Need to change MUL key to PRI key - mysql

Question

Initially the Primary Key was
(caseId,osName)
+--------------------+---------------+------+-----+-------------------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------------------+---------------+------+-----+-------------------+-------+
| createdBy | varchar(200) | NO | | NULL | |
| createdOn | timestamp | NO | | CURRENT_TIMESTAMP | |
| osName | varchar(200) | NO | | NULL | |
| caseId | varchar(100) | NO | MUL | NULL | |
| issueType | varchar(100) | NO | | NULL | |
| observationState | varchar(60) | YES | | NULL | |
| problemSolving | int(11) | NO | | NULL | |
| structure | int(11) | NO | | NULL | |
| logicalTransition | int(11) | NO | | NULL | |
| ownership | int(11) | NO | | NULL | |
| expectationSetting | int(11) | NO | | NULL | |
| empathy | int(11) | NO | | NULL | |
| valueAdd | int(11) | NO | | NULL | |
| comments | varchar(2000) | NO | | NULL | |
| version | int(11) | YES | | 0 | |
| showcase_escalate | varchar(30) | YES | | NULL | |
| ringback_state | varchar(200) | YES | | NULL | |
| trt_state | varchar(200) | YES | | NULL | |
| srp_state | varchar(200) | YES | | NULL | |
| id | int(11) | NO | PRI | NULL | |
+--------------------+---------------+------+-----+-------------------+-------+
In the above schema, id was AUTO INCREMENT. I removed that so I could make caseId as PRIMARY KEY. But since there are multiple records with same caseId I am not able to make the change.
At the end I just want caseId as my Primary Key.
Initially the Primary Key was (caseId,osName).

Answer 1

As defined in the MySQL Glossary:
primary key
A set of columns -- and by implication, the index based on this set of columns -- that can uniquely identify every row in a table. As such, it must be a unique index that does not contain any NULL values.
InnoDB requires that every table has such an index (also called the clustered index or cluster index), and organizes the table storage based on the column values of the primary key.
When choosing primary key values, consider using arbitrary values (a synthetic key) rather than relying on values derived from some other source (a natural key).
See Also clustered index, index, natural key, synthetic key.
Since you have multiple records with the same value for caseId, caseId alone cannot "uniquely identify every row" in your table; and therefore caseId alone cannot be a PRIMARY KEY.