-`ALTER TABLE [db].name ADD PROJECTION name AS SELECT <COLUMN LIST EXPR> [WHERE] [GROUP BY] [ORDER BY]` - Adds projection description to tables metadata.
-`ALTER TABLE [db].name DROP PROJECTION name` - Removes projection description from tables metadata and deletes projection files from disk.
-`ALTER TABLE [db.]table MATERIALIZE PROJECTION name IN PARTITION partition_name` - The query rebuilds the projection `name` in the partition `partition_name`. Implemented as a [mutation](../../../../sql-reference/statements/alter/index.md#mutations).
Projection manipulation is supported only for tables with [`*MergeTree`](../../../../engines/table-engines/mergetree-family/mergetree.md) engine (including [replicated](../../../../engines/table-engines/mergetree-family/replication.md) variants).