The PRIMARY KEY INDEX is the most common type of index found in relational databases. In MySQL PRIMARY KEY is a field in a table that uniquely identifies each record in that table. In other words, all values in the indexed column(s) must occur once. A PRIMARY KEY must be unique and cannot contain NULL values. MySQL allows to create only one PRIMARY KEY in a table. It must be named "PRIMARY". AUTO_INCREMENT columns mostly use this type. The primary keys are almost always added when creating the table. However, these can also be created using some other sql statements.
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