It was said in the video that the similarity between the two types (primary and unique) of constraints is that both can be applied to multiple columns, not just a single column. So how do you apply a primary key if there can be only one primary key in a table?

Hi Edu! Thanks for reaching out. There's the possibility to create a composite primary key, where not just a single column, but multiple columns define the uniqueness of the primary key. For instance, if a single column determines the primary key, all values in the column must be different. E.g. (id_001), (id_002) etc. Otherwise, if you are using a composite key, the id column may be combined with, e.g. the date column, so the primary keys become of the following type: (id_001, '2021-10-24'), (id_001, '2021-10-25'), (id_002, '2021-10-24'), (id_002, '2021-10-30') etc. Hope this helps. Best, Martin

Hello, how do we combine two columns to primary key ? Is there a specific format to define composite primary key? tx

Hi Anuradha! Thanks for reaching out. When you define your table the syntax for a composite primary key is the following: PRIMARY KEY (column_1, column_2, ...) Hope this helps. Best, Tsvetelin