MySQL Constraints - SQL Introduction - Phptpoint.com

MySQL Constraints

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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 and unique both prevent the duplicate entry .

Primary : it can never be null. only one field may be primary. it is row based index

Unique: multiple fields may be unique, may be null. it is column based

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.