root@host# mysql -u root -p password;
Enter password:*******
mysql> use TUTORIALS;
Database changed
mysql> SELECT * FROM tcount_tbl;
+-----------------+----------------+
| tutorial_author | tutorial_count |
+-----------------+----------------+
| mahran     |       20 |
| mahnaz     |      NULL |
| Jen       |      NULL |
| Gill      |       20 |
| John Poul    |       1 |
| Sanjay     |       1 |
+-----------------+----------------+
6 rows in set (0.01 sec)
mysql> SELECT * from tutorials_tbl;
+-------------+----------------+-----------------+-----------------+
| tutorial_id | tutorial_title | tutorial_author | submission_date |
+-------------+----------------+-----------------+-----------------+
|      1 | Learn PHP   | John Poul    | 2007-05-24   |
|      2 | Learn MySQL  | Abdul S     | 2007-05-24   |
|      3 | JAVA Tutorial | Sanjay     | 2007-05-06   |
+-------------+----------------+-----------------+-----------------+
3 rows in set (0.00 sec)
mysql>

現(xiàn)在，加入這兩個表我們可以編寫一個SQL查詢。這個查詢會選擇所有的作者從表tutorials_tbl，從tcount_tbl會拿起相應數(shù)量的教程。

mysql> SELECT a.tutorial_id, a.tutorial_author, b.tutorial_count
  -> FROM tutorials_tbl a, tcount_tbl b
  -> WHERE a.tutorial_author = b.tutorial_author;
+-------------+-----------------+----------------+
| tutorial_id | tutorial_author | tutorial_count |
+-------------+-----------------+----------------+
|      1 | John Poul    |       1 |
|      3 | Sanjay     |       1 |
+-------------+-----------------+----------------+
2 rows in set (0.01 sec)
mysql>

在PHP腳本中使用聯(lián)接：

可以使用任何上述的SQL查詢的PHP腳本。只需要通過PHP函數(shù)mysql_query()執(zhí)行SQL查詢，然后用常規(guī)方法獲取結果。
例子:

試試下面的例子：

<?php
$dbhost = 'localhost:3036';
$dbuser = 'root';
$dbpass = 'rootpassword';
$conn = mysql_connect($dbhost, $dbuser, $dbpass);
if(! $conn )
{
 die('Could not connect: ' . mysql_error());
}
$sql = 'SELECT a.tutorial_id, a.tutorial_author, b.tutorial_count
    FROM tutorials_tbl a, tcount_tbl b
    WHERE a.tutorial_author = b.tutorial_author';

mysql_select_db('TUTORIALS');
$retval = mysql_query( $sql, $conn );
if(! $retval )
{
 die('Could not get data: ' . mysql_error());
}
while($row = mysql_fetch_array($retval, MYSQL_ASSOC))
{
  echo "Author:{$row['tutorial_author']} <br> ".
     "Count: {$row['tutorial_count']} <br> ".
     "Tutorial ID: {$row['tutorial_id']} <br> ".
     "--------------------------------<br>";
} 
echo "Fetched data successfully\n";
mysql_close($conn);
?>

MySQL左連接:

一個簡單的連接和一個MySQL左連接是不同的。一個MySQL LEFT JOIN提供了額外的考慮到在左邊的表。

如果做了LEFT JOIN，得到的所有記錄以同樣的方式相匹配，此外，得到一個額外的記錄每個不匹配的記錄，在左表中的聯(lián)接 - 從而保證了每一個作者得到關聯(lián)（本例子中）：
實例:

試試下面的例子就明白了LEFT JOIN：

root@host# mysql -u root -p password;
Enter password:*******
mysql> use TUTORIALS;
Database changed
mysql> SELECT a.tutorial_id, a.tutorial_author, b.tutorial_count
  -> FROM tutorials_tbl a LEFT JOIN tcount_tbl b
  -> ON a.tutorial_author = b.tutorial_author;
+-------------+-----------------+----------------+
| tutorial_id | tutorial_author | tutorial_count |
+-------------+-----------------+----------------+
|      1 | John Poul    |       1 |
|      2 | Abdul S     |      NULL |
|      3 | Sanjay     |       1 |
+-------------+-----------------+----------------+
3 rows in set (0.02 sec)

需要做更多的實踐才能熟悉JOINS。這是一個復雜的概念，在MySQL/SQL將變得更加清晰。

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