A primary key is a column which uniquely identifies the records in a table. In a broad sense, a primary key is the mixture of a unique key and an index: A collumn with a primary key is indexed to deliver a faster query, and doesn't allow duplicate values to ensure specific data.
Most programmers recommend all tables having a primary key (and only one) to enhance the speed of queries and overall database performance.
An example of a primary key may be found in a table named "departments," which might have a collumn named "department_number" that uniquely identifies each department in the table with a number.
A foreign key is a column (the child collumn) in a table which has a corresponding relationship and a dependency on another collumn (the parent collumn) that is usually in a different table. Parent collumns can have multiple child collumns, but a child collumn can only have one parent collumn.
The child collumn is the collumn with the foreign key; the parent collumn does not have the foreign key "set" on it, but most databases require the parent collumn to be indexed.
Foreign keys are made to link data across multiple tables. A child collumn cannot have a record that its parent collumn does not have.
Say a table named "employees" has 20 employees (rows) in it. There are 4 departments in the "departments" table. All 20 employees must belong to a department, so a collumn in the "employees" table named "department" would point to the primary key in the "departments" table using a foreign key. Now all employees must belong to a department as specified by the "departments" table. If a department isn't specified in the "departments" table, the employee cannot be assigned to it.
A candidate key would be any key which could be used as the primary key, which means that the combination of the columns, or just the single column would create a unique key. You would then need to determine which of these candidate keys would work best as your primary key.
Most programmers recommend all tables having a primary key (and only one) to enhance the speed of queries and overall database performance.
An example of a primary key may be found in a table named "departments," which might have a collumn named "department_number" that uniquely identifies each department in the table with a number.
A foreign key is a column (the child collumn) in a table which has a corresponding relationship and a dependency on another collumn (the parent collumn) that is usually in a different table. Parent collumns can have multiple child collumns, but a child collumn can only have one parent collumn.
The child collumn is the collumn with the foreign key; the parent collumn does not have the foreign key "set" on it, but most databases require the parent collumn to be indexed.
Foreign keys are made to link data across multiple tables. A child collumn cannot have a record that its parent collumn does not have.
Say a table named "employees" has 20 employees (rows) in it. There are 4 departments in the "departments" table. All 20 employees must belong to a department, so a collumn in the "employees" table named "department" would point to the primary key in the "departments" table using a foreign key. Now all employees must belong to a department as specified by the "departments" table. If a department isn't specified in the "departments" table, the employee cannot be assigned to it.
A candidate key would be any key which could be used as the primary key, which means that the combination of the columns, or just the single column would create a unique key. You would then need to determine which of these candidate keys would work best as your primary key.
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