LEFT OUTER JOIN

A LEFT OUTER JOIN is one of the JOIN operationss that allow you to specify a join clause. It preserves the unmatched rows from the first (left) table, joining them with a NULL row in the shape of the second (right) table.

Syntax

TableExpression LEFT [ OUTER ] JOIN TableExpression
{
    ON booleanExpression 
    }

The scope of expressions in either the ON clause includes the current tables and any tables in query blocks outer to the current SELECT. The ON clause can reference tables not being joined and does not have to reference either of the tables being joined (though typically it does).

Example 1

--match cities to countries in Asia

SELECT CITIES.COUNTRY, CITIES.CITY_NAME, REGION 
FROM Countries 
LEFT OUTER JOIN Cities
ON CITIES.COUNTRY_ISO_CODE = COUNTRIES.COUNTRY_ISO_CODE
WHERE REGION = 'Asia';

-- use the synonymous syntax, LEFT JOIN, to achieve exactly 
-- the same results as in the example above

SELECT  COUNTRIES.COUNTRY, CITIES.CITY_NAME,REGION 
FROM COUNTRIES 
LEFT JOIN CITIES 
ON CITIES.COUNTRY_ISO_CODE = COUNTRIES.COUNTRY_ISO_CODE
WHERE REGION = 'Asia';

Example 2


-- Join the EMPLOYEE and DEPARTMENT tables, 
-- select the employee number (EMPNO), 
-- employee surname (LASTNAME), 
-- department number (WORKDEPT in the EMPLOYEE table
-- and DEPTNO in the DEPARTMENT table) 
-- and department name (DEPTNAME) 
-- of all employees who were born (BIRTHDATE) earlier than 1930

SELECT EMPNO, LASTNAME, WORKDEPT, DEPTNAME 
   FROM SAMP.EMPLOYEE LEFT OUTER JOIN SAMP.DEPARTMENT 
   ON WORKDEPT = DEPTNO 
   AND YEAR(BIRTHDATE) < 1930;

-- List every department with the employee number and 
-- last name of the manager,
-- including departments without a manager

SELECT DEPTNO, DEPTNAME, EMPNO, LASTNAME
      FROM DEPARTMENT LEFT OUTER JOIN EMPLOYEE
	    ON MGRNO = EMPNO;