--- 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` — 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` — 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/special/index.md#table_engines-virtual_columns) - [merge](../../../sql-reference/table-functions/merge.md) table function [Original article](https://clickhouse.com/docs/en/operations/table_engines/special/merge/)