MySQL：LEFT JOIN选择

Question

how can I join two tables and make the second optional?

SELECT t1.title,SUM(t2.seconds) AS seconds 
FROM operation t1 
  LEFT JOIN workhours t2 ON t1.id = t2.pid AND t1.status='0' 
ORDER BY t1.tstamp DESC

This query shows only one result but there is a second one withour any worktime on it. How can I make sure that the data from t1 will always be displayed even when there is nothing in t2?

Answer 1

You need to add GROUP BY t1.title :

SELECT t1.title,SUM(t2.seconds) AS seconds 
FROM operation t1 LEFT JOIN workhours t2 ON t1.id = t2.pid AND t1.status='0' 
GROUP BY t1.title
ORDER BY t1.tstamp DESC

In general, if title is not unique, you should group by unique key.

Answer 2

如果你需要总是得到第一个的结果与第二个结果不匹配，那么你必须使用Left Outer Join如果另一个表与第二个表没有任何匹配的结果，它将始终显示结果如果第二个匹配或不匹配，则第一个独立。

Answer 3

It seems that the issue may be in the "SUM(t2.seconds)" parameter on your select.

Try adding ISNULL(SUM(t2.seconds), 0).

If that doesn't work, you might try a SQL IF statement or CASE and check a column on t2 for nulls.