Mysql primary KEY,

What is a composite primary key for a data table
The so-called composite primary key means that the primary key of your table contains more than one field composition
Like what
CREATE TABLE Test
(
Name varchar (19),
ID number,
Value varchar (10),
Primary KEY (Name,id)
)

The combination of the name and ID fields above is the composite primary key of your test table.
It appears because your Name field may have duplicate names, so add the ID field so you can keep your records unique.
In general, the less field length and number of fields the primary key is, the better

KEY

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 primary KEY,