ydb/yql/essentials/docs/en/syntax/use.md

USE

Specifying the "database". As a rule, one of the backend clusters is used as a database. This database will be used by default to search for tables whenever the database hasn't been specified explicitly.

If the query doesn't include USE, then the cluster must be specified at the beginning of the table path in the format cluster.`path/to/table`, for example cluster.`home/yql/test`. Backticks are used to automatically escape special characters (in this case, slashes).

Usually the cluster name is specified explicitly, but you can use an expression for it. For example, this will let you use the parameters declared by DECLARE. In this case, USE must have the notation USE yt:$cluster_name, where $cluster_name is the named expression of the String type. Alternatively, you can specify the cluster right at the beginning of the table path in the format yt:$cluster_name.`path/to/table`.

As far as USE is concerned, you can add it inside actions or subquery templates. The value of the current cluster is inherited by declarations of nested actions or subqueries. The scope of USE is terminated at the end of the action or subquery template where it has been declared.

Examples

USE my_cluster;

{% note info %}

USE doesn't guarantee that the query will be executed against the specified cluster. The query might be executed against other cluster if it doesn't use any input data (for example, USE foo; SELECT 2 + 2; ) or if the full path to the table on other cluster has been specified (for example, USE foo; SELECT * FROM bar.path/to/table;).

{% endnote %}