aggregate function

Author: amantiwari8861

Diff for: first.sql

@@ -379,5 +379,6 @@ select * from products order by productname desc;
 drop table products;
 drop table categories;
 
+use mydatabase;
 
-
+select * from categories join products;

Diff for: groupby.sql

@@ -0,0 +1,27 @@
+create database groupdb;
+use groupdb;
+create table students(roll int primary key auto_increment,
+name varchar(30),course varchar(30),marks varchar(40),dept varchar(30));
+
+select * from students;
+truncate students;
+
+select * from students group by dept ;
+
+select  count(*) from students;
+select distinct count(*) from students;
+select min(marks) from students;
+select max(marks) from students;
+select avg(marks) from students;
+select sum(marks)/avg(marks) as 'count' ,count(*) from students;
+
+set @num1=50,@num2=30;
+
+select @num1+@num2 as 'sum';
+
+prepare stmt1 from 
+'select ? + ? as sum ;';
+
+execute stmt1 using @num1,@num2;
+
+deallocate prepare stmt1;

Diff for: join.sql

@@ -101,3 +101,36 @@ union
 select * from categories c
 right join products p
 on c.categoryid=p.categoryid;
+
+
+-- ----------------------------
+-- 04/08/2021 joining multiple tables 
+use mydatabase;
+
+create table user_info(userid int primary key auto_increment,
+username varchar(20),password varchar(20),productid varchar(20),
+foreign key(productid) references products(product_id));
+
+insert into user_info(username,password,productid) values('aman','123','201'),('rohan','456','203'),('rohit','789','204');
+
+select * from categories c
+inner join products p on c.categoryid=p.categoryid
+inner join user_info u on u.product_id=p.product_id;
+
+-- or
+alter table user_info rename column productid to product_id;
+
+select * from categories c
+inner join products p using(categoryid)
+inner join user_info u using(product_id);
+-- --------------------------------
+
+select * from categories c
+left join products p
+on c.categoryid=p.categoryid
+where p.categoryid is null
+union
+select * from categories c
+right join products p
+on c.categoryid=p.categoryid
+where c.categoryid is null;