SQL Server null table join

问题

I have two tables one of it is LEAGUE, another is MATCH , in MATCH table there is a column as refLeague now I want to get total matches of week

For example

Id TotalMatches
-- ------------
1   12
2    0
3    6

If there is no match, I want to write 0 as table

 SELECT l.Id ,COUNT(m.Id) as TotalMatches 
 FROM LEAGUE l
 LEFT JOIN MATCH m ON l.Id = m.refLeauge
 WHERE 
     m.MatchDate >= DATEADD(dd, DATEDIFF(dd, 1, GETDATE()) / 7 * 7 + 1, 0) 
     AND m.MatchDate < DATEADD(dd, DATEDIFF(dd, -6, GETDATE())/7 * 7 + 1, 0) 
     AND l.refSport = 1
 GROUP BY l.Id

I wrote this query but it is not giving any result due to no rows in Match table, but it must be written 0

Example

Id TotalMatches
-- ------------
 1      0
 2      0
 3      0

Where is my mistake?

回答1:

Move the right table filters to ON condition

Non matching records will have NULL values in m.MatchDate which will be filtered by the condition in Where clause . Implicitly it will be converted to INNER JOIN. So the condition should be moved to ON clause which tells what are the records to be joined with LEAGUE instead of filtering the result

SELECT l.id, 
       Count(m.id) AS TotalMatches 
FROM   league l 
       LEFT JOIN match m 
              ON l.id = m.refleauge 
                 AND m.matchdate >= Dateadd(dd, Datediff(dd, 1, Getdate()) / 7 * 7 + 1, 0) 
                 AND m.matchdate < Dateadd(dd, Datediff(dd, -6, Getdate()) / 7 * 7 + 1, 0) 
WHERE  l.refsport = 1 
GROUP  BY l.id

回答2:

The where is breaking the left join

 SELECT l.Id, COUNT(m.Id) as TotalMatches 
 FROM LEAGUE l
 LEFT JOIN MATCH m 
   ON l.Id = m.refLeauge
  and m.MatchDate >= dateadd(dd, datediff(dd, 1, getdate()) / 7 * 7 + 1,0) 
  AND m.MatchDate <  dateadd(dd, datediff(dd,-6, getdate()) / 7 * 7 + 1,0) 
 where l.refSport=1
 GROUP BY l.Id

/ 7 * 7 = 1

When I started this answer the other answer was not yet posted

来源：https://stackoverflow.com/questions/44862219/sql-server-null-table-join

标签

sql

sql-server

tsql

left-join