The query does not create data on the disk, but assumes that data is already in the appropriate places, and just adds information about the specified table, dictionary or database to the server. After executing the `ATTACH` query, the server will know about the existence of the table, dictionary or database.
If a table was previously detached ([DETACH](../../sql-reference/statements/detach.md) query), meaning that its structure is known, you can use shorthand without defining the structure.
This query is used when starting the server. The server stores table metadata as files with `ATTACH` queries, which it simply runs at launch (with the exception of some system tables, which are explicitly created on the server).
If the table was detached permanently, it won't be reattached at the server start, so you need to use `ATTACH` query explicitly.
Allows to attach non-replicated MergeTree table as ReplicatedMergeTree. ReplicatedMergeTree table will be created with values of `default_replica_path` and `default_replica_name` settings. It is also possible to attach a replicated table as a regular MergeTree.
Note that table's data in ZooKeeper is not affected in this query. This means you have to add metadata in ZooKeeper using `SYSTEM RESTORE REPLICA` or clear it with `SYSTEM DROP REPLICA ... FROM ZKPATH ...` after attach.
If you are trying to add a replica to an existing ReplicatedMergeTree table, keep in mind that all the data in converted MergeTree table will be replicated as well.