上一篇简要介绍了图数据库的一些基本内容(初识SQL Server2017 图数据库(一)),本篇通过对比关系型一些语法来体现图数据库模式的一些优点,比如查询方便,语句易理解等。
在图数据库模型上构建查询的优势:
T-SQL 带给图表查询一些新的语法。在SELECT语句中我们有一些特殊的语句来关联点和边。让我们来演练一些,构建查询语句检索发帖和回复,如下:
- 我们检索每个记录的两个部分,发帖和回复,因此我们需要在FROM子句中引用两次ForumPosts’表,这个地方可以采用一些有意义的别名:
FROM dbo.ForumPosts ReplyPost, dbo.ForumPosts RepliedPost
- 尽管我们能选择任何别名,但是在处理图对象时最好选择有意义的名字。
- 我们需要“posts”之间的关系,而这个关系就是表Reply_to’。语法如下:
FROM dbo.ForumPosts ReplyPost, dbo.Reply_to, dbo.ForumPosts RepliedPost
- 在WHERE 子句中,我们需要关联所有的表,用下面这种MATCH语句来实现关联:
FROM dbo.ForumPosts ReplyPost, dbo.Reply_to, dbo.ForumPosts RepliedPost
WHERE MATCH(ReplyPost-(Reply_to)->RepliedPost)
- 这个语法很有意思:“-”破折号表示边的$From_id字段表示关系,然后“->”破折号和大于号用边的$To_id字段表示关系。
- 因为知道那个别名有reply,那个别名有replied post,我们可以构建一个查询字段列表:
FROM dbo.ForumPosts ReplyPost, dbo.Reply_to, dbo.ForumPosts RepliedPost
WHERE MATCH(ReplyPost-(Reply_to)->RepliedPost)
- 在关系型模型中相同功能的查询如下:
select RepliedPost.PostId,RepliedPost.PostTitle, ReplyPost.PostId as ReplyId, ReplyPost.PostTitle as ReplyTitle from Forum.ForumPosts ReplyPost, Forum.ForumPosts RepliedPost where ReplyPost.PostId=RepliedPost.ReplyTo
- 这些查询很相似,当然MATCH的语法更容易理解。
- 执行完上面语句查询结果如下:
- 我们加上写这个回复贴人的名字。需要在FROM子句中添加‘ForumMembers’节点和‘Written_By’这个边。语句如下:
FROM dbo.ForumPosts ReplyPost, dbo.Reply_to, dbo.ForumPosts RepliedPost, dbo.ForumMembers RepliedMember, Written_By RepliedWritten_By
- 还要添加MATCH语句的内部关系:
WHERE MATCH(ReplyPost-(Reply_to)->RepliedPost-(RepliedWritten_by)->RepliedMember)
- 这就可以在SELECT列表中添加回帖人的名字,最终的查询如下:
-- Posts 、members 和replies SELECT RepliedPost.PostId,RepliedPost.PostTitle,RepliedMember.MemberName, ReplyPost.PostId as ReplyId, ReplyPost.PostTitle as ReplyTitle FROM dbo.ForumPosts ReplyPost, dbo.Reply_to, dbo.ForumPosts RepliedPost, dbo.ForumMembers RepliedMember, Written_By RepliedWritten_By WHERE MATCH(ReplyPost-(Reply_to)->RepliedPost-(RepliedWritten_by)->RepliedMember)
- 在关系型模型中的对应查询如下:
SELECT RepliedPost.PostId,RepliedPost.PostTitle, ReplyPost.PostId as ReplyId, ReplyPost.PostTitle as ReplyTitle, RepliedMember.MemberName FROM Forum.ForumPosts ReplyPost, Forum.ForumPosts RepliedPost, Forum.ForumMembers RepliedMember WHERE ReplyPost.PostId=RepliedPost.ReplyTo and RepliedPost.OwnerId=RepliedMember.MemberId
- 结果如下所示:
- 还缺少回复对象的名字。像上面一样增加‘ForumMembers’ 和 ‘Written_By’在FROM子句中:
From dbo.ForumPosts ReplyPost, dbo.Reply_to, dbo.ForumPosts RepliedPost,
dbo.ForumMembers RepliedMember, Written_By RepliedWritten_By,
