php - What is a Parent table and a Child table in Database? - Stack Overflow
A relational table is a table of columns or fields that describe a listing (or rows) of In relational databases, a one-to-many relationship occurs when a parent record in one table can potentially reference several child records in another table. Does terdata store parent child relationship at dictonary tables? if yes view, but returns the internal IDs of databases, tables, and columns. If normalized tables are a relational database's foundation, then relationships are You have only one mother, but she may have several children. When you move a field to a new table, you'll probably add that new table's.
A table is made up of rows and columns.
- A Quick-Start Tutorial on Relational Database Design
- Parent-Child Dimension
- Relational databases: Defining relationships between database tables
A row is also called a record or tuple. A column is also called a field or attribute. A database table is similar to a spreadsheet.
Foreign key - Wikipedia
However, the relationships that can be created among the tables enable a relational database to efficiently store huge amount of data, and effectively retrieve selected data. Database Design Objective A well-designed database shall: This is because duplicate data not only waste storage spaces but also easily lead to inconsistencies. Ensure Data Integrity and Accuracy: Databases are usually customized to suit a particular application.
No two customized applications are alike, and hence, no two database are alike. Guidelines usually in terms of what not to do instead of what to do are provided in making these design decision, but the choices ultimately rest on the you - the designer.
Define the Purpose of the Database Requirement Analysis Gather the requirements and define the objective of your database, e. Drafting out the sample input forms, queries and reports, often helps.
What is a relational table?
Gather Data, Organize in tables and Specify the Primary Keys Once you have decided on the purpose of the database, gather the data that are needed to be stored in the database. Divide the data into subject-based tables. Choose one column or a few columns as the so-called primary key, which uniquely identify the each of the rows.
Primary Key In the relational model, a table cannot contain duplicate rows, because that would create ambiguities in retrieval. To ensure uniqueness, each table should have a column or a set of columnscalled primary key, that uniquely identifies every records of the table.
For example, an unique number customerID can be used as the primary key for the Customers table; productCode for Products table; isbn for Books table. A primary key is called a simple key if it is a single column; it is called a composite key if it is made up of several columns.
The primary key is also used to reference other tables to be elaborated later. You have to decide which column s is to be used for primary key. The decision may not be straight forward but the primary key shall have these properties: The values of primary key shall be unique i. For example, customerName may not be appropriate to be used as the primary key for the Customers table, as there could be two customers with the same name.
The primary key shall always have a value. In other words, it shall not contain NULL. Consider the followings in choose the primary key: The primary key shall be simple and familiar, e. The value of the primary key should not change.
Primary key is used to reference other tables. If you change its value, you have to change all its references; otherwise, the references will be lost. For example, phoneNumber may not be appropriate to be used as primary key for table Customers, because it might change.
Primary key often uses integer or number type. But it could also be other types, such as texts. However, it is best to use numeric column as primary key for efficiency.
Primary key could take an arbitrary number. This arbitrary number is fact-less, as it contains no factual information.
Unlike factual information such as phone number, fact-less number is ideal for primary key, as it does not change. Primary key is usually a single column e. But it could also make up of several columns. You should use as few columns as possible. Let's illustrate with an example: Name may not be unique. Phone number and address may change.
News, Tips, and Advice for Technology Professionals - TechRepublic
Hence, it is better to create a fact-less auto-increment number, say customerID, as the primary key. Create Relationships among Tables A database consisting of independent and unrelated tables serves little purpose you may consider to use a spreadsheet instead. The power of relational database lies in the relationship that can be defined between tables.
The most crucial aspect in designing a relational database is to identify the relationships among tables. The types of relationship include: In a "company" database, a manager manages zero or more employees, while an employee is managed by one and only one manager. In a "product sales" database, a customer may place many orders; while an order is placed by one particular customer.
This kind of relationship is known as one-to-many. One-to-many relationship cannot be represented in a single table. For example, in a "class roster" database, we may begin with a table called Teachers, which stores information about teachers such as name, office, phone and email. To store the classes taught by each teacher, we could create columns class1, class2, class3, but faces a problem immediately on how many columns to create.
On the other hand, if we begin with a table called Classes, which stores information about a class courseCode, dayOfWeek, timeStart and timeEnd ; we could create additional columns to store information about the one teacher such as name, office, phone and email. However, since a teacher may teach many classes, its data would be duplicated in many rows in table Classes. To support a one-to-many relationship, we need to design two tables: We can then create the one-to-many relationship by storing the primary key of the table Teacher i.
The column teacherID in the child table Classes is known as the foreign key. A foreign key of a child table is a primary key of a parent table, used to reference the parent table. Take note that for every value in the parent table, there could be zero, one, or more rows in the child table. For every value in the child table, there is one and only one row in the parent table. Dimension Schema The dimension schema of a parent-child hierarchy depends on a self-referencing relationship present on the dimension main table.
For example, the following diagram illustrates the DimOrganization dimension main table in the AdventureWorksDW sample database. In this dimension table, the ParentOrganizationKey column has a foreign key relationship with the OrganizationKey primary key column.
In other words, each record in this table can be related through a parent-child relationship with another record in the table. This kind of self-join is generally used to represent organization entity data, such as the management structure of employees in a department. Hierarchies and Levels Dimensions that do not have a parent-child relationship construct hierarchies by grouping and ordering attributes.
These dimensions derive the level names for their hierarchies from the attribute names. However, parent-child dimensions construct parent-child hierarchies by examining the data that the dimension main table contains, and then evaluating the parent-child relationships between the records in the table.
For more information about parent-child hierarchies, see User Hierarchies. Parent-child hierarchies do not derive the names for the levels in a parent-child hierarchy from the attributes that are used to create the hierarchy.
Instead, these dimensions create level names automatically by using a naming template-a string expression you can specify at the level of the parent attribute that controls how the attribute generates the attribute hierarchy.