HOWTO: Mysql JOIN

MySQL LEFT JOIN Explanation

(blogged from: http://www.tizag.com/mysqlTutorial/mysqlleftjoin.php )

How is a LEFT JOIN different from a normal join? First of all, the syntax is quite different and somewhat more complex. Besides looking different, the LEFT JOIN gives extra consideration to the table that is on the left.

Being "on the left" simply refers to the table that appears before the LEFT JOIN in our SQL statement. Nothing tricky about that.

This extra consideration to the left table can be thought of as special kind of preservation. Each item in the left table will show up in a MySQL result, even if there isn't a match with the other table that it is being joined to.

MySQL Join and LEFT JOIN Differences

Here are the tables we used in the previous Mysql Joins lesson.

MySQL family and food Tables:

Position	Age
Dad	41
Mom	45
Daughter	17
Dog

Meal	Position
Steak	Dad
Salad	Mom
Spinach Soup
Tacos	Dad

We executed a simple query that selected all meals that were liked by a family member with this simple join query:

Simplified MySQL Query:

SELECT food.Meal, family.Position
FROM family, food 
WHERE food.Position = family.Position

Result:

Dad - Steak
Mom - Salad
Dad - Tacos

When we decide to use a LEFT JOIN in the query instead, all the family members be listed, even if they do not have a favorite dish in our food table.

This is because a left join will preserve the records of the "left" table.

MySQL LEFT JOIN Example

The code below is the exact same as the code in the previous lesson, except the LEFT JOIN has now been added to the query. Let's see if the results are what we expected.

PHP and MySQL Code:

// Make a MySQL Connection
// Construct our join query
$query = "SELECT family.Position, food.Meal ".
"FROM family LEFT JOIN food ".
 "ON family.Position = food.Position";
 
$result = mysql_query($query) or die(mysql_error());


// Print out the contents of each row into a table
while($row = mysql_fetch_array($result)){
 echo $row['Position']. " - ". $row['Meal'];
 echo "
";
}
?>

Display:

Dad - Steak
Dad - Tacos
Mom - Salad
Daughter -
Dog -

Success! The LEFT JOIN preserved every family member, including those who don't yet have a favorite meal in the food table! Please feel free to play around with LEFT JOIN until you feel like you have a solid grasp of it. This stuff isn't easy!

Another example for 'JOIN', 'LEFT JOIN' and 'RIGHT JOIN' is here:

http://www.wellho.net/mouth/158_MySQL-LEFT-JOIN-and-RIGHT-JOIN-INNER-JOIN-and-OUTER-JOIN.html