我需要连接两个表(实际上是两个视图),这样对于左视图的每个选定行,右视图中都有一个行数 . 听起来像LEFT JOIN,但是在SQLite(这个测试database)和LEFT JOIN查询中:
SELECT TARGET.session_id session_id, TARGET.labeltype_id labeltype_id, TARGET.label_id label_id, count(SECONDARY.label_id) NOlabels
FROM segment_extended TARGET LEFT JOIN segment_extended SECONDARY
WHERE TARGET.session_id = SECONDARY.session_id AND TARGET.lt_name= "Word" AND SECONDARY.lt_name ="Comments"
AND ((SECONDARY.start <= TARGET.start AND TARGET.END <= SECONDARY.END) OR (TARGET.start <= SECONDARY.start AND SECONDARY.END <= TARGET.END))
AND TARGET.label != '' AND SECONDARY.label != ''
GROUP BY TARGET.session_id,TARGET.labeltype_id, TARGET.label_id;
我得到的只是我期望的一小部分:
2 3 3 1
2 3 9 1
更扩展的查询会给出正确的结果:
SELECT session_id, labeltype_id, label_id, max(NOlabels) NOlabels
FROM (SELECT TARGET.session_id session_id, TARGET.labeltype_id labeltype_id, TARGET.label_id label_id, count(SECONDARY.label_id) NOlabels
FROM segment_extended TARGET , segment_extended SECONDARY
WHERE TARGET.session_id = SECONDARY.session_id AND TARGET.lt_name= "Word" AND SECONDARY.lt_name ="Comments"
AND ((SECONDARY.start <= TARGET.start AND TARGET.END <= SECONDARY.END) OR (TARGET.start <= SECONDARY.start AND SECONDARY.END <= TARGET.END))
AND TARGET.label != '' AND SECONDARY.label != ''
GROUP BY TARGET.session_id,TARGET.labeltype_id, TARGET.label_id
UNION
SELECT TARGET.session_id session_id, TARGET.labeltype_id labeltype_id, TARGET.label_id label_id, 0 NOlabels
FROM segment_extended TARGET
WHERE TARGET.lt_name= "Word"
AND TARGET.label != ''
GROUP BY TARGET.session_id,TARGET.labeltype_id, TARGET.label_id)
GROUP BY session_id, labeltype_id, label_id
ORDER BY session_id,labeltype_id, label_id
session_id labeltype_id label_id NOlabels
2 3 2 0
2 3 3 1
2 3 4 0
2 3 5 0
2 3 7 0
2 3 8 0
2 3 9 1
2 3 10 0
但似乎不必要地复杂化 . 左连接我做错了什么?
2 回答
您的加入不是左连接 .
如果没有与连接条件匹配的行,则左连接会为右表添加NULL值 . 但是,查询没有连接条件,并且WHERE条件不受LEFT JOIN子句的影响 .
用ON替换WHERE .
在进行左连接时,您必须将左连接中的空值计为0记录,但仍包含它们 . 您可以在内部查询中使用CASE构造完成此操作,然后在外部group-by中使用SUM聚合函数 .