_table` is satisfied, so the `_table` column acts as an index.

**See Also**

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