You have a Fabric tenant that contains a warehouse named Warehouse1.

GRANT ALTER ON SCHEMA::schemaA TO User1;

The ALTER permission allows a user to modify the schema of an object, and granting ALTER on a schema will allow the user to perform operations like TRUNCATE TABLE on any object within that schema. It is the correct permission to grant to User1 for truncating tables in schemaA.

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GRANT Schema Permissions

Permissions That Can Be Granted on a Schema