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MySQL Constraints


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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