understanding primary key integrity constraint

Author: techysanoj

primary_key_integrity_constraint.sql

@@ -0,0 +1,26 @@
+use temp_db;
+
+show tables;
+
+-- here primary key simplye means = NOT NULL + UNIQUE
+-- it is used to uniquely identify a tuple/record
+
+create table temp1(id int PRIMARY KEY, fullname varchar(15));
+
+insert into temp1 values(1, 'SANOJ'); -- no error
+insert into temp1 values(2, 'Kumar'); -- no error
+
+insert into temp1 values(1, 'DEEPAK'); -- gives error
+
+insert into temp1 values(NULL, 'DEEPAK kumar'); -- gives error
+
+
+-- tables can also be defined like this
+create table temp2(id int, fullname varchar(15), primary key(id)); -- here id is primary key
+
+create table temp3(id int, fullname varchar(15), primary key(id, fullname)); -- here id and fullname is primary key
+
+insert into temp3 values(1, 'SANOJ'); -- no error
+insert into temp3 values(1, 'DEEPAK'); -- no error because combination is primary key
+insert into temp3 values(NULL, 'DEEPAK') -- gives error because column can not contain null values
+

unique_integrity_constrain.sql

@@ -7,6 +7,10 @@ show tables;
 create table stu_2(stu_id int UNIQUE, stu_name varchar(15));
 
 create table stu_3(stu_id int, stu_name varchar(15), unique(stu_id)); -- here we have provided the constraint at column level
+-- here it means stu_id combinaton must be unique what if i have written
+create table stu_4(stu_id int, stu_name varchar(15), unique(stu_id, stu_name));
+-- it means combination of id and name must be unique
+-- like 1, sanoj is different than 1, deepak
 
 -- here stu_id must be unique