MySQL key PRI UNI MUL

Source: Internet
Author: User
Tags mul

1. If key is empty, then the column value can be duplicated, indicating that the column has no index, or is a non-leading column of a non-unique composite index
2. If key is a PRI, then the column is part of the primary key
3. If key is uni, then the column is the first column (the leading column) of a unique value index and must not contain a null value (NULL)
4. If key is Mul, the value of the column can be repeated, which is a leading column of a non-unique index (the first column) or a component of a unique index but can contain null values

If the definition of a column satisfies many of the 4 cases mentioned above, for example, a column is both a PRI and uni
Then "desc table name", the displayed key value is displayed according to the priority level Pri->uni->mul
So at this point, the PRI is displayed

A unique index column can be displayed as a PRI, and the column cannot contain null values, and the table does not have a primary key

A unique index column can be displayed as Mul if multiple columns make up a unique composite index
Because although the multiple-column combination of indexes is unique, such as Id+name is unique, no single column can have duplicate values
As long as Id+name is the only one.

MySQL key PRI UNI MUL

Related Article

Contact Us

The content source of this page is from Internet, which doesn't represent Alibaba Cloud's opinion; products and services mentioned on that page don't have any relationship with Alibaba Cloud. If the content of the page makes you feel confusing, please write us an email, we will handle the problem within 5 days after receiving your email.

If you find any instances of plagiarism from the community, please send an email to: info-contact@alibabacloud.com and provide relevant evidence. A staff member will contact you within 5 working days.

A Free Trial That Lets You Build Big!

Start building with 50+ products and up to 12 months usage for Elastic Compute Service

  • Sales Support

    1 on 1 presale consultation

  • After-Sales Support

    24/7 Technical Support 6 Free Tickets per Quarter Faster Response

  • Alibaba Cloud offers highly flexible support services tailored to meet your exact needs.