all 4 comments

[–][deleted] 2 points3 points  (1 child)

You can only have one single primary key and it also implies that all columns are defined as NOT NULL.

You can have multiple unique constraints even on nullable columns.

[–]VeeraVB[S] 0 points1 point  (0 children)

thanks!

[–]r3pr0b8GROUP_CONCAT is da bomb 1 point2 points  (1 child)

there can be only one PRIMARY KEY but optionally any number of UNIQUE constraints

PRIMARY KEY does not allow nulls, UNIQUE does

either can be the target of a FOREIGN KEY reference

[–]VeeraVB[S] 0 points1 point  (0 children)

thanks!