"dual" is a keyword in oracle sql.   It represents a fake table from which you can select whatever you want.   It is extremely useful for a lot of etl tasks.   For example if you need to generate a list of one hundred numbers you can do something like this   SELECT ROWNUM rn FROM dual CONNECT BY 1=1 AND ROWNUM < 1001   (rownum is another special keyword in oracle to have an id of the row)   Another example is this      select 1 id, 'a' desc from dual union select 2 id, 'b' desc from dual   creating on the fly a simple table with 2 rows   1,'a' 2,'b'   and so on.   This on the fly generated table data can then be used and joined with other table and so on.   At the moment with impala i've create a real table to do the job inserting all the needed data   Beleive me that this fake table became very handy in a lot of situations   bye ... View more