Do you need a primary key for this?

Question

MySQL

There is a table-binding user-social network. The structure is:
user_id , service_id , id_in_service .
Do I need a primary key for this case?

Answer 1

primary key is always needed.
If, according to the logic of your application, only one user in each service_id can correspond to one user_id, then PK for the first two fields.
If there are several, then pk into 3 fields.
The order of the fields when specifying the PK of the link table is important. Very important for mysql/innodb, because the table is clustered by PK, and any access to the secondary key results in a search by PK (except for covering indexes). Choose the order based on the most frequent requests to the table, so that the maximum requests can go by the primary key.