How to delete unique constraints in mysql

The "ALTER TABLE" statement and the "DROP" keyword can be used to uniquely constrain in mysql. The syntax is "ALTER TABLE table name DROP INDEX constraint name;"; "ALTER TABLE" can be added Reduce constraints, when used together with "DROP", can delete constraints.

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

In mysql, you can use the "ALTER TABLE" statement and the "DROP" keyword to unique (unique) constraints.

The "ALTER TABLE" statement changes the structure of the original table, such as adding or deleting columns (constraints), changing the original column type, renaming columns or tables, etc.

When the "ALTER TABLE" statement is used together with the "DROP" keyword, deletion of constraints can be achieved.

unique (unique) constraint means that the value of the field cannot appear repeatedly in all records. For example, after adding a unique constraint to the id field, the id value of each record is unique and cannot be repeated. If the id value of one of the records is '0001', then there cannot be another record with the id value of '0001' in the table.

Unique constraints are similar to primary key constraints in that they can ensure the uniqueness of columns. The difference is that there can be multiple unique constraints in a table, and the column where the unique constraint is set is allowed to have null values, but there can only be one null value. There can only be one primary key constraint in a table, and no null values ​​are allowed. For example, in the user information table, in order to avoid duplicate user names in the table, the user name can be set as a unique constraint.

Method to delete unique (unique) constraints

The syntax format for deleting unique constraints in MySQL is as follows:

ALTER TABLE <表名> DROP INDEX <唯一约束名>;

Example:

View the constraints in the data table tb_dept1

DESC tb_dept1;

You can see that in the data table tb_dept1, the name of the specified department is unique; now delete the unique name in the data table tb_dept1 Constraint unique_name

ALTER TABLE tb_dept1 DROP INDEX unique_name;

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