how to add constraint in postgresql using alter code example

Example 1: postgresql add column with constraint

ALTER TABLE customers 
ADD COLUMN contact_name VARCHAR NOT NULL;

Example 2: alter constraint postgres

/* Drop your constraint */
alter table yourtable drop constraint constraint_name;

/* Create new constraint */
alter table yourtable add constraint "constraint_name" FOREIGN KEY (some_id) REFERENCES yourtable(some_id) on delete cascade;