MySQL constraint_ Unique constraint

-- Unique constraint ：unique( Restricted fields can only be unique , Can't repeat )
-- Usually in non business fields （id only ）
CREATE TABLE stu(
   id INT,
   NAME VARCHAR(20),
   phone_number VARCHAR(11) UNIQUE -- Unique constraint
);
-- Insert
INSERT INTO stu(id,NAME,phone_number)VALUES(1,' Zhang San ',133);
INSERT INTO stu(id,NAME,phone_number)VALUES(2,' Li Si ',133); -- Duplicate entry '133' for key 'phone_number'
DELETE FROM stu WHERE id=2;
DELETE FROM stu WHERE id=1;
--   Delete unique constraint
-- alter table Table name drop index The field name of the unique constraint
-- alter table stu modify phone_number varchar(11);// It is equivalent to changing only one field

ALTER TABLE stu  DROP INDEX phone_number;
-- Add unique constraints ( Ensure that there is no duplicate data before adding a unique constraint )
ALTER TABLE stu MODIFY phone_number VARCHAR(11) UNIQUE;

SELECT*FROM stu