constraints.md

Primary key

create table employee (emp_id int PRIMARY KEY, emp_name varchar);  - creating primary key


alter table employee add constaint p_key PRIMARY KEY (emp_id);  - updating a primary key

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Foreign key

 create table employee (emp_id int PRIMARY KEY, emp_name varchar, dept_id int); `

 create table departments (id serial, dept_name varchar, dept_id int references employees(dept_id) ` - creating foreign key

 alter table employees add constaint f_key Foreign key (dept_id) references employees (dept_id); `

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Not null

create table employees (id serial, f_name varchar NOT NULL, age int);  - creating not null constraint

alter table employees alter column age set NOT NULL;  - updating an existing column

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Check

create table shop(id serial, pr_name varchar, price int check(price > 0)); 

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Unique

create table employees1 (id serial, name varchar UNIQUE, age int);