The DUAL table is jointly created by OceanBase Database and the data dictionary. The DUAL table is located in the schema of the SYS user, but is accessible to all users by its name.
The DUAL table has one column named DUMMY of the VARCHAR2(1) data type and one row with the value X. When you use the SELECT statement to calculate a constant expression, you can select data from the DUAL table for convenience. The DUAL table has only one row and therefore the constant is returned only once. Alternatively, you can select a constant, pseudocolumn, or expression from any table, but the value is returned as many times as there are rows in the table.