SQL Joining by field that *may* be empty

问题

In my SQL CE database I have three tables: customer, list and customerlist (a junction table between customer and list - as it is a many-to-many relationship).

I am trying to run a query that will display all current lists with also the number of customers who are currently subscribed to that list (counting from customerlist table).

Here is my current query:

select list.listid, count(customerlist.customerid) as numppl, list.ShortDesc
from list inner join customerlist on list.listid=customerlist.listid
group by list.ShortDesc, list.listid
order by numppl desc

The current structure of this database is:

[Customer]           [List]             [CustomerList]
CustomerId           ListId             CustomerListId
Name                 ShortDesc          CustomerId
Other details                           ListId

This currently returns all the lists who have customers currently assigned to them - but not lists which are empty. Empty lists are hidden.

I would like to modify this query to also display empty lists but I am struggling. My desired output is:

Name      numppl
listA     375
listB     45
listC     0

(In the example above, listC is currently not being returned).

Any thoughts on how to also show listC in the query?

回答1:

Use LEFT JOIN instead with ISNULL to replace NULL with 0:

SELECT 
  list.listid, 
  ISNULL(count(customerlist.customerid), 0) AS numppl, 
  list.ShortDesc
FROM list 
LEFT JOIN customerlist ON list.listid = customerlist.listid
GROUP BY list.ShortDesc, 
         list.listid
ORDER BY numppl DESC;

SQL Fiddle Demo

Update

For SQL Server CE, try this:

SELECT 
  list.listid, 
  SUM(CASE WHEN customerlist.customerid IS NULL THEN 0 ELSE 1 END) AS numppl, 
  list.ShortDesc
FROM list 
LEFT JOIN customerlist ON list.listid = customerlist.listid
GROUP BY list.ShortDesc, 
         list.listid
ORDER BY numppl DESC;

回答2:

You must use LEFT JOIN syntax.

select list.listid,  case when count(customerlist.customerid) is null then 0 else count(customerlist.customerid) end as numppl, list.ShortDesc
from list left join customerlist on list.listid=customerlist.listid
group by list.ShortDesc, list.listid
order by numppl desc

Or you can use RIGHT JOIN :

select list.listid, case when count(customerlist.customerid) is null then 0 else count(customerlist.customerid) end as numppl, list.ShortDesc
from customerlist right join list on customerlist.listid=list.listid
group by list.ShortDesc, list.listid
order by numppl desc

Use COALESCE :

select list.listid,  coalesce(count(customerlist.customerid),0) as numppl, list.ShortDesc
from list left join customerlist on list.listid=customerlist.listid
group by list.ShortDesc, list.listid
order by numppl desc

来源：https://stackoverflow.com/questions/14976931/sql-joining-by-field-that-may-be-empty

标签

sql

sql-server-ce