The System table system.one contains only one row of Data, and this row has a single Column named dummy with Type: UInt8 and Value 0. When you Run a SELECT Search without specifying a FROM clause (such as SELECT 0), the System will Default to use this table as the query carrier. Its function is similar to the DUAL table in Other Database manage Systems (DBMS), used for simple queries in Scenes where there is None Table.

-- No need to write FROM, the Database will Default to use system.one 
SELECT 0; 

-- Equivalent to explicitly specifying FROM system.one 
SELECT 0 FROM system.one;