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SQL Keys

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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

Customer_id

Customer_name 

Email

1

Nitu

deynitu1998@gmail.com

2

Sumit

10sumitsarkar@gmail.com

3

Hariom

hariomprajapati9097@gmail.com

 


Table 2

ORDER_ID

ORDER_DATE

AMOUNT

CUSTOMER_ID

1

'2020/5/6'

55

1

2

'2020/06/06'

100

1

3

'2020/03/12'

203

2

4

'2019/05/13'

76

3



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

MySql