dbo.ForumMembers ReplyMember, Written_By ReplyWritten_By
- 接下来,修改MATCH子句,‘ReplyMember’需要关联‘ReplyPost’,但是如何去处理这个关系而不影响其他关系?需要用不同的方式来实现:
WHERE MATCH(ReplyMember<-(ReplyWritten_By)-ReplyPost-(Reply_to)->RepliedPost-(RepliedWritten_by)->RepliedMember)
- 注意这个符号“<-”与之前的相反方向,但是意义是相同的:一个在边表的$to_id与节点表的关系。
- 最终,还需增加写着回复的成员姓名,代码如下:
-- Posts and members and their replies and members SELECT RepliedPost.PostId, RepliedPost.PostTitle,RepliedMember.MemberName, ReplyPost.PostId as ReplyId, ReplyPost.PostTitle as ReplyTitle, ReplyMember.MemberName [ReplyMemberName] FROM dbo.ForumPosts ReplyPost, dbo.Reply_to, dbo.ForumPosts RepliedPost, dbo.ForumMembers RepliedMember, Written_By RepliedWritten_By, dbo.ForumMembers ReplyMember, Written_By ReplyWritten_By WHERE MATCH(ReplyMember<-(ReplyWritten_By)-ReplyPost-(Reply_to)->RepliedPost-(RepliedWritten_by)->RepliedMember)
- 结果集如下:
- 在关系型查询的对应语句:
SELECT RepliedPost.PostId,RepliedPost.PostTitle, RepliedMember.MemberName, ReplyPost.PostId as ReplyId, ReplyPost.PostTitle as ReplyTitle, ReplyMember.MemberName FROM Forum.ForumPosts ReplyPost, Forum.ForumPosts RepliedPost, Forum.ForumMembers RepliedMember, Forum.ForumMembers ReplyMember WHERE ReplyPost.PostId=RepliedPost.ReplyTo and RepliedPost.OwnerId=RepliedMember.MemberId and ReplyPost.OwnerId=ReplyMember.MemberId
- 在这个时候,可能在关系型模式里面随着关系的增多读取就会越困难,而在图数据模式中MATCH子句相对就容易很多。让我们看一下在图数据模式中一些有趣又有用的地方。
统计每篇帖子的回复数
SELECT distinct RepliedPost.PostID,RepliedPost.PostTitle, RepliedPost.PostBody, count(ReplyPost.PostID) over(partition by RepliedPost.PostID) as TotalReplies FROM dbo.ForumPosts ReplyPost, Reply_To, dbo.ForumPosts RepliedPost WHERE MATCH(ReplyPost-(Reply_To)->RepliedPost)
在这个语句中我们统计了每一篇回复的数量,但是仅仅在一个层面中,并不是在整个回复的树结构里面。
根贴(主贴)的列表
我们通过下面不使用MATCH的语句得到所有的根贴:
SELECT Post1.PostId,Post1.PostTitle FROM dbo.ForumPosts Post1 WHERE $node_id not in (select $from_id from dbo.Reply_To
MATCH语法只是允许我们关联三个或者更多的实体(比如两个节点和一个关系)。当我们只想关联其中两个的时候,只需要一个常规的连接或者子查询。如上面的语句一样。
在结果中添加‘Level’字段
添加一个‘Level’字段,显示树结构。在T-SQL中有一个简单的语法,叫做CTE实现递归。但是有一个问题,不能使用MATCH语法在一个派生表上,此时可以使用CTE。如果有必要,可以在CTE中使用MATCH,但是反之就不行了,有这样的限制。下面展示一下使用常规的关系仅仅使用CTE来迭代,代码如下:
with root as ( select $node_id as node_id,RootPosts.PostId, RootPosts.PostTitle, 1 as Level, 0 as ReplyTo from dbo.ForumPosts RootPosts where $node_id not in (select $from_id from dbo.reply_to) union all select $node_id,ReplyPost.PostId, ReplyPost.PostTitle, Level+1 as [Level], root.PostId as ReplyTo from dbo.ForumPosts ReplyPost, reply_to, root where ReplyPost.$node_id=reply_to.$from_id and root.node_id=reply_to.$to_id ) select PostId,PostTitle, Level, ReplyTo from root
