Adding constraints to existing columns

Описание: This video explains how to add constraints to existing columns in the table.
link to HOW to add constraint while creating table: https://    • ADD CONSTRAINTS to a new table  
link to how to add new columns with constraints :    • adding  constraints to NEW columns in an e...  
link to how to add constraints to existing column    • Adding constraints to existing columns  
how to drop all constraints on the table:    • drop primary key,not null,unique,default a...  
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