The hierarchical model is characterized by using a "directed tree" data structure to represent various entities and their relationships. Each node in the tree represents a record type, and the tree structure indicates the connections between entity types.
The network model, on the other hand, represents entity types and their relationships through a network structure.
Advantages of the hierarchical model include a clear structure with simple node connections. Once you know the parent node of a particular node, you can understand the entire model structure. However, querying child nodes must go through the parent node, as the hierarchical model imposes an order on the children of each node, which implicitly controls the database access path.
Advantages of the network model include its ability to directly represent complex relationships in the real world and the capacity to show various complex connections between entities. However, the structure is more complex, and the Data Definition Language (DDL) and Data Manipulation Language (DML) are intricate, making it difficult for users to work with. Moreover, the larger the application environment, the more complex the database structure becomes, which is not conducive to users understanding it.
In the network model, the limitations of the hierarchical model regarding the inability to represent non-hierarchical structures are removed. Nodes can have multiple parents, transforming the directed tree into a directed graph that describes the network model.
Data manipulation in the hierarchical model mainly involves queries, insertions, deletions, and updates. When performing insertions, deletions, and updates, it is necessary to satisfy the integrity constraints of the hierarchical model. When inserting data, if there is no corresponding parent node value, the child node value cannot be inserted. When deleting data, if the parent node value is deleted, the corresponding child node values are also deleted simultaneously.详情
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嗨，我叫魏嘉哲，我喜欢吃米饭但是我不喜欢吃西红柿。我的朋友喜欢吃Hi, my name is Wei Jiazhe I like to eat rice, but I do not like to eat tomatoes. My friends like to eat chicken but he does not like to eat carrots.鸡肉但是他不喜欢吃胡萝卜。
写一篇英语作文(我喜欢吃什么,我不喜欢吃什么。)
Hello, my name is Dale. I enjoy eating hamburgers and apples for breakfast. However, I am not a fan of bread and milk. For lunch, I prefer rice, fruit salad, and chicken. Noodles are not something I enjoy. As for dinner, I like to have porridge and vegetables, but I d...
用英语写一篇小短文介绍自己喜欢吃什么
用英语写一篇小短文 介绍一下自己喜欢吃什么喝什么 I am a healthy boy.l like eating a lot of healthy food.For breakfast,I like eggs,bread and milk.For lunch,I like fish,meat,vegetables and rice.And for dinner,I have chicken,tomatoes and noodles.I like French fries very much,...
我喜欢的或者不喜欢的食物的英语作文.150词左右
My favorite food is breakfast.I love eating soft scramble puffy eggs with bacon,sausage and ham followed by a tall glass of orange juice.Normally I cook my,own breakfast but when I feel lazy,I don’t think about cooking.All I think about is,a place where I can get a delic...
初一英语作文写出一日三餐你和同伴喜欢吃什么不喜欢吃什么.50词左右
午餐时间，我吃得很简单。一个有蔬菜和肉的午餐盒就行了。我不吃。也吃吧很多。或者到下午我可能会睡着。餐车是我最好的能量来源。我喜欢海鲜很多虾龙虾，各种贝类美味。酒或者酒，其中之一，我更喜欢米饭而不是面包晚餐。西红柿用炒鸡蛋做调料，加些糖，酸甜的味道，总是给我最好的胃口！
写出我和我的家人喜欢和不喜欢的食品英语作文
He doesn't like ice cream.My mother is an English teacher.She likes music and going shopping.She likes having bread ,eggs and milk.She doesn't like potatoes.And I like playing basketball.My favorite friut are bananas and peaches.I like eating hamburgers.I dislike vegetables.
写一篇英语作文。假如你是Dale介绍一日三餐喜欢吃什么和不喜欢吃什么...
Hello,I am Dale.I like hamburgers and apples for breakfast.But I don't like bread and milk.For lunch,I like rice、fruit salad and chicken.I don't like noodlles.For dinner,I like porridge and vegetable.And I don't like eggs.
英语作文我喜欢打篮球吃水果沙拉,我不喜欢吃汉堡
I was in sixth grade this year, my mother said I changed. I used naughty, do not love learning. Now, I'm civilized and sensible, hard to learn. My body also grow taller and stronger than before. Character, cheerful, and pay a lot of friends.I like to play basketball ...
我喜欢吃香蕉汉堡,不喜欢吃西红柿,爸爸妈妈喜欢吃英语作文
Bananas are the fruit of my favorite.Banana skin yellow, but also some blue, blue is not yet ripe bananas of. Banana crescent like heaven, like the lake boats and sickle. On some banana peels have Many black specks, you must not think that it has gone bad, listening to the...
写一篇关于家庭喜欢吃什么和不喜欢的食物的英语作文(
it is very delicious.Although cake is very sweet,it looks very beautiful.On my birthday,my mother bought a cake for me.L'm very happy.There are some things on the cake.For example ,some fruit and some chocolate .l also like chocolate very much .Cake is my favourite .
