For example we have two tables: products and buyers with the following structures.
Table products:
mysql> SELECT * FROM buyers;
Mysql starts with the left table (buyers). For each row from the table buyers mysql scans the table products, finds the id of the product and returns the product name. Then the product name is joined with the matching row from the table buyers. For unmatched rows it returns null.
To make it simpler, the above query is same as (except the unmatched rows are not returned):
Table products:
mysql> SELECT * FROM products; +----+--------------+--------------+ | id | product_name | manufacturer | +----+--------------+--------------+ | 1 | Shoes | Company1 | | 2 | Laptop | Company2 | | 3 | Monitor | Company3 | | 4 | DVD | Company4 | +----+--------------+--------------+ 4 rows in set (0.00 sec)Table buyers:
mysql> SELECT * FROM buyers;
+----+------+------------+----------+ | id | pid | buyer_name | quantity | +----+------+------------+----------+ | 1 | 1 | Steve | 2 | | 2 | 2 | John | 1 | | 3 | 3 | Larry | 1 | | 4 | 3 | Michael | 5 | | 5 | NULL | Steven | NULL | +----+------+------------+----------+ 5 rows in set (0.00 sec)
Left Join
mysql> SELECT buyer_name, quantity, product_name FROM buyers LEFT JOIN products ON buyers.pid=products.id; +------------+----------+--------------+ | buyer_name | quantity | product_name | +------------+----------+--------------+ | Steve | 2 | Shoes | | John | 1 | Laptop | | Larry | 1 | Monitor | | Michael | 5 | Monitor | | Steven | NULL | NULL | +------------+----------+--------------+ 5 rows in set (0.00 sec)
What happened?
Mysql starts with the left table (buyers). For each row from the table buyers mysql scans the table products, finds the id of the product and returns the product name. Then the product name is joined with the matching row from the table buyers. For unmatched rows it returns null.
To make it simpler, the above query is same as (except the unmatched rows are not returned):
mysql> SELECT buyers.buyer_name, buyers.quantity, products.product_name FROM buyer s,products WHERE buyers.pid=products.id; +------------+----------+--------------+ | buyer_name | quantity | product_name | +------------+----------+--------------+ | Steve | 2 | Shoes | | John | 1 | Laptop | | Larry | 1 | Monitor | | Michael | 5 | Monitor | +------------+----------+--------------+ 4 rows in set (0.00 sec)
Right Join
mysql> SELECT buyer_name, quantity, product_name FROM buyers RIGHT JOIN products ON buyers.pid=products.id; +------------+----------+--------------+ | buyer_name | quantity | product_name | +------------+----------+--------------+ | Steve | 2 | Shoes | | John | 1 | Laptop | | Larry | 1 | Monitor | | Michael | 5 | Monitor | | NULL | NULL | DVD | +------------+----------+--------------+ 5 rows in set (0.00 sec)What happens here is Mysql starts with the Right table (products). For each id from the table products MySQL scans the left table - buyers to find the matching pid. When it finds the matching pid it returns the buyer_name and the quantity. For unmatched rows it returns null. From my example above it returns NULL for DVD because no one bought DVD.
