SQL LEFT JOIN

What is a LEFT JOIN in SQL?

A LEFT JOIN performs a join starting with the first (left-most) table.
Then, any matched records from the second table (right-most) will be included.
LEFT JOIN and LEFT OUTER JOIN are the same.

The SQL LEFT JOIN syntax

The general LEFT JOIN syntax is

SELECT column-names
  FROM table-name1 LEFT JOIN table-name2 
    ON column-name1 = column-name2
 WHERE condition

The general LEFT OUTER JOIN syntax is

SELECT column-names
  FROM table-name1 LEFT OUTER JOIN table-name2 
    ON column-name1 = column-name2
 WHERE condition

ORDER
Id
OrderDate
OrderNumber
CustomerId
TotalAmount

CUSTOMER
Id
FirstName
LastName
City
Country
Phone

SQL LEFT JOIN Example

Problem: List all customers and the
total amount they spent irrespective
whether they placed any orders
or not.

SELECT OrderNumber, TotalAmount, FirstName, LastName, City, Country
  FROM Customer C LEFT JOIN [Order] O
    ON O.CustomerId = C.Id
 ORDER BY TotalAmount

Note: The ORDER BY TotalAmount shows the customers without orders first (i.e. TotalMount is NULL).

Result: 832 records

OrderNumber	TotalAmount	FirstName	LastName	City	Country
NULL	NULL	Diego	Roel	Madrid	Spain
NULL	NULL	Marie	Bertrand	Paris	France
542912	12.50	Patricio	Simpson	Buenos Aires	Argentina
542937	18.40	Paolo	Accorti	Torino	Italy
542897	28.00	Pascale	Cartrain	Charleroi	Belgium
542716	28.00	Maurizio	Moroni	Reggio Emilia	Italy
543028	30.00	Yvonne	Moncada	Buenos Aires	Argentina
543013	36.00	Fran	Wilson	Portland	USA