Why do we use SQL constraints? Which constraints we can use while creating a table in SQL?

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Ashutosh Kumar Verma · Answer

SQL Constraints ;
and  SQL Constraints is a set of rules that specifies for the data in database table. Constraints are specified when a table is created or after table is created with SQL ALTER command.
Syntax- 
and  and  CREATE TABLE table_name(
and  and  column1 datatype constraint,
and  and  column2 datatype constraint,
and and and and ----------------------------------------
and and and and );
Use of SQL Constraints:
and  and  SQL Constraints is used to provide a set of rules to columns in database tables. SQL Constraints can provide in table during creation of table or alter of tables.
SQL Constraints can either column level or table level. The column level constraints are apply on single column and table level column are applied on whole tables.and 
There are several types of SQL Constraints are used when a table is create, which are as follow-
1-and  NOT NULL Constraints-and and and and and This constraint is used when no column is to be kept empty. NOT NULL constraints prevent the value of a column from being null. When this constraint is used in a column, then null value cannot be provided to that column.
2-and and Primary key constraints-and and and and  These constraints uniquely identify each row (record) in the database table. A primary key can never contain a null value. It always contains a unique value.
3-and  Foreign key constraints-and and and and and Foreign key constraints are used to provide a link between the data in two tables. This constraint is used to enforce referential integrity.
4-and  and Check constraints- and and and and Check constraints are used to limit the value set in the column. It checks all the value of column based on condition.
5-and  and UNIQUE Constraints-and and and and and Unique constraints shows the uniqueness of the value of the column. Unique constraints ensure that the column or field contains only unique values, which means that all the values of the column will be different.
6-and  DEFAULT Constraints-and and and and and This constraint is used to provide default value to the column. When no value is provided to a column, then Default Constraints are used.
7-and  and INDEX-and and and and  Index is used to quickly retrieve or create data in any database table.

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Why do we use SQL constraints? Which constraints we can use while creating a table in SQL?

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