11 examples of primary key
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Example Of Primary Key Computer Science Essay
2290 words (9 pages) Essay in Computer Science
5/12/16 Computer Science Reference this
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Database is a structure that is generally divided in two respects, namely a flat database and a relational database. Relational database easier to understand than the flat database. Because relational database has a simple design and it is easy to do data operations. Database also has a relational structure found in tables to store data. On each table is it consists of columns and rows and a column to define the type of information that should be saved. Basically the database is very useful when the time to retain the information that will be categorized logically. For example, if you want to save the information on the data, the database enterprise or school, it can be in a group with matters in business become more tables. In the database there is some relationship, the relationship is as one to one, many to many, one to many,
Primary Key
The key is a set of columns that can be used to identify or access a particular row or rows. Identified key explanatory tables, indexes, constraints or reference. The same column can be part of more than one key.
A unique key is a key that is constrained so that no two values ​​are the same. Unique key columns cannot contain NULL values​​. For example, the column number of employees can be defined as a unique key, because each value in column identifies only one employee. No two employees can have the same number of workers.
The mechanism used to enforce uniqueness of the so-called index unique key. Unique index table is a column, or a collection of ordered columns, each of which identifies value (specify function) unique rows. A unique index can contain NULL values​​.
The key is one unique key defined on the table, but was selected as the first important key. There can be only one primary key on the table.
A primary index is automatically created for the primary key. The main index used by the database manager for efficient access to table rows, and allows the database manager to enforce the uniqueness of the primary key. (You can also specify non-primary index key columns efficiently access the data when processing the query.)
If the table does not have a “natural” unique key, or if the order of arrival is a method used to distinguish unique rows, use a time stamp as part of the key can help.
Example of Primary Key
PROJNO (Primary Key)
PROJNAME
DEPTNO
MA2100
Weld Line Automation
D01
MA2110
Weld Line Programming
D11
Example 1
If each column in the table contains duplicate values​​, it cannot determine the key with just one column. A key with more than one column is a composite key. The combination of column values ​​should define a unique entity. If a composite key cannot be easily defined, it can also be considered to create a new column with unique values​​.
EMPNO
(Primary Key)
PROJNO
(Primary Key)
ACTNO
(Primary Key)
EMPTIME
EMSTDATE
(Primary Key)
000250
AD3112
60
1.0
1982-01-01
000250
AD3112
60
5
1982-02-01
000250
Answer:
STUDENT {SID, FNAME, LNAME, COURSEID} Candidate Key. Candidate keys are those keys which is candidate for primary key of a table. Primary Key. Such type of candidate key which is chosen as a primary key for table is known as primary key. Foreign Key. Alternate Key. Composite Key.