Is the primary key unique in mysql?

The primary key is unique. All database primary keys are unique. Conversely, if you want to set a certain field of a table as the primary key, the data in the field must be unique. This is a necessary condition. The primary key value can uniquely identify each row in the table, through which the entity integrity of the table can be enforced; the role of the primary key is to determine the uniqueness of the data, mainly used for foreign key associations with other tables, and for the identification of this record Modification and deletion.

The operating environment of this tutorial: windows7 system, mysql8 version, Dell G3 computer.

All database primary keys are unique. On the other hand, if you want to set a certain field of a table as the primary key, the data in the field must be unique. This is a necessary condition.

Primary key in mysql

The full name of the primary key (PRIMARY KEY) is "primary key constraint", which is the most frequently used constraint in MySQL. Under normal circumstances, in order to facilitate the DBMS to find records in the table faster, a primary key will be set in the table.

Primary keys are divided into single-field primary keys and multi-field joint primary keys. This section will explain the creation, modification and deletion of these two primary key constraints respectively.

The following points should be noted when using primary keys:

• Each table can only define one primary key.

• The primary key value must uniquely identify each row in the table and cannot be NULL, that is, there cannot be two rows of data with the same primary key value in the table. This is the principle of uniqueness.

• A field name can only appear once in the joint primary key field table.

• The joint primary key cannot contain unnecessary redundant fields. When a field in the joint primary key is deleted, if the primary key composed of the remaining fields still satisfies the uniqueness principle, then the joint primary key is incorrect. This is the principle of minimization.

Add primary key constraints when modifying the table

Primary key constraints can not only be created while creating the table, but also Added when modifying the table. However, it should be noted that null values ​​are not allowed in fields set as primary key constraints.

The syntax format for adding primary key constraints when modifying the data table is as follows:

ALTER TABLE <数据表名> ADD PRIMARY KEY(<字段名>);

View the table structure of the tb_emp2 data table. The SQL statement and running results are as follows.

DESC tb_emp2;

Example:

Modify the data table tb_emp2, set the field id as the primary key, SQL statement and running result as follows.

ALTER TABLE tb_emp2 ADD PRIMARY KEY(id);

Normally, when you want to set the primary key constraint of a field in the table when modifying the table, you must ensure that the value in the field set as the primary key constraint cannot have Repeated and guaranteed to be non-empty. Otherwise, the primary key constraint cannot be set.

Delete primary key constraints

When a primary key constraint is not needed in a table, it needs to be deleted from the table. It is much easier to delete a primary key constraint than to create one.

The syntax format for deleting primary key constraints is as follows:

ALTER TABLE <数据表名> DROP PRIMARY KEY;

Example:

Delete primary key constraints, SQL statements and running results in the tb_emp2 table as follows.

ALTER TABLE tb_emp2 DROP PRIMARY KEY;

Since there can only be one primary key constraint in a table, you can delete the primary key constraint in a table without specifying the primary key name.

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