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Standardization makes your data model flexible and that makes working with your data much easier.
A good database design starts with a list of the data that you want to include in your database and what you want to be able to do with the database later on.
The next step is to determine the relationships between the entities and to determine the cardinality of each relationship. The cardinality shows how much of one side of the relationship belongs to how much of the other side of the relationship.
The true cardinality can be calculated through assigning the biggest values for left and right, for which 'N' or 'M' are greater than '1'. The relationships Sales --> Customers, and Sales --> Products are mandatory, but the other way around this is not the case.
In an ERD (see next chapter) this type of relationship is a line that goes out of the entity and returns with a nice loop to the same entity. In the example there are two many-to-many relationships that need to be solved: 'Products Sales', and 'Products Shops'.
The Products Shops relationship shows which products are available in which the shops, also known as 'stock'.
About the products that you sell, you want to know, for example, what the price is, what the name of the manufacturer is, and what the type number is. In the ERD the primary key attributes are indicated by the text 'PK' behind the name of the attribute.
The standard data types that every database knows, and are most-used, are: CHAR, VARCHAR, TEXT, FLOAT, DOUBLE, and INT.
The first form of normalization states that there may be no repeating groups of columns in an entity.
The second form of normalization states that all attributes of an entity should be fully dependent on the whole primary key.
This entity is not according the second normalization form, because in order to be able to look up the date of a sale, I do not have to know what is sold (productnr), the only thing I need to know is the sales number. The third form of normalization states that all attributes need to be directly dependent on the primary key, and not on other attributes. In this case the price of a loose product is dependent on the ordering number, and the ordering number is dependent on the product number and the sales number. There are more normalization forms than the three forms mentioned above, but those are not of great interest for the average user. Primary key - one or more columns within a table that together form a unique combination of values by which each record can be pointed out separately. Build your next data model with DeZign for Databases trial software, available for download directly from Datanamic's download section.
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The relationship is the connection between the entities, just like in the real world: what does one entity do with the other, how do they relate to each other? First, you need to state for each relationship, how much of one side belongs to exactly 1 of the other side. There are four entities and each entity has a relationship with every other entity, so each entity must have three relationships, and also appear on the left end of the relationship three times. These are relationships that are already indicated by other relationships, although not directly. But there are also relationships from customers to sales and from sales to products, so indirectly there already is a relationship between customers and products through sales.
What a M:N relationship says is that a number of records from one table belongs to a number of records from another table.
Customers all have a customer number, products all have a unique product number and the sales have a sales number. In the example only the entity 'shop' does not have an obvious candidate for the PK, so we will introduce a new attribute for that entity: shopnr. Please note that the attribute 'products' is no longer necessary in 'Sales', because 'sold products' is now included in the link-table. VARCHAR is the same as CHAR, the difference is that VARCHAR only takes as much space as necessary. A lot of databases have variations of the INT, such as TINYINT, SMALLINT, MEDIUMINT, BIGINT, INT2, INT4, INT8. It does generate some overhead because you usually get more tables, but it enables you to do many things with your data model without having to adjust it. We could have created an entity 'sales' with attributes for each of the products that were bought. If you would have to sell 4 products, than you would have to start a second sale or adjust your data model by adding 'product4' attributes. This means that each attribute of an entity can only be identified through the whole primary key. The date is dependent on the sales number, and the quantity is dependent on the sales number and the sold product.
This seems to be what the second form of normalization states, but in the second form is actually stated the opposite. Foreign Key-columns can only contain values that exist in the Primary Key column that they refer to.
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A much-used notation is the 'crowfeet' notation, where entities are represented as rectangles and the relationships between the entities are represented as lines between the entities. All attributes part of a primary key must have a value in every record (which cannot be left empty) and the combination of the values within these attributes must be unique in the table. Each of these data is unique and each record will contain a value, so these attributes can be a primary key.
For example in the Sales_details entity we could use the combination of the PK's of the sales and products entities as the PK of Sales_details. The foreign key of an entity can also be part of the primary key, in that case the attribute will be indicated with 'PF' behind its name.
In the link-table another field was added, 'quantity', that indicates how many products were sold. A few are standardized, but many databases have their own data types that all have their own advantages.
If you define a CHAR(10) you can save up to ten positions maximum, but if you only use two positions the database will still save 10 positions. In the second form of normalization you point out attributes through the PK, in the third form of normalization every attribute needs to be dependent on the PK, and nothing else.
If you stick to the design rules and the normalization mentioned in this article, you will create a design that works great for most applications.
Shirt comes in white with artistic "painted" heart in stars & stripes Our NB shirt is 100% polyester birdseye knit flatback Pique mesh fabric. It is important to know what these rules are, but more importantly is to know why these rules exist, otherwise you will tend to make mistakes! If the information you want to include doesn't fit into these categories, than it is probably not an entity but a property of an entity, an attribute. If there is a 'many' on the left side, this will be indicated with 'M', if there is a 'many' on the right side it is indicated with 'N'.
In this case, products are only purchased through a sale, so the relationships 'Customers Products' can be deleted. This entity has a many-to-one relationship with Sales, and a many-to-one relationship with Products. Of the sales you know when they happened, in which shop, what products were sold, and the sum total of the sale.
Often an integer column is used for the primary key so a record can be easily found through its number. In this way we enforce that the same product (type) can only be used once in the same sale. This is usually the case with the link-entities, because you usually link two instances only once together (with 1 sale only 1 product type is sold 1 time).
The quantity field was also added in the stock-table, to indicate how many products are still in store. Some databases offerthe possibility to define your own data types, in case the standard types cannot do the things you need. A regular INT is 4 bytes (INT4) and fits figures from -2147483647 to +2147483646, or if you define it as UNSIGNED from 0 to 4294967296. In these cases you should always create a new entity that you link to the old one via a one-to-many relationship. In logical models this is called an associative entity and in physical database terms this is called a link table or junction table.
The side of the relationship that is mandatory for the other to exist will be indicated through a dash on the line. The INT8, or BIGINT, can get even bigger in size, from 0 to 18446744073709551616, but takes up to 8 bytes of diskspace, even if there is just a small number in it. What will be included precisely is not of importance yet; it is still only about what you want to save.



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