绘图上下文中的WPF清除区域？(WPF Clear Region on a Drawing Context?)

 我认为你的主要潜在困惑是，当（例如）只有A被突出显示为红色时，你将这意味着“查询只返回来自A数据”，但实际上它意味着“查询只返回那些数据A有记录的情况 “。 该查询可能仍包含来自B的数据（对于B没有记录的情况，查询将替换NULL 。） 
 
 
 
 同样，下面的图片只包含了来自B圈的数据，那么为什么A包含在join语句中呢？ 
 
 
 如果你的意思是 - A的图像完全是白色的， B的部分有A与A不重叠的红色月牙形，那么： A出现在查询中的原因是， A是它的发现方式B中的记录需要排除。 （如果A没有出现在查询中，那么Venn图不会有A ，它只会显示B ，并且没有办法将期望的记录与不需要的记录区分开来。） 
 
 
 
 这个图像看起来像是圆圈B是sql语句的主要焦点，但是从A开始（从A选择，加入B），sql语句本身给我传达了相反的印象，即A是焦点的sql语句。 
 
 
 完全正确。 出于这个原因， RIGHT JOIN是比较少见的; 虽然使用LEFT JOIN的查询几乎总是可以重新排序以使用RIGHT JOIN （反之亦然），但通常人们会使用LEFT JOIN而不是RIGHT JOIN来编写查询。 

I think your main underlying confusion is that when (for example) only A is highlighted in red, you're taking that to mean "the query only returns data from A", but in fact it means "the query only returns data for those cases where A has a record". The query might still contain data from B. (For cases where B does not have a record, the query will substitute NULL.)
 
 
 
Similarly, the image below that only includes data from the B circle, so why is A included at all in the join statement?
 
 
If you mean — the image where A is entirely in white, and there's a red crescent-shape for the part of B that doesn't overlap with A, then: the reason that A appears in the query is, A is how it finds the records in B that need to be excluded. (If A didn't appear in the query, then Venn diagram wouldn't have A, it would only show B, and there'd be no way to distinguish the desired records from the unwanted ones.)
 
 
 
The image makes it seem like circle B is the primary focus of the sql statement, but the sql statement itself, by starting with A (select from A, join B), conveys the opposite impression to me, namely that A would be the focus of the sql statement.
 
 
Quite right. For this reason, RIGHT JOINs are relatively uncommon; although a query that uses a LEFT JOIN can nearly always be re-ordered to use a RIGHT JOIN instead (and vice versa), usually people will write their queries with LEFT JOIN and not with RIGHT JOIN.