If we have to alter or modify the primary key of a table in an existing mysql table, the following steps will help you.

For a table without primary key

For single primary key

ALTER TABLE table_name ADD PRIMARY KEY(your_primary_key);

For composite primary key

ALTER TABLE table_name ADD PRIMARY KEY(key1, key2, key3);

 

For a table with already existing primary key

For single primary key

ALTER TABLE table_name DROP PRIMARY KEY, ADD PRIMARY KEY(primary_key);

For composite primary key

ALTER TABLE table_name DROP PRIMARY KEY, ADD PRIMARY KEY(key1, key2, key2);

 

Hope this is helpful 🙂