当left join返回null时，'where'条件失败

Question

When left join fails then due to vlu.status=1 query does not return anything. I want result even if left join fails.

If i remove vlu.status=1 then it returns right result, but i have to use vlu.status=1 when left join does not fails.

select vb.first_name,vb.last_name,DATE_FORMAT(vb.created_date,'%m-%d-%Y') as Created_On,
     concat(la.first_name,' ',la.last_name) as Loan_Agent, vl.loan_number, 
     count(vs.id) as Num_Deliveries from vid_borrowers vb 

         inner join vid_loans vl on vl.borrower_id= vb.id 
         left join vid_delivery_schedules vs on vs.borrower_id = vb.id
         left join vid_loan_agents la on la.id=vl.loan_officer_id 
         left join vid_users vlu on vlu.id=la.user_id 

     where vb.bank_id=6
       AND STR_TO_DATE(vb.created_date, '%Y-%m-%d') between  '2014-12-01' and '2014-12-16'
     and  vlu.status=1
     group by vb.first_name, vb.last_name, la.first_name, la.last_name, vl.loan_number

Answer 1

Put the condition inside the join and remove it from the WHERE clause; doing so creates a NULL row if the condition fails before the WHERE gets involved.

 ...
 left join vid_users vlu on vlu.id=la.user_id and vlu.status = 1
 where vb.bank_id=6
   AND STR_TO_DATE(vb.created_date, '%Y-%m-%d') between  '2014-12-01' and '2014-12-16'
 group by vb.first_name, vb.last_name, la.first_name, la.last_name, vl.loan_number

You could let the WHERE take care of it as well by checking for NULL values, but IMHO it's better to perform the filter as soon as possible.

Answer 2

The problem here is that by applying a LEFT JOIN and a WHERE filter is that the condition where the LEFT JOIN fails returns NULL for vlu.status , which is then filtered out in the WHERE (since the criteria is vlu.status = 1 ). You need to change either:

WHERE ...
AND (vlu.status = 1 OR vlu.status IS NULL) -- NULL for the Left Join

OR move the vlu.status filter into the LEFT JOIN criteria

LEFT JOIN vid_users vlu on vlu.id=la.user_id AND vlu.status = 1

More on this here

Answer 3

The coalesce function takes a list of expressions and returns the first result that isn't null. You can use it to replace nulls with a default value:

coalesce(vlu.status = 1, true)