Constraints in sql

RULES ABOUT THE COLUMNS IN TABLE
NOT NULL 	 # Ensures a column cannot have a NULL value
UNIQUE 		 # Ensures all values in a column are unique
PRIMARY KEY  # Identifies a record in a table, is NOT NULL & UNIQUE
FOREIGN KEY  # References a unique record from another table
CHECK		 # Ensures all column values satisfy a condition
DEFAULT		 # Set a default value for a column if none is entered
INDEX		 # Quick way of retrieving records from database

Sql constarint

It creates a new constraint on an existing table, which is used to specify
rules for any data in the table.
Example: Adds a new PRIMARY KEY constraint named ‘user’ on columns
ID and SURNAME.
ALTER TABLE users
ADD CONSTRAINT user PRIMARY KEY (ID, SURNAME);

Similar codes

4 votes273 views2 answers
Hosts file
TypeScript
14 votes296 views2 answers
Upgrade to typescript in react js
TypeScript
0 vote331 views1 answer
Vue bootstrap tooltip
TypeScript
1 vote263 views1 answer
Track changes of input in angular
TypeScript
0 vote257 views1 answer
Nestjs get request header in guard
TypeScript
1 vote259 views1 answer
Mysql database exists or not
TypeScript
0 vote280 views1 answer
Fiber absorption loss measurement
TypeScript
1 vote234 views1 answer
Ionic pasword visible inside ion-input
TypeScript
0 vote273 views1 answer
Additional data structures
TypeScript
0 vote274 views4 answers
How to implement read more and readless in angular
TypeScript