constraints in sql

Author: mayankSystems

src/main/java/dev/mayankg/db_concepts/mysql/exc02/sql_constraints.sql

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+-- Create a contacts table with unique phone numbers
+CREATE TABLE contacts (
+    name VARCHAR(100) NOT NULL,
+    phone VARCHAR(15) NOT NULL UNIQUE
+);
+
+-- Insert a valid contact
+INSERT INTO contacts (name, phone)
+VALUES ('billybob', '8781213455');
+
+-- This will cause an error due to UNIQUE constraint violation on phone
+-- INSERT INTO contacts (name, phone)
+-- VALUES ('billybob', '8781213455');
+
+-- Create a users table with a CHECK constraint on age
+CREATE TABLE users (
+    username VARCHAR(20) NOT NULL,
+    age INT CHECK (age > 0)
+);
+
+-- Attempt to create a palindromes table with a CHECK using REVERSE (this will likely fail in many RDBMS as REVERSE is not allowed in CHECK)
+CREATE TABLE palindromes (
+    word VARCHAR(100) CHECK (REVERSE(word) = word)
+);
+
+-- Safer version with named CHECK constraint on users2 table
+CREATE TABLE users2 (
+    username VARCHAR(20) NOT NULL,
+    age INT,
+    CONSTRAINT age_not_negative CHECK (age >= 0)
+);
+
+-- Another attempt at a palindrome table using a named CHECK constraint
+CREATE TABLE palindromes2 (
+    word VARCHAR(100),
+    CONSTRAINT word_is_palindrome CHECK (REVERSE(word) = word)
+);
+
+-- Create a companies table with a composite UNIQUE constraint on name and address
+CREATE TABLE companies (
+    name VARCHAR(255) NOT NULL,
+    address VARCHAR(255) NOT NULL,
+    CONSTRAINT name_address UNIQUE (name, address)
+);
+
+-- Create a houses table with a CHECK constraint to ensure sale price ≥ purchase price
+CREATE TABLE houses (
+    purchase_price INT NOT NULL,
+    sale_price INT NOT NULL,
+    CONSTRAINT sprice_gt_pprice CHECK (sale_price >= purchase_price)
+);