wp-cli with sharded databases

why-do-i-need-a-filename.hmm

# Be *very* careful with this.

# In your wp-config.php file

$db_name = getenv( 'WP_CLI_DB_NAME' );
define( 'DB_NAME', ( !empty( $db_name ) ? $db_name : 'default_global' ) ); # Default often 'wpmu_global'

# Then, when calling wp-cli, if the tables you want are in a shard called prefix_3f
# And you want to grab the comments table for a site with an ID of 10175
WP_CLI_DB_NAME=prefix_3f; export WP_CLI_DB_NAME; wp db export --tables=wp_10175_comments

As an addition here, for certain wp-cli commands, such as wp db tables you'll also need to include the --url flag which needs to point at one of the sites that is run from the tables in the shard you're after.