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MySQL Constraints

Constraints are the utilities that restrict entry of data into the table if the specified condition is not fulfilled. They are primarily of the following type -- Primary key, foreign key, check, unique, not null

Primary Key : Primary key prevents duplicate entry. It is called row based constraints(it means inside a table there is only one field can be declared primary ) No null Value can be entered .

Unique Key : It will not let enter a duplicate value in the field. But a Null Value can be entered. It is called column based constraint.

Foreign Key : A column is called foreign key when it refers to the primary key column of another table.

Not null : It will not allow entering null Value in the field. But duplicate value can be entered.

MySQL Index

indexing is basically of four types : => Primary => Unique => Index => full-text

Primary Key

Primary Key prevent the duplicate entry.
Only one field can be defined Primary
it is row based Constraint

Unique Key

Unique Key also prevent the duplicate entry.
More than one field can be defined unique
it is column based Constraint

Index and full-texx

Index and full-text both are used for filtration. when you need to search , if you have to write the full name of field i.e. indexing. on the other hand if the searching is based on the basis of characters it is full-text.

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