How to ALTER TABLE to add CHECK constraint?

Question

I want to ALTER my already created ZIPCODE table so that the sales_tax_applied field can never have a negative value. Here is the code for my already created table:

CREATE TABLE ZIPCODE 
( 
  city VARCHAR(50) NOT NULL, 
  state CHAR(2) NOT NULL, 
  zipcode VARCHAR(20) NOT NULL, 
  sales_tax_applied DECIMAL(10,4) NOT NULL, 
  PRIMARY KEY (zipcode) 
);

What is the code for me to be able to do this? NOTE: all fields must be NOT NULL.

Thank you

Answer 1

You would do:

alter table zipcode add constraint chk_zipcode_sales_tax_applied
    check (sales_tax_applied >= 0);

You can also add this into the create table statement in multiple ways, such as:

CREATE TABLE ZIPCODE ( 
  city VARCHAR(50) NOT NULL, 
  state CHAR(2) NOT NULL, 
  zipcode VARCHAR(20) NOT NULL, 
  sales_tax_applied DECIMAL(10,4) NOT NULL, 
  PRIMARY KEY (zipcode),
  constraint chk_zipcode_sales_tax_applied check (sales_tax_applied >= 0)
);