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Constraints in SQL


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SQL Constraints

In this blog, I’m explaining about constraints in SQL.

Description:

 SQL constraints are used to restrict the type of data that can insert into a database table. Constraints can be defined in two ways.

 

Column Level    : limits on column data

Table Level        : Limits on table data

 

Types of SQL constraints

 

·         Not null

·         Unique key

·         Primary key

·         Foreign key

·         Check

·         Default

 

Not Null Constraints

This  constraint ensures that  all rows  in the database table must contain value for the  column which specified as not null  means  a null value is not allowed in the column.

Syntax:

Create table table_name

( col1 datatype[CONSTRAINT cons_name] not null,

Col2 datatype,

)

 

Unique key Constraints

Unique key  is a set  of one or more columns of a table that uniquely identify each record in database table. It is like a Primary key but it can accept only one null value and it can not have duplicate values.

Syntax:

Create table table_name

( col1 datatype[CONSTRAINT cons_name] unique,

Col2 datatype,

)

 

Primary key Constraints

Primary key is a set  of  one  or more  columns of a table that  uniquely identify each in database table. It can not  accept null, duplicate  values.

Syntax:

Create table table_name

( col1 datatype[CONSTRAINT cons_name] PRIMARY KEY,

Col2 datatype,

)

 

 

 

Foreign Key Constraints

Foreign key is a field in database table that is Primary key in another table. It  can accept multiple null ,duplicate values.

Syntax:

Create table table_name

( col1 datatype[CONSTRAINT cons_name] references

 reference_table_name (reference_table_column_name),

col2 datatype

)

 

Check Constraints

This constraint defines a business rules on a column in the  database table that each row of the table must  follow this rule.

Syntax:

Create table table_name

(

 Col1 datatype[CONSTRAINT cons_name] check(condition)

Col2 datatype

)

 

Default constraints

The Default constraint is used to insert a default value into a column.

The default value will be added  to all new records, if  no  other value is specified.

 

Syntax:

Create table table_name

( Col1 datatype,

Col2  datatype Default Getdate()

)


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