检索一个帖子中的所有回复
使用CTE递归语法,我们可以用一种树结构检索一个帖子的所有回复。如果使用常规的语法不能在检索帖子1的时候检索贴子3,因为3是对2的回复,而2是对1的回复。使用CTE.当查询帖子1的所有回复时能检索贴子3。代码如下:
with root as ( select $node_id as node_id,RootPosts.PostId,RootPosts.PostTitle, 1 as Level, 0 as ReplyTo from dbo.ForumPosts RootPosts where PostId=1 union all select $node_id,ReplyPost.PostId, ReplyPost.PostTitle, Level+1 as [Level],root.PostId as ReplyTo from dbo.ForumPosts ReplyPost, reply_to, root where ReplyPost.$node_id=reply_to.$from_id and root.node_id=reply_to.$to_id ) select PostId,PostTitle, Level, ReplyTo from root
我们也可以反过来做,在树状结构中按顺序检索所有父贴。由于CTE不支持OUTER join,所以要在外部添加,代码如下:
with root as ( select LeafPost.$node_id as node_id,LeafPost.PostId, LeafPost.PostTitle from dbo.ForumPosts LeafPost where LeafPost.PostId=3 -- Single post union all select RepliedPost.$node_id as node_id,RepliedPost.PostId, RepliedPost.PostTitle from dbo.ForumPosts RepliedPost, Reply_to, root where root.node_id=Reply_to.$from_id and Reply_to.$to_id=RepliedPost.$node_id ) select root.PostId,root.PostTitle, RepliedPost.PostId ParentPostId from root left join reply_to on root.node_id=reply_to.$from_id left join dbo.ForumPosts RepliedPost on reply_to.$to_id=RepliedPost.$node_id
检索一个用户所有帖子
查询一个用所有的信息,与帖子不同,这不需要树,要简单不少:
-- Peter回复的所有帖子 SELECT distinct RepliedPost.PostID,RepliedPost.PostTitle, RepliedPost.PostBody FROM dbo.ForumPosts ReplyPost, Reply_To, dbo.ForumPosts RepliedPost, dbo.ForumMembers Members,Written_By WHERE MATCH(Members<-(Written_By)-ReplyPost-(Reply_To)->RepliedPost) and Members.MemberName='Peter' -- Peter发的所有帖子 SELECT ReplyPost.PostID,ReplyPost.PostTitle,ReplyPost.PostBody, RepliedPost.PostId ReplyTo FROM dbo.ForumPosts ReplyPost, Reply_To, dbo.ForumPosts RepliedPost, dbo.ForumMembers Members,Written_By WHERE MATCH(Members<-(Written_By)-ReplyPost-(Reply_To)->RepliedPost) and Members.MemberName='Peter'
或许你注意到上面两个查询的不同,就是在展示字段上是否使用DISTINCT。这个去重是因为Peter回复同一个帖子可以超过一次。
在模型中检索Likes(点赞)
这个查询是有意思的:‘Likes’边是成员和发帖表的关系。每一个关系都是唯一的,并不受其他关系影响。代码如下:
