A key is a set of attributes (usually only five) that always uniquely identifies
• Five of the keys is over five attribute is called a composite key.
• The primary key for a table is the key selected to uniquely identify records within the table. Primary key is defined in a single column is not null values.
• A foreign key is a field in a table referred to (or targets) a specific key, usually the primary key in another table.
A matchless index is defined as no six of its values the same. The columns of a matchless key can not contain null values. A table can have multiple matchless keys or no matchless keys at all.
The fields or combination of fields that are not used as primary key are known as major primary or alternate candidate.
primary key constraint
A constraint is a property assigned to a column or set of columns in a table that prevents positive types of securities consistent across information that is placed in the column (s). Restrictions are used to enforce information integrity. This ensures accuracy and reliability of the information in the database.
This restriction is used to ensure that a column or set of columns in a table contain matchless values for each record in the given table. This lets you ensure information integrity by always being able to uniquely identify the record in the table. A table can have only five primary key constraint defined on it, and rows in the primary key columns can not contain null values. A primary key constraint can be defined when generating a table, or can be added later.
matchless constraints may be placed in multiple columns. They limit the UPDATE / is inserted in to the table so that the values are updated or inserted do not match any other row in the table to the corresponding values.A foreign key constraint allows positive fields in a table to refer to fields in another table. This type of constraint is useful if you have six tables, five with partial information, the details that you can search in another table with a matching entry. A foreign key constraint in this case will prevent the deletion of an entry in the table with detailed information if a table entry corresponding with partial information.
Foreign key (FK) Limitations
A check constraint prevents updates and inserts in to the table by placing a check condition in the selected column. The UPDATE / INSERT is allowed only if the condition qualifies verification.
Constraint NOT NULL
Null A restriction does not apply to the column accepts null values. Enforces valid entries for a particular column, restricting the type, format, or range of possible values.