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

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ATTACH Statement

This query is exactly the same as CREATE, but

• Instead of the word CREATE it uses the word ATTACH.
• 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 table to the server. After executing an ATTACH query, the server will know about the existence of the table.

If the table was previously detached (DETACH), meaning that its structure is known, you can use shorthand without defining the structure.

ATTACH TABLE [IF NOT EXISTS] [db.]name [ON CLUSTER cluster]

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 system tables, which are explicitly created on the server).