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SQL Joins

A SQL JOIN is used to combine two or more tables, based on a related column between tables.

Let's take a look selection from the "Orders" table:

OrderId CustomerId OrderDate
1001 1 2020-05-22
1002 2 2020-05-22
1003 3 2020-05-22

Then, look at a selection from the "Customers" table:

CustomerId CustomerName Country
1 John USA
2 Bill USA
3 Jimmy USA

Now, the relationship between Orders table and Customers table is the "CustomerId" column.

Then, we can create the following SQL statement that contains an INNER JOIN, It'll select records that have matching values in both tables:


SELECT o.OrderId, c.CustomerName, o.OrderDate
FROM Orders as o
INNER JOIN Customers as c ON o.CustomerId=c.CustomerId

and above SQL statement will produce following result like this:

OrderId CustomerName OrderDate
1001 John 2020-05-22
1002 Bill 2020-05-22
1003 Jimmy 2020-05-22

 

Types of SQL JOINs

  1. INNER JOIN : Return records that have matching value in both tables.
  2. LEFT OUTER JOIN : Returns all records from the left table, and the matched records from the right table
  3. RIGHT OUTER JOIN : Returns all records from the right table, and the matched records from the left table
  4. FULL OUTER JOIN : Returns all records when there is a match in either left or right table
     
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