How to show 0 when no row found

问题:

I have a SQL query in which I am passing sysdate to the query problem is that when there is no matching date in table with sysdate then it don't shows the zero even if there is nvl applied here is my query

select * from molasses where trunc(trn_dte) = trunc(sysdate) 

But it show data only when current date is present in table but I want to show zero if no data found in table.please help me to do this in oracle 10 g. Because some times the situation is like above and I have to display zero when no data found

回答1:

This is weird and I wouldn't use it, it's more of a hack:

SELECT col1, col2, ..., colN              --- numeric columns FROM molasses WHERE trunc(trn_dte) = trunc(sysdate)  UNION ALL  SELECT 0, 0, ..., 0 FROM dual WHERE NOT EXISTS       ( SELECT *         FROM molasses          WHERE trunc(trn_dte) = trunc(sysdate)       ) ; 

One only wonders what the application/user will understand when there is exactly one row in the table and all the values are zero.

---

I think this would work, too:

SELECT m.col1, m.col2, ..., m.colN              --- numeric columns FROM dual LEFT JOIN molasses m   ON trunc(m.trn_dte) = trunc(sysdate) ; 

and show Nulls instead of (the wanted) 0s. Using the COALESCE() function could easily fix that, as well.

回答2:

Something like this would work, but I don't think it is a very good idea:

select nvl(t.a, n.a) as a, nvl(t.b, n.b) as b, ... from molasses t right outer join (select 0 as a, 0 as b, ... from dual) n on 1 = 1 

回答3:

This is a very unusual question. Why should a query return 0's when no data present? Usually it's up to the client software/report generator to show something meaningful when no data is present, not the query itself.

回答4:

Assuming your table has four numeric columns, you could write:

select * from molasses where trunc(trn_dte) = trunc(sysdate) union all select 0, 0, 0, 0 from dual where ( select count(*) from molasses where trunc(trn_dte) = trunc(sysdate) ) = 0 

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