SQL Keys

Primary keys

The primary key constraint uniquely identifies each record in a table.

The primary key must contain a unique value, and cannot contain a null value.

Syntax -

create table table_name (
column1_name data_type (range) not null,
column2_name data_type (range) not null,
column3_name data_type (range) not null,
  ………..,
primary key(column name)
);

Foreign key

A foreign key is a key used to link two table together.

A foreign key is a field ( or collection of field ) in one table that refer to the  primary key in another table.

Example

Table 1

Table 2

Syntax for table 1 -

create table table_name (
column1_name not null auto increment primary key,
……………,
);

Syntax for table 2 -

create table  table_name (
column1_name  data_type  not null auto increment primary key,
column2_name  datatype (range),
…………………,
foreign key ( column_name // column name must same as table 1 primary key)
references table1_name (column name // table 1 column name which is primary key)
);

Note:- Create a column with the same name as table1 primary key column name in table 2  and then put it in foreign key(column name).                            

Table 1

Table 2