--点赞的帖子或者被别人点赞的帖子。 SELECT Post.PostID,Post.PostTitle,Member.MemberName FROM dbo.ForumPosts Post, Likes, dbo.ForumMembers Member WHERE MATCH(Member-(Likes)->Post) -- 点赞的人或者被人点赞 SELECT Member.MemberId,Member.MemberName LikeMember, LikedMember.MemberName LikedMemberName FROM dbo.ForumMembers Member, Likes, dbo.ForumMembers LikedMember WHERE MATCH(Member-(Likes)->LikedMember)
还可以很容易地聚合信息,以获得每个帖子或每个成员的总的Likes。
--每个帖子总的likes select Post.PostId,Post.PostTitle, count(*) totalLikes from dbo.ForumPosts Post,Likes, dbo.ForumMembers Members where Match(Members-(Likes)->Post) group by PostId,PostTitle --每个成员总的点赞数select LikedMembers.MemberId,LikedMembers.MemberName, count(*) totalLikes from dbo.ForumMembers Members,Likes, dbo.ForumMembers LikedMembers where Match(Members-(Likes)->LikedMembers) group by LikedMembers.MemberId, LikedMembers.MemberName
用户点赞并且回复帖子
我们也可以创建一些更有趣的查询,例如,查找这些点赞并回复的人,如下:
SELECT Member.MemberName,Member.Memberid,
LikedPost.PostId,LikedPost.PostTitle,
ReplyPost.PostTitle ReplyTitle
FROM dbo.ForumPosts LikedPost, Reply_To, dbo.ForumPosts ReplyPost,
Likes, dbo.ForumMembers Member, Written_By
WHERE MATCH(Member-(Likes)->LikedPost<-(Reply_To)-ReplyPost-(Written_By)->Member)
注意,对于‘Member’节点使用了两次在同一个MATCH表达式中。这形成了一种过滤:点赞并且有回复的成员,需要在‘LikedPost’和‘ReplyPost’中都有记录才可以。
那么在关系型模式中代码如下:
select Likes.MemberId,Members.MemberName from Forum.Likes Likes, Forum.ForumPosts Posts, Forum.ForumMembers Members where Likes.MemberId=Posts.OwnerId and Posts.ReplyTo=Likes.PostId and Members.MemberId=Likes.MemberId
看起来这种写法更难理解和读懂。
回帖给多个帖子的成员
SELECT Members.MemberId, Members.MemberName, Count(distinct RepliedPost.PostId) as Total FROM dbo.ForumPosts ReplyPost, Reply_To, dbo.ForumPosts RepliedPost, Written_By,dbo.ForumMembers Members WHERE MATCH(Members<-(Written_By)-ReplyPost-(Reply_To)->RepliedPost) GROUP BY MemberId, Members.MemberName Having Count(RepliedPost.PostId) >1
回帖个一个帖子多次的成员:
SELECT Members.MemberId, Members.MemberName, RepliedPost.PostId RepliedId,count(*) as TotalReplies FROM dbo.ForumPosts ReplyPost, Reply_To, dbo.ForumPosts RepliedPost, Written_By,dbo.ForumMembers Members WHERE MATCH(Members<-(Written_By)-ReplyPost-(Reply_To)->RepliedPost) GROUP BY MemberId,MemberName,RepliedPost.PostId Having count(*) >1
上述两种语句中唯一的不同就是展示结果的聚合。
总结
通过上述构建在图数据模式下的查询和关联,对比了常规语句以及在关系模式下的相同查询,不难发现无论是在易读性,逻辑理解上还是在性能上都有很大提高。当然这只是第一个版本,所以难免有很多问题, 下一篇我讲介绍这个版本存在的一部分问题。
发布者:全栈程序员-用户IM,转载请注明出处:https://javaforall.cn/119695.html原文链接:https://javaforall.cn
【正版授权,激活自己账号】: Jetbrains全家桶Ide使用,1年售后保障,每天仅需1毛
【官方授权 正版激活】: 官方授权 正版激活 支持Jetbrains家族下所有IDE 使用个人JB账号...