MySQL-----Unique Index

Unique index:

Single-column unique index and federated unique index

The index is for faster lookups.

A unique index is a constraint condition. For example, the primary foreign key.

Constraints for Unique indexes:

Constraint cannot be duplicated (can be empty)

Primary key cannot be duplicated (cannot be empty)

Speed up Search

CREATE TABLE T1 (

ID int .....,

Num INT,

Unique uq1 (num)

Limit num to be unique. is a unique index that adds a multivalued value in (), which can be a federated unique index, but the federated unique data cannot be exactly the same. For example, id num XX

1 1 1

2 2 1

Num and XX Groups of data cannot appear exactly the same.

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MySQL-----Unique Index