tableau/SQL when no data show 0 or string in table

Question

this is my program , and i want to show 0 when server no data

    select
      a.toolid ,
      a.chamber,
      b.datetime,
      b.value
   from “table1” a
   left join “table2”b
   on a.toolid = b.toolid
   where b.datetime between ‘2022-06-01’ and now()

i test isnull and zn, but all can't use

Answer 1

The restrictions on the table2 table in the WHERE clause should be moved to the ON clause of the join. Then, use COALESCE to display a default value for those table2 columns whose values are not present.

SELECT
    a.toolid,
    a.chamber,
    b.datetime,          -- can use COALESCE here too, see below
    COALESCE(b.value, 0) AS value
FROM table1 a
LEFT JOIN table2 b
    ON a.toolid = b.toolid AND
       b.datetime BETWEEN '2022-06-01' AND NOW();

Note that we could also use COALESCE on b.datetime , but then we would need to display a default date. If we wanted instead to display default text, then we would have to first cast b.datetime to text.