Sql Join Understanding
by reading this content you can come to know how join is working in sql to get output faster and safer.
SQL Joins are used to relate information in different tables. A Join condition is a part of the sql query that retrieves rows from two or more tables. A SQL Join condition is used in the SQL WHERE Clause of select, update, delete statements.
The Syntax for joining two tables is:
SELECT col1, col2, col3...
FROM table_name1, table_name2
WHERE table_name1.col2 = table_name2.col1;
If a sql join condition is omitted or if it is invalid the join operation will result in a Cartesian product. The Cartesian product returns a number of rows equal to the product of all rows in all the tables being joined. For example, if the first table has 20 rows and the second table has 10 rows, the result will be 20 * 10, or 200 rows. This query takes a long time to execute.
Lets use the below two tables to explain the sql join conditions.
database table "product";
product_id product_name supplier_name unit_price
100 Camera Nikon 300
101 Television Onida 100
102 Refrigerator Vediocon 150
103 Ipod Apple 75
104 Mobile Nokia 50
database table "order_items";
order_id product_id total_units customer
5100 104 30 Infosys
5101 102 5 Satyam
5102 103 25 Wipro
5103 101 10 TCS
SQL Joins can be classified into Equi join and Non Equi join.
1) SQL Equi joins
It is a simple sql join condition which uses the equal sign as the comparison operator. Two types of equi joins are SQL Outer join and SQL Inner join.
For example: You can get the information about a customer who purchased a product and the quantity of product.
2) SQL Non equi joins
It is a sql join condition which makes use of some comparison operator other than the equal sign like >, <, >=, <=
you can below link check are information about join and types of join and join to where use to example are show
->left outer join
->right outer join
->full outer join
(4)left outer join where null
(5)right outer join where null
(6)outer